Java concepts and questions

JAVA notes

Prepared by

Farag Zakaria Safy Saad

Master Student @ Cairo University

Faculty of Computers & Information
ITI-JAVA Intake 30

farag_cs2005@yahoo.com

This is my own study notes; please keep free to send me any errors exist or any other top
you need to add.

1. Key word & reserved word
key word is a word whose meaning is defined by the programming language as int,…
Reserved word  they have no meaning by programming language as goto,const
You cannot use any key word or reserved word in defining variables
2. Allowed access modifiers for the public class are public , final , abstract , strictfp only
3. Defining memory heap size when running java applications
-Xms32m defines minimum heap space as 32 mega
-Xmx32m defines maximum heap space as 32 mega.
Ex.
java -Xms32m –Xmx512m Test // run Test class with minimum heap size 32mega
and maximum heap size 512 mega bytes
4. Garbage collection JVM parameters
-XX:+UseSerialGC run serial garbage collection.
-XX:+UseParallelGC runs parallel garbage collection.
-XX:+UseParallelOldGC runs parallel compact collector
-XX:ParallelCMSThreads=n runs concurrent Mark and Sweep collector with
n threads.
-XX:+UseConcMarkSweepGC runs Concurrent Mark and Sweep collector.
5. strictfp
Marking aclass as strictfp means that any method code in the class will conform
to the IEEE 754 standard rules for floating point s
6. All wrapper classes have no default constructor
Wrapper classes constructors are WrapperType(Type) , Wrapper(String).
see next example.
7. Byte wrapper class constructors are Byte(byte), Byte(String)
ex. Byte b = new Byte(); // compilation error undefined constructor Byte()
Byte b = new Byte(3); // compilation error
Byte b = new Byte((byte)3); // true
Byte b = 21; // true

8. All data types are signed except char or boolean
char is integral but unsigned  its range from 0 to (2^7-1) // ^ stands for power
9. Identifiers
Identifier must begin with one of ( $,_,letter ) only
Ex. int %t = 1; compilation error illegal identifier
10. Array Declaration
int marks[] ; // [] doesn't matter before array name or after it.
int []marks;
in methods also 
int[] myMethod()
int myMethod()[] //// [] doesn't matter before method name or after it
int myMethod[] () // Compilation error
//////////////
int[] marks; // declaration to the compiler
marks = new int[2]; // construction of array in runtime
== when an array constructed it's all elements are initialized to their defaults
11. Static import introduced in java 1.5
Ex. instead of using Math.PI
you can do = import static Math.PI
and use PI in your code without referring to class Math
Ex. double pi = PI
12. Variable types
1. Member variable  these are the non static variables of the class when an object
is constructed(belongs to specific object).
The Compiler initialize them to their default values.
2. Automatic variables(local variables)  are the variables of a method when it
executed. Must be explicity initialized before being used.
3. Class variables (static variables)  exists even if the class is not instanciated
(no object created from this class).
Compiler initialize them to their default values;

class ABC
{
int x; // member variable initialized with 0;
int z; // class variable initialized with 0;

public void printZ()
{
int z;
System.out.println(z); /// compilation error z must be initialized
}
}
13. Compilation units in java source file
a. package declaration

b. import statements
c. class, interface , enum definitions

You compile with the file name but run with the class name
for example if you have file Good.java which contains
class ABC
{
public static void main(String args[])
{}
}
compilation command is ====== javac Good.java to run === java ABC
14. Evaluation order

Ex.
int[] a = {4,4};
int b = 1;
a[b] = b = 0; // evaluation from left to right requires that a[b] evaluated first then b

so  a[1] = b; and b = 0;  then a[1]=0
15. Difference between & and &&
in & both left operand and right operands are evaluated
in && the right operand is not evaluated if the left operand is false because the
whole operation is false
boolean a = true;
boolean b = false;
if ( b && (a = false) )
{
System.out.println("test");
}
System.out.println("a = " + a + " , b = " + b); // prints a = true , b = false

boolean a = true;
boolean b = false;
if ( b & (a = false) )
{
System.out.println("test");
}
System.out.println("a = " + a + " , b = " + b); // prints a = false , b = false

16. Difference between | and ||
| will evaluate two operands
|| if the first operand is evaluated to true the second operand will not evaluated
Example
public static void test(String s)
{
if ( str == null | str.length() == 0)

{
System.out.println("String is empty");
}
}

when called with test(null) the first operand is evaluated to ture and the second
operand will be evaluated so it will through NullPointerException

//
public static void test(String s)
{
if ( str == null // str.length() == 0)
{
System.out.println("String is empty");
}
}

when called with test(null) the first operand is evaluated to true and prints String is
empty

17. switch(x)
x must be one of (1. byte 2. short 3. char 4. int)
Each of the values specified in the case statements must be of a type compatible
with or assignable to the switch expression. Each case value must be a compile
time constant/expression, not a variable.
Ex.
int x = 10;
final int a = 5;
int b = 10;
final int c; //c is final variable but not constant.
c = 10;

switch (x) {
case a: // a must be final (compile time constant)
case b: // compilation error (b must be constant)
case c: // compilation error c is not a constant
default:
}
18. Conversion & Casting
All conversion rules are enforced at compile time.
Conversion occurs in the three situations
1. Assignment.
2. Method call.
3. Arithmetic promotions.
19. You cannot cast Boolean to any type or vice versa

20. Arithmetic promotion rules
1. For unary operators (++ , -- , …….)
- if operand is byte,short,char it is converted to an int.
- if it is any other type it is not converted.
2. For binary operands
- if one of the operands is double, float, long, the second is converted to the same
type ELSE Both operands converted to int.
21. + operator with strings and numbers
The + operator is syntactically left-associative, the expression a + b + c is always
regarded as (a + b) + c.
Ex.
System.out.println(1 + 2 + " fiddlers"); // prints 3 fiddlers
// it is computed as ((1+2)+"fiddlers")  3 + "fiddlers" = 3 fiddlers
System.out.println("fiddlers " + 1 + 2); // prints fiddlers 12
// ( ("fiddlers " + 1 ) + 2)  ("fiddlers 1" + 2) = "fiddlers 12"
22. Implicit narrowing conversion
The implicit narrowing conversion work with the method return statement
constants.
Ex.
class ABC
{
byte method_1()
{
return 126; // ok no problems, although 126 is an integer not byte
}
}
Ex.2
class ABC
{
byte method_1()
{
final int m = 127;
return m; // ok no problems, although m is an integer not byte
}
}
Ex.3
class ABC
{
byte method_1()
{
final int m = 128;
return m; // compilation error 128 is greater than a byte.
}
}
Ex.4
class ABC
{

byte method_1()
{
int m = 126;
return m; // compilation error m is not final.
// return type must be of type byte or final variable and its value must
be allocated to byte value.
}
}

23. Final variable in enhanced for loop
Ex.
String[] strArr = {"A", "B", "C", "D"};

for (final String s : strArr)
{ // compiles because it is local variable to the for loop
System.out.println(s);
}
this code compiles and run without errors because it is equal to the following code.

String[] strArr = {"A", "B", "C", "D"};
for (int index = 0; index < strArr.length; index++) {
final String s = strArr[index]; // compiles because it is local variable to the for loop
System.out.println(s);
}
24. Enumeration
is a set of constants(special kind of class)
syntax
enum WeekDays
{
Saturday,Sunday,Monday,Tuesday,Wednesday,Thursday,Friday
}; // ; is optional
enum properties
1. Cannot be defined in a method
Ex.
public void print()
{
enum A{} // compilation error enum A cannot be local
}
2. Enum cannot extends anther because multiple inheritance is not allowed in JAVA
Ex.
enum Day{}
enum Week extends Day{} // compilation error

Defining constructors
enum WeekDays
{
Saturday(1),Sunday(2),Monday(3),Tuesday(4),Wednesday(5),Thursday(6),Friday(7);
// ; is mandatory
int order;

WeekDays(int order)
{
this.order = order;
}
public int getOrder()
{
return order;
}
}; // ; is optional
Enum types must not be declared abstract; doing so will result in a compile-time
error.
Ex.
abstract enum Dog{D1,D2}; // Compile Error
// Illegal modifier for the member enum Dog; only public, protected, private & static are
// permitted

It is a compile-time error for an enum type E to have an abstract method m as a
member unless E has one or more enum constants, and all of E's enum constants
have class bodies that provide concrete implementations of m. It is a compile-time
error for the class body of an enum constant to declare an abstract method
Ex.
enum Dog
{
abstract void print(); // Compilation error Enum Dog must have constants
// because it defines abstract method
}
Ex. 2
enum Dog
{
D1, D2; // compilation error D1,D2 must implement abstract method print()
abstract void print();
}
The correct code of the previous
enum Dog
{
D1
{
void print(){}
}
,
D2
{
void print(){}
};
abstract void print();
}

25. Method overriding
- Done at runtime
- you cannot override methods with their return type only except the following case.
In Java 5.0, it introduces a new facility called covariant return type. You can
override a method with the same signature but returns a subclass of the object
returned.
covariant returns - that is, the specialization of the return type to a subtype (but only for
reference types)
Ex.
class A
{
A getObject()
{
System.out.println("A");
return new A();
}
}
class B extends A
{
B getObject() // Ok because B is subclass of A
// if anything other than A,B it will give compilation error
{
System.out.println("B");
return new B();
}
}
in main method
A a = new B();
a.getObject(); // will print B
- you cannot override methods to be more restricted in visibility.
- if you override method that throws exception, the overriding method must
1. Throws the same exception or children of that exception or doesn't throws
any exceptions.
2. It may throw any runtime exception as RuntimeException,
IndexOutOfBoundsException, …

class ABC
{
protected void method() throws FileNotFoundException
{}
}

class ABChild extends ABC
{
void method() throws NotSerializableException // two errors here
{ // 1. You cannot reduce method scope (method must be protected or public)

} // 2. Method must throws FileNotFoundException or one of its' children
} // This method can throw any runtime exception

//////////////
interface X
{
void print();
}
class YZ implements X
{
protected void print() // compilation error you cannot reduce method visibility
{ // because default access modifier for interface methods is public
}
}

--------------
A compile-time error occurs if a static method hides an instance method(non-
static methods)
BUT for variables it is permissible for a static variable to hide an instance variable
Ex.
class A
{
int x = 90;
void print(){}
}
class B extends A
{
static int x = 90; // ok no errors
static void print(){} // compile time error
// This static method cannot hide the instance method from A
// This method can hide print() method in class A if it is static.
}

26. Constructors
-- cannot be static
-- cannot be final
 only allowed access modifiers for constructors are public, protected, private,
default
Default constructors created if there is no any constructor in the class with the same
access modifier of the class otherwise it is not created.
Ex.
class ABC
{
public ABC(int x) // because existence of this constructor default const. doesn't
{} created
}

if you tried ABC a = new ABC(); in main it will give compilation error because
ABC() is undefined.
this() used to call another constructor in the same class and must appear in
the first line of the constructor
super() used to call the constructor of the super class and must appear in the first
line of the constructor.
Ex.
class A
{
A(int x) // because this constructor default didn't created
{}
}

class B extends A
{
B() // compilation error because there is embedded super() and default constructor for
A is not created
{} // to remove this error put call super(0); in B constructor
}

when creating an object from a class the compiler creates the super and member
classes first then creates the current object
27. Accessing protected member of super class from subclass via super class reference.
A method in subclass has access to the protected instance members/methods of its
superclass, but only if it accesses them via a subclass object
Ex.
package points;
public class Point {
protected int x, y;
}

package threePoint;
import points.Point;
public class Point3d extends Point {
protected int z;
public void delta(Point p) {
p.x += this.x; // compile-time error: cannot access p.x
p.y += this.y; // compile-time error: cannot access p.y
// x,y can be accessed but using reference from subclass Point3d because it is
// outside its package

}
public void delta3d(Point3d q) {
q.x += this.x;
q.y += this.y;

q.z += this.z;
}
}
28. Objects and classes
Data Hiding  defining the scope visibility of variables to public, protected, private
Encapsulation  aggregation of data and behavior.
(data hiding) + How to access these data(setters, getters, ….).

is-a  relationship indicates super class (Inheritance).
has-a  relationship describes the member variables.(Aggregation).

Overloading is the reuse of function name with different types and numbers of
parameters.(done at compilation time)
 Can't be done with return type only
Public level classes cannot be private or protected because it has no meaning
Ex.
public class XYZ
{}
private class ABC /// compilation error private is not allowed here
{}
Inner classes
is a class inside class or method.
- when you create an object from inner class you must have an object from the
outer class.
- It is a compile-time error if a static class contains a usage of a non-static
member of an enclosing class.
- Member interfaces are always implicitly static. It is permitted but not required
for the declaration of a member interface to explicitly list the static modifier
class Outer
{
class Inner
{
void print()
{System.out.println("Inner");}
}
}
// to create object from Inner inside main
Outer out = new Outer();
Inner inner = out.new Inner(); // or Inner in = new Outer().new Inner();
Local Inner classes (inside methods)
class Outer
{
int a;
void print(final int z , int x)
{
final int xn = 9;
int abc = 0;

class Inner // allowed to access members of Outer class and final variables
{ //inside the method print and final variables of its parameters
} // only a , z, xn variables are visible to Inner class
}
}

Static Inner classes
class Outer
{
static class Inner
{
}
}
// to create object from Inner  Outer.Inner in = new Outer.Inner();

////////////////////// Local inner classes override inner classes (see shadowing)
class Outer
{
class Inner
{
void print()
{ System.out.println("Inner class"); }
}
void printMessage()
{
class Inner
{
void print()
{ System.out.println("Local Inner class"); }
}

Inner in = new Inner();
in.print(); //// will print  Local Inner class
} // if local Inner class is not exist it will print  "Inner class"
}

inner classes cannot have static declarations unless they are compile-time constant
fields
Ex.
class Outer
{
class Inner
{
static final int x = 3; // OK no compilation errors
static int x; // compilation error  inner classes can't have static declarations
static void getX() // compilation error
{
return x;

}
}
static class NestedButNotInner
{
static int z=3; // OK no errors, not an inner class
}
interface NeverInner{} // interfaces are never inner because they are static by
// default
}

- Nested classes that are not inner classes may declare static members freely, in
accordance with the usual rules of the Java programming language
- Member interfaces are always implicitly static so they are never considered to be
inner classes.

Annonymous classes can be declared to extends another class or to implement a
single interface BUT not both.
Annonymous classes cannot have constructors because the name of the constructor
is the same as the name of the class and they didn't have names.

Nesting Classes
A compile-time error occurs if a class has the same simple name as any of its
enclosing classes or interfaces
Ex.
class Test
{
class Test{} /// compilation error The nested type Test cannot hide an enclosing type
}
29. Class Instance Initializers
It is compile-time error if an instance initializer of a named class can throw a
checked exception unless that exception or one of its supertypes is explicitly
declared in the throws clause of each constructor of its class and the class has at
least one explicitly declared constructor.

An instance initializer in an anonymous class can throw any exceptions.
Ex.
class Test
{
{
throw new Exception(); // compilation error
}
}
30. Final fields
1. A field can be declared final. Both class and instance variables
(static and non-static fields) may be declared final.
2. It is a compile-time error if a blank final class variable is not definitely

assigned by a static initializer of the class in which it is declared.
3. A blank final instance variable must be definitely assigned at the end of every
constructor of the class in which it is declared; otherwise a compile-time error
occurs.
Ex.
class Test
{
final int x;
public Test()
{
x = 10; // x must be assigned value in the constructor or compile time error occur
}
}
Ex.2
class Test
{
static final int x;
public Test()
{
x = 3; // compile time error  final field x cannot be assigned
}
static
{
x = 10; // x must be assigned value in the static initializer or compile time error occur
}
}

31. Interface
is a collection of constants and abstract methods.
interface MyInterface
{
// final , static , public  only allowed for variables
// variables must have initial value
public static final int x = 0; // public static final is default for variables
// public abstract are default for methods //
}

interface ABC
{
int a; //// compilation error a must be initialized because its default is
// public static final
}

Static by default so that they cannot be defined in Inner classes or inside methods
Ex.
class Outer
{

class Inner
{
interface X // Compilation error
{}
}
}
protected and private declarations are not allowed in interfaces, only allowed public
and abstract
interface X
{
protected void print(); // Compilation error protected and private is not allowed.
}
32. Static class loading vs dynamic class loading
Static class loading done using new operator
Ex. Car c = new Car(); // throws NoClassDefFoundException if Car cannot be found
Dynamic class loading done using reflection Class.forName(String classname)
// throws ClassNotFoundException
33. Static blocks (or static initializer)
executed only once before constructors to initialize static fields.

34. Null interfaces (marker interfaces), (Empty interfaces)
They don't have any function declaration on them just an empty interface.
They are treated differently in compiler such as Serializable interface.
Ex. interface ABC { }
35. Abstract class
// is a collection of abstract and non-abstract methods and variables

Abstract class Interface
May contain implemented methods All methods are abstract
Variables can be modified inside objects Variables are final static by default
Cannot extends an interface Cannot implements another interface
Can implements an interface Can extends another interface(one
or more)
May have no methods May have no methods (Marker
interface)
May have a constructor Cannot have constructors
Static by default so that cannot
defined inside Inner classes because
static is not allowed in Inner classes.
36. Abstract classes and interfaces are away to defer implementation to sub classes
They CANNOT be final because their methods should be overridden by children
classes.
37. … operator  for method parameters only
used to pass variable length of arguments to method
void print(int … x) // you can call it with print(1), print(1,2),print(1,2,…….)
// or print(new int[]{1,2,3,4});
// void print(int x…)  gives compilation error since … is not defined

/////////////
void print(int… x){}
void print(int x){} // No errors
void print(int[] x){} // compilation error (duplicate method) the same as print(int…x)
38. Checked Exceptions
are exceptions that doesn't descend from RuntimeException which must be handled
by try and catch or throws-declaration.
39. Runtime Exceptions(Unchecked Exceptions).
avoidable exceptions that are your own fault.(they should never happen)
Ex. RuntimeException, ArrayIndexOutOfBoundsException,……………..

Class Error and its subclasses also are unchecked.
40. Finally block in exceptions
finally{} block executed in all circumstances.
what prevents finally block from completion
a. death of the current thread.
b. Execution of System.exit().
b. Turning off the computer.
41. Exception’s subclasses have to be caught first before the General Exception
Ex.
FileNoFoundException is inherited from the IOException
try
{
} catch (IOException exp) // compilation error
{
}
catch(FileNoFoundException exp) // Need to be catched
// first before IOException
// Unreachable catch block for FileNotFoundException.
It is already handled by the catch block for IOException
{
}
42. If there is an overriding method in the sub class and this method throws an
exception in the super class then it must throw the same exception which it throws
in the super class or throw an exception which is a sub class of the exception which
it throws in the super class.
Ex.
class ABC
{
public void printABC() throws MyException
{}
}

class XYZ extends ABC
{

public void printABC() // it must throws MyException or any child of it or throws
// None or any runtime exception
}
43. Volatile
A field may be declared volatile, in which case the Java memory model ensures
that all threads see a consistent value for the variable.
When any thread accesses volatile fields it performs automatic synchronization
(automatic locking).
A compile-time error occurs if a final variable is also declared volatile.
Ex.
class ABC
{
final volatile int x = 10; // compilation error
}
44. Static
If the keyword this or the keyword super occurs in an initialization expression for
a class variable, then a compile-time error occurs.
Ex.
class ABC
{
int x = 10;
}
class XYZ extends ABC
{
int a = 10;
static int z = super.x; // compilation error (super and this can't be used in static context)
static int y = this.a; // compilation error
}

45. Initializers for Instance Variables (related to classes)
Initialization expressions for instance variables may use the simple name of any
static variable declared in or inherited by the class, even one whose declaration
occurs textually later.
Ex.1
class Test
{
float f = j; // no compilation error
static int j = 10;
}
Ex.2
class Test
{
int i = j; // compilation error -- incorrect forward reference
int j = 10;
}

46. Abstract Methods
1. It is a compile-time error for a private method to be declared abstract.
2. It is a compile-time error for a static method to be declared abstract.
3. It is a compile-time error for a final method to be declared abstract.
4. An abstract class can override an abstract method by providing another
abstract method declaration.
47. Native Methods
- A method that is native is implemented in platform-dependent code(in another
programming language).
- A compile-time error occurs if a native method is declared abstract.
Ex.
class Test
{
abstract native void print(); // compile error
}
48. Run method of threads doesn't throws any checked exceptions but it may throw any
runtime exception
class ABC implements Runnable
{
public void run() throws Exception // compilation error
}

class ABC implements Runnable
{
public void run() throws RuntimeException // NO ERRORS
public void run() throws ArrayIndexOutOfBoundsException // NO ERRORS
}

49. Assertion
provide a convenient mechanism for verifying that a class's method are called
correctly and used commonly to check
1. Preconditions
2. Post conditions
3. Class invariants (a constraint on a class's state that must be met before and after
execution of any non-private method of a class).
ex. assert Exp1;
assert Exp1:Exp2;
// Exp1 must be boolean type
// Exp2 may have any type;
50. Parameters passing in java
In java  parameters passing is by value BUT if you pass a reference the jvm makes
a copy of the reference (You can use it inside function but you cannot assign it
another object).
class Value
{
public int i = 15;

}
public class Test
{
public static void main(String[] args)
{
Test t = new Test();
t.first();
}
public void first()
{
int i = 5;
Value v = new Value();
v.i = 25;
second(v,i);
System.out.println(v.i); // prints 20 the last value assigned to v.i in
// second() function
}
public void second(Value v, int i)
{
i = 0;
v.i = 20;
Value val = new Value();
v = val; // ok no compilation error but visible inside function only
System.out.println(v.i); // prints 15 because v assigned to a new object
}
}
51. instanceof  used to test type of objects not references
public class Test
{
static Test t1;
{

Test t2 = new Test();
if ( t1 instanceof Test)
{
System.out.println(" t1 instanceof Test = true");
}
else
{
System.out.println(" t1 instanceof Test = false"); // will be printed
// because t1 is a reference (doesn't refer to any object)
}

if ( t2 instanceof Test)
{
System.out.println("t2 instanceof Test = true"); // will be printed because t2
// holds an object
}
}
}

// Example 2
class Cup
{
}
class PassionCup
{}

in main
Cup c = new PassionCup();
PassionCup pc = new PassionCup();
if ( c instance of PassionCup )
{
System.out.println("c instanceof PassionCup is true"); // will be printed because instanceof
// test objects not references
}

if ( pc instanceof Cup )
{
System.out.println("pc instanceof Cup is true"); // will be printed because of
// PassionCup extends Cup
}

52. Auto boxing
Boxing is the automatic assignment of a primitive value to a compatible wrapper
type.
Ex. Integer x = 1;

Unboxing  is the automatic extraction of a wrapped value.
Integer z = 1;
int y = z;

Boxing pool
If the value that is being boxed is true, false, a byte, a char in the range
u0000 to u007f, or an int or short number between -128 and 127 then the
object is pooled.

Ex.
Integer i1 = 2;
Integer i2 = 2;
Integer i3 = 200;
Integer i4 = 200;

System.out.println( i1 == i2 ); // will print true because objects are not pooled
System.out.println( i3 == i4 ); // will print false because objects are not pooled so
references are not equal.

53. Mutable & Immutable
Mutable  once the object is created you can edit it.
Immutable  once the object is created you can't edit or modify it.
Ex. String contains immutable string
StringBuffer  contains mutable strings and is thread safe (all of its methods
are synchronized).
StringBuffer sb = "abc"; // compilation error cannot convert from String to StringBuffer

StringBuilder  contains mutable strings but is not thread safe.
(faster than StringBuffer)
StringBuilder sb = "ab" ; // compilation error cannot convert from String to StringBuilder
To apply Immutable concept to your own defined objects.
Ex. if you have class Person with first name and last name and you need to make
sure that all objects of this class are immutable see the following class code
class Person
{
String firstName;
String lastName;
public Person(String firstName, String lastName)
{
this.firstName = firstName;
this.lastName = lastName;
}
String getFirstName()
{
return firstName;
}
String getLastName()
{
return lastName;
}
}

because there is no setters in this class, once an object is created you cannot modify
it

54. Final classes cannot be extended and final methods cannot be overridden

55. The class must be abstract if
1. The class has one or more abstract methods.
2. The class inherits abstract methods and doesn't provide an implement to any of
them.
3. Class implements an interface and don't implement every method.
56. Static features (static variables, static methods)
Static features belong to the class not to its instances so it exists before any object
(instance) of the class is created.
Static Methods
1. May access only the static data of its class, it may not access non-static data.
2. May call only the static methods of its class, it may not call non-static methods.
3. Static methods has no this pointer (reference).
4. Static methods may not be overridden by non-static methods

57. Serialization is the process of storing the state of a java object by converting it to
byte stream.
Rules of serialization
1. Static fields are not serialized because they are not belonging to any object.
2. Transient fields are not serialized.
3. Fields from the base class are handled only if those are serializable.

What is preserved when an object is serialized?
1. Only the object's data are preserved.
2. Methods and constructors are not part of the serialized stream.
3. Class information is included.

What is the difference between serializable and externalizable
Serializable -- provides the default serialization mechanism of the compiler(JVM).
Externalizable -- provides customized serialization mechanism which allows you to
implement your own protocols through the methods
writeExternal (…) and readExternal (…)

Version Control
All serialized objects are automatically assigned a unique identifier.
If the identifier of the class doesn't equal the identifier of the serialized object a
java.io.InvalidClassException is thrown.

Ex. class ABC implements Serializable
{
int x = 20;
}
then you create an object ABC myObject = new ABC() ;
and wrote it to a file the compiler assigns default value for hidden variable
serialVersionUID .
And after that another variable is added to the class.

class ABC implements Serializable
{
int x = 20;
int y = 300;
}
-- at this point the compiler assigns another value to serialVersionUID
if you tried to read myObject now java.io.InvalidClassException will be thrown
To prevent this exception you should add serialVersionUID to ABC class

Ex.
{
private static final long serialVersionUID = 1;
int x = 20;
}

if you create and object ABC myObject = new ABC(); and wrote it to a file.
and you add another data to ABC class
{
private static final long serialVersionUID = 1;
int x = 20;
int y = 300;
}
Now if you need to read the stored old object from file the compiler checks
if serialVersionUID of the stored object is equal to serialVersionUID of the new class
the compiler reads the old object and initialize the new features to their default
vales (y will be initialized to 0 NOT 300 )

58. Objects of some system-level classes are not serializable
59. Threads
- Single threaded java program has one entry point (main function) and one exit
point.

- Multi-threaded java program has one entry point (main function) and multiple end
points.

60. A thread can enter the waiting state by the following ways:
1. Invoking its sleep() method,
2. By blocking on I/O
3. By unsuccessfully attempting to acquire an object’s lock
4. By invoking an object’s wait() method.
5. It can also enter the waiting state by invoking its (deprecated) suspend() method.

61. What is the difference between preemptive scheduling and time slicing?

Under preemptive scheduling, the highest priority task executes until it enters the
waiting or dead states or a higher priority task comes into existence. Under time
slicing, a task executes for a predefined slice of time and then re-enters the pool of
ready tasks. The scheduler then determines which task should execute next, based
on priority and other factors.

62. What invokes a thread’s run() method?
After a thread is started, via its start() method of the Thread class, the JVM invokes
the thread’s run() method when the thread is initially executed.
63. What is daemon thread and which method is used to create the daemon thread?
Daemon threads are threads with low priority and runs in the back ground doing the
garbage collection operation for the java runtime system. The setDaemon() method
is used to create a daemon thread. These threads run without the intervention of
the user. To determine if a thread is a daemon thread, use the accessor method
isDaemon()
When a standalone application is run then as long as any user threads are active the
JVM cannot terminate, otherwise the JVM terminates along with any daemon
threads which might be active. Thus a daemon thread is at the mercy of the runtime
system. Daemon threads exist only to serve user threads.

64. How would you implement a thread pool?
public class ThreadPool implements ThreadPoolInt
This class is an generic implementation of a thread pool, which takes the following
input
a) Size of the pool to be constructed
b) Name of the class which implements Runnable and constructs a thread pool with
active threads that are waiting for activation. Once the threads have finished
processing they come back and wait once again in the pool.
This thread pool engine can be locked i.e. if some internal operation is performed on
the pool then it is preferable that the thread engine be locked. Locking ensures that
no new threads are issued by the engine. However, the currently executing threads
are allowed to continue till they come back to the passivePool.
65. Is there a separate stack for each thread in Java?
Yes. Every thread maintains its own separate stack, called Runtime Stack but they
share the same memory. Elements of the stack are the method invocations,
called activation records or stack frame. The activation record contains pertinent
information about a method like local variables.
66.
67. How you can run Garbage collection?
I can run GC through calling System.gc() or Runtime.getRuntime().gc();
but this didn't guarantee that GC will run it only put its thread in ready state.
GC informs the object when it is about destroying it by calling its finalize() method
68. Reflection
Reflection is commonly used by programs which require the ability to examine or
modify the runtime behavior of applications running in the Java virtual machine.
see Class,Method, Constructor,Member  to know the attributes ,methods and
constructors of an object in runtime
69. Collections
Set - didn't contain duplicates and may contain at most one null value.

- A Set stores elements in an unordered way and does not contain duplicate elements.
Map  is set of objects that map key with value, cannot contain duplicate keys
List - set of elements that maintain its insertion order.
- stores elements in an ordered way but may contain duplicate elements.
70. Generics
An enhancement to the type system that supports operations on objects of various
types while providing compile-time type safety.
It adds compile-time type safety to the Collections Framework and eliminates the
drudgery of casting.
Ex.
ArrayList<String> myList = new ArrayList<String>();
the previous generics restrict the type of objects in this ArrayList to String at compile time
and prevent casting Objects to String.

Sub-typing is not allowed in generics
Ex. ArrayList<Object> myList = new ArrayList<String>(); // compile error
// cannot covert from ArrayList<String> to ArrayList<Object>

Inheritance relationship is maintained between collection elements.
Ex.
class A
{}
class B extends A
{}
class C extends B
{}
ArrayList<A> myList = new ArrayList<A>();
ArrayList<A> myList = new ArrayList<B>(); // compilation errors
myList.add(new A()); // ok no errors
myList.add(new B()); // ok no errors
myList.add(new C()); // ok no errors

Wildcards ?  means collection of unknown type
Ex.
public void printCollection(ArrayList<Object> list)
{}

ArrayList<Object> list1 = new ArrayList<Object>();
ArrayList<String> list2 = new ArrayList<String>();
when calling printCollection function
printCollection(list1); // ok no errors
printCollection(list2); // compilation error
to prevent the last error see the following code
public void printCollection(ArrayList<?> list) // ? maps to any type
{
// the only allowed here is to read elements of list as Objects
// add elements to list is not allowed(list.add(new Object()) gives compilation errors)

}


Bounded wildcards <? extends ...>
bounds unknown type to be subtype of another type
Ex.
public void printCollection(ArrayList<? extends Object> list) // maps to any subtype of Object
{
// the only allowed here is to read elements of list as Objects
// add elements to list is not allowed(list.add(new Object()) gives compilation errors)
}


71. Iterator : Enables you to cycle through a collection in the forward direction only, for
obtaining or removing elements

ListIterator : It extends Iterator, allow bidirectional traversal of list and the
modification of elements.

72. Properties class
The properties class is a subclass of Hashtable that can be read from or written to a
stream. It also provides the capability to specify a set of default values to be used.
73. Enumeration: It is series of elements. It can be use to enumerate through the
elements of a vector, keys or values of a hashtable. You cannot remove elements
from Enumeration.

74. What method should the key class of Hashmap override?
The methods to override are equals() and hashCode().
75. What is the difference between Enumeration and Iterator?
The functionality of Enumeration interface is duplicated by the Iterator interface.
Iterator has a remove() method while Enumeration doesn’t. Enumeration acts as
Read-only interface, because it has the methods only to traverse and fetch the
objects, where as using Iterator we can manipulate the objects also like adding and
removing the objects.
So Enumeration is used whenever we want to make Collection objects as Read-only.
76.
77. Shadowing(Hiding)
A variable is shadowed if there is another variable with the same name that is closer

in scope. In other words, referring to the variable by name will use the one closest in
scope , the one in the outer scope is shadowed.
Ex.1
class A
{
String name="A";

String getName()
{
return name;
}
}
class B extends A
{
String name="B"; // shadows the name variable of class A
String getName() // really this is the same as overriding but for variables
and declarations
{
return name;
}
}

A a = new B();
System.out.println(a.getName()); // will print B due to shadowing

Ex.2
class Outer
{
class Inner
{
void print()
{ System.out.println("Inner class"); }
}
void printMessage()
{
class Inner
{
void print()
{ System.out.println("Local Inner class"); }
}

Inner in = new Inner(); // due to shadowing this is the Inner class inside this
method
in.print(); //// will print  Local Inner class
}
}

78. Double indirection
in most JVMs, the reference value is actually the address of and address. The second
address refers to the real data.
79. Code exercises
difference between null and "" in string
String s = null;
s+= "abc";
System.out.println(s); /// will print nullabc;

String s = "";
s+= "abc";
System.out.println(s); /// will print abc;

Division by zero
Dividing integer by zero
int i=12;
System.out.println( i/0 ); // will give ArithmaticException division by zero
// with integers
Dividing floats and doubles by zero
double d = 12;
System.out.println( d/0 ); // gives Infinity with floating and double variables

Division remainder with floats and doubles
int i=12;
System.out.println( i%0 ); // will give ArithmaticException division by zero
// with integers

Division remainder with floats and doubles
double d = 12;
System.out.println( d%0 ); // gives NaN with floating and double variables

Casting Object to interface
Ex.1
{
}
class MyObject
{
}
in main()
MyObject o = new MyObject();
MyInterface myInterface = (MyInterface)o; // no compilation errors but it will throw
// ClassCastException at runtime
The idea is that even if the class does not implement the interface, but one of its
subclass might. If the actually object class does not implement the interface then

you will get a ClassCastException error at runtime.

Ex.2
{
}
final class MyObject
{
}
in main()
MyObject o = new MyObject();
MyInterface myInterface = (MyInterface)o; // compilation error cannot cast MyObject
// to MyInterface

The compiler knows at compile time exactly what interfaces are implemented by the
final class. If the compiler can determine at compile time that the final class can
never be instanceof the interface, that's a compile time error.
Static with null references
Ex.
public class Test
{
static String name="Farag";
static Test getName()
{
System.out.println("Getting Name");
return null;
}
{
System.out.println(getName().name); // it will executes without NullPointerException
}
}
when getName() returns null reference and calling static variable name using this
reference the compiler recognizes that static variable name is not associated with an
object so it will execute it without throwing NullPointerException
casting array of primitives to array of objects.
Ex.
Object c = new long[4]; // OK
Object[] c1 = new Long[4]; // OK
Object[] c = new long[4]; // compilation error cannot convert from long[] to Object[]
80. Thread safe and non thread safe java classes

Thread safe Not Thread safe
Vector ArrayList
Hashtable Hashmap
StringBuffer StringBuilder

81. Vector vs. ArrayList

Vector ArrayList
Synchronized Not synchronized
Implement the List interface. Implement the List interface.
implemented using dynamically resizable implemented using dynamically resizable
arrays arrays
providing fast random access and fast providing fast random access and fast
traversal traversal
data is retrieved using the elementAt() data is retrieved using the get() method
method
has a default size of 10 has no default size

82. Difference between Hashtable & HashMap

Hashtable HashMap
Doesn't store null values Store null values
Ex. ht.put("f",null); will throw Ex. hm.put("f",null); compiles and runs OK
NullPointerException at runtime without exceptions
Synchronized Not synchronized
doesn’t allow nulls allows null values as key and value
does not guarantee that the order of the
map will remain constant over time.
enumerator for the Hashtable isn’t fail- Iterator in the HashMap is fail-safe
safe
can be synchronized by

Map m =
Collections.synchronizeMap(hashMap);

83. Fail-safe is relevant from the context of iterators. If an iterator has been created on
a collection object and some other thread tries to modify the collection object
“structurally”, a concurrent modification exception will be thrown. It is possible for
other threads though to invoke “set” method since it doesn’t modify the collection
“structurally”. However, if prior to calling “set”, the collection has been modified
structurally, “IllegalArgumentException” will be thrown.
84. String pooling & computation
if you create String as String s="my string"; the JVM checks the string literal pool first. If the
string already exists in the pool, a reference to the pooled instance returned. If the string
does not exist in the pool, a new String object instantiates, then it is placed in the pool.
 Concatenation of constant Strings computed in compile time.
 Concatenating string expressions is computed at runtime.
Ex.
String hello = "Hello";
String lo = "lo";
System.out.println( hello == ( "Hel" + "lo") ); // prints true (pool computed at compile time)

System.out.println( hello == ( "Hel" + lo ) ); // prints false (pool computed at runtime)

String computation rules
1. Literal strings within the same class in the same package represent references to the
same String object.
2. Literal strings within different classes in the same package represent references to
the same String object.
3. Literal strings within different classes in different packages likewise represent
references to the same String object.
4. Strings computed by constant expressions are computed at compile time and then
treated as if they were literals.
5. Strings computed by concatenation at run time are newly created and therefore
distinct.
85. JDBC
JDBC architecture decouples an abstraction from its implementation so that the
implementation can vary independent from the abstraction this is an example of adapter
design pattern.
JDBC API provides the abstraction and the JDBC drivers provide the implementation. So new
drivers can be plugged-in to the JDBC API without changing the client code.
Statements.
1. Statement (regular statement)
Statement stmt = myConnection.createStatement();
stmt.executeQuery(""); or stmt.execute(""); or stmt.executeUpdate("");
2. PreparedStatement more efficient due to pre-compilation of sql.
PreparedStatement pstmt =
myConnection.prepareStatement("select * from emp where id=?");
more secure because it prevent sql injection attack
3. CallableStatement used to call stored procedure
CallableStatement callableStmt = myConnection.prepareCall("{call proc_showbooks}");
callableStmt.executeQuery();
Ex.
CallableStatement cs = myConnection.prepareCall("{ call abc(?, ?)}");
cs.setLong(1, customerID);
cs.registerOutParameter(2, java.sql.Types.NVARCHAR);
cs.execute();
String customerName = cs.getString(2);

--- create or replace procedure abc(custId in number, custName out nvarchar2(100))
86. DataSource

DataSource DriverManager
a DataSource object has properties that
identify and describe the data source it
represents
Work with JNDI and managed separately Managed by the application code
from application (Ex. by Application Server)
An application does not need to hardcode Driver information are hardcoded in
driver information application code
Connection pooling and distributed No connection pooling

transactions No distributed transactions
87.
88. What is the difference between DOM and SAX parsers

DOM SAX
Object based Event Based
Creates tree of xml in memory Don't so it is faster
Read/write Read only

89. Weak reference is one that does not prevent the referenced object from being
garbage collected.
You might use them to manage a HashMap to look up a cache of objects.
90. public class Test
{
public void print(String match)
{
System.out.println("--== String ==--");
}

public void print(StringBuffer match)
{
System.out.println("--== String Buffer ==--");
}

{
t. print (null); // Compilation error The method match(String) is ambiguous
}
}
91. public class Test
{
public void print(Object match)
{
System.out.println("--== Object ==--");
}

public void print(String match)
{
System.out.println("--== String ==--");
}

{
t. print (null); // will print --== String ==--
}
}

92. A hidden class (static) method can be invoked by using a reference whose type
is the class that actually contains the declaration of the method. In this respect,
hiding of static methods is different from overriding of instance methods.
The example:

class Super {
static String greeting() {
return "Goodnight";
}

String name() {
return "Farag";
}
}

class Sub extends Super {
static String greeting() {
return "Hello";
}

String name() {
return "Ali";
}
}

public class Test {
public static void main(String[] args) {
Super s = new Sub(); // Super has the declaration of static method
System.out.println(s.greeting() + ", " + s.name());
// prints Goodnight, Ali
}
}

93. final volatile int x; // compile time error
// The field x can be either final or volatile, not both
94. public class Doubler {
static int two() { return two(1); }
private static int two(int i) { return 2*i; }
}

class Test extends Doubler {
public static long two(long j) {return j+j; }
System.out.println(two(3));
System.out.println(Doubler.two(3)); // compile-time error
// The method two(int) from the type Doubler is not visible
}
}
95. class ColoredPoint {
int x, y;
byte color;
void setColor(byte color) { this.color = color; }
}
class Test {

ColoredPoint cp = new ColoredPoint();
byte color = 37;
cp.setColor(color);
cp.setColor(37); // compile-time error
}
}
Here, a compile-time error occurs for the second invocation of setColor, because
no applicable method can be found at compile time. The type of the literal 37 is
int, and int cannot be converted to byte by method invocation conversion.
96. Overloading Ambiguity
class Point
{
int x, y;
}
class ColoredPoint extends Point
{
int color;
}
class Test
{
static void test(ColoredPoint p, Point q)
{
System.out.println("(ColoredPoint, Point)");
}
static void test(Point p, ColoredPoint q) {
System.out.println("(Point, ColoredPoint)");
}
ColoredPoint cp = new ColoredPoint();
test(cp, cp); // compile-time error
}
}
This example produces an error at compile time. The problem is that there are two
declarations of test that are applicable and accessible, and neither is more specific
than the other. Therefore, the method invocation is ambiguous.

Solutions is to add third method
static void test(ColoredPoint p, ColoredPoint q)
{
System.out.println("(ColoredPoint, ColoredPoint)");
}

===================================================================

General Questions(allapplabs)

97. What if the main method is declared as private?
The program compiles properly but at runtime it will give "Main method not public."
message.
98. What if the static modifier is removed from the signature of the main method?
Program compiles. But at runtime throws an error "NoSuchMethodError".
99. What if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error "NoSuchMethodError".

100. If I do not provide any arguments on the command line, then the String array
of Main method will be empty or null?
It is empty. But not null.
Ex.
class Test
{
{
System.out.println(args.length); // if you run with java Test
// it will print 0
}
}
101. Can I import same package/class twice? Will the JVM load the package twice
at runtime?
One can import the same package or same class multiple times. Neither compiler nor
JVM complains abt it. And the JVM will internally load the class only once no matter
how many times you import the same class.
102. Does importing a package imports the subpackages as well? e.g. Does
importing com.MyTest.* also import com.MyTest.UnitTests.*?
No you will have to import the subpackages explicitly. Importing com.MyTest.* will
import classes in the package MyTest only. It will not import any class in any of it's
subpackage.

103. Can main method be declared final?
Yes, the main method can be declared final, in addition to being public static.
104.
105. S
106. S
107. S
108. A
109.
110.
111.

To be seen later

Thread.setDefaultUncaughtExceptionHandler(new
Thread.UncaughtExceptionHandler()

References

1. The Java Language Specification, Third Edition
http://java.sun.com/docs/books/jls/

2. http://www.allapplabs.com/
3. Java community process
http://jcp.org/en/jsr/platform?listBy=2&listByType=platform
4. JAVA Docs
5. http://www.interview-questions-java.com/
6. http://java.boot.by/scjp-tiger/index.html
7. Certification books and sites
8.

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