Java 06

Java Tutorial

Extending Classes and Interfaces

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Inheritance in Java
Inheritance is a compile-time mechanism in Java that allows you to extend a class (called the base
class or superclass) with another class (called the derived class or subclass). In Java,
inheritance is used for two purposes:
1. class inheritance - create a new class as an extension of another class, primarily for the purpose
of code reuse. That is, the derived class inherits the public methods and public data of the
base class. Java only allows a class to have one immediate base class, i.e., single class
inheritance.
2. interface inheritance - create a new class to implement the methods deﬁned as part of an
interface for the purpose of subtyping. That is a class that implements an interface “conforms
to” (or is constrained by the type of) the interface. Java supports multiple interface inheritance.
In Java, these two kinds of inheritance are made distinct by using different language syntax. For
class inheritance, Java uses the keyword extends and for interface inheritance Java uses the
keyword implements.
public class derived-class-name extends base-class-name {
// derived class methods extend and possibly override
// those of the base class
}
public class class-name implements interface-name {
// class provides an implementation for the methods
// as specified by the interface
}

Greg Lavender

Slide 1 of 12

6/15/99

Java Tutorial


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Example of class inhertiance
package MyPackage;
class Base {
private int x;
public int f() { ... }
protected int g() { ... }
}
class Derived extends Base {
private int y;
public int f() { /* new implementation for Base.f() */ }
public void h() { y = g(); ... }
}
In Java, the protected access qualiﬁer means that the protected item (ﬁeld or method) is visible to a
any derived class of the base class containing the protected item. It also means that the protected
item is visible to methods of other classes in the same package. This is different from C++.
Q: What is the base class of class Object? I.e., what would you expect to get if you executed the
following code?
Object x = new Object();
System.out.println(x.getClass().getSuperclass());

Greg Lavender

Slide 2 of 12

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Order of Construction under Inheritance
Note that when you construct an object, the default base class constructor is called implicitly, before
the body of the derived class constructor is executed. So, objects are constructed top-down under
inheritance. Since every object inherits from the Object class, the Object() constructor is always
called implicitly. However, you can call a superclass constructor explicitly using the builtin super
keyword, as long as it is the ﬁrst statement in a constructor.
For example, most Java exception objects inherit from the java.lang.Exception class. If you wrote
your own exception class, say SomeException, you might write it as follows:
public class SomeException extends Exception {
public SomeException() {
super(); // calls Exception(), which ultimately calls Object()
}
public SomeException(String s) {
super(s); // calls Exception(String), to pass argument to base class
}
public SomeException (int error_code) {
this("error”); // class constructor above, which calls super(s)
System.err.println(error_code);
}
}

Greg Lavender

Slide 3 of 12

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Abstract Base Classes
An abstract class is a class that leaves one or more method implementations unspeciﬁed by
declaring one or more methods abstract. An abstract method has no body (i.e., no implementation). A
subclass is required to override the abstract method and provide an implementation. Hence, an
abstract class is incomplete and cannot be instantiated, but can be used as a base class.
abstract public class abstract-base-class-name {
// abstract class has at least one abstract method
public abstract return-type abstract-method-name ( formal-params );
... // other abstract methods, object methods, class methods
}
public class derived-class-name extends abstract-base-class-name {
public return-type abstract-method-name (formal-params) { stmt-list; }
... // other method implementations
}
It would be an error to try to instantiate an object of an abstract type:

abstract-class-name obj = new abstract-class-name();

// ERROR!

That is, operator new is invalid when applied to an abstract class.

Greg Lavender

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Example abstract class usage
abstract class Point {
private int x, y;
public Point(int x, int y) { this.x = x; this.y = y; }
public void move(int dx, int dy)
{ x += dx; y += dy; plot(); }
public abstract void plot(); // has no implementation
}
abstract class ColoredPoint extends Point {
private int color;
protected public ColoredPoint(int x, int y, int color)
{ super(x, y); this.color = color; }
}
class SimpleColoredPoint extends ColoredPoint {
public SimpleColoredPoint(int x, int y, int color) { super(x,y,color); }
public void plot() { ... } // code to plot a SimpleColoredPoint
}
Since ColoredPoint does not provide an implementation of the plot method, it must be declared
abstract. The SimpleColoredPoint class does implement the inherited plot method. It would be an
error to try to instantiate a Point object or a ColoredPoint object. However, you can declare a Point
reference and initialize it with an instance of a subclass object that implements the plot method:
Point p = new SimpleColoredPoint(a, b, red); p.plot();

Greg Lavender

Slide 5 of 12

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Interfaces
An abstract class mixes the idea of mutable data in the form of instance variables, non-abstract
methods, and abstract methods. An abstract class with only static final instance variables and all
abstract methods is called an interface. An interface is a specification, or contract, for a set of
methods that a class that implements the interface must conform to in terms of the type signature of
the methods. The class that implements the interface provides an implementation for each method,
just as with an abstract method in an abstract class.
So, you can think of an interface as an abstract class with all abstract methods. The interface itself
can have either public, package, private or protected access defined. All methods declared in an
interface are implicitly abstract and implicitly public. It is not necessary, and in fact considered
redundant to declare a method in an interface to be abstract.
You can define data in an interface, but it is less common to do so. If there are data fields defined in
an interface, then they are implicitly defined to be:
• public.
• static, and
• final
In other words, any data defined in an interface are treated as public constants.
Note that a class and an interface in the same package cannot share the same name.
Methods declared in an interace cannot be declared final. Why?

Greg Lavender

Slide 6 of 12

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Interface declaration
Interface names and class names in the same package must be distinct.
public interface interface-name {
// if any data are defined, they must be constants
public static final type-name var-name = constant-expr;
// one or more implicitly abstract and public methods
return-type method-name ( formal-params );
}
An interface declaraion is nothing more than a specification to which some class that purports to
implement the interface must conform to in its implementation. That is, a class the implements the
interface must have defined implementations for each of the interface methods.
The major reason for interfaces being distinct elements in Java is that you often want to define some
operation to operate on objects that all conform to the same interface. So, you can define some code in
a very general way, with a guarantee that by only using the methods defined in the interface, that all
objects that implement the interface will have defined implementations for all the methods.
For example, you might define a Shape interface that defines an area() method, and so you would
expect that any class that implements the Shape interface, would define an area method. So, if I
have a list of references to objects that implement the Shape interface, I can legitimately invoke the
area method, abstractly, on each of these objects and expect to obtain as a result a value that
represents the area of some shape.

Greg Lavender

Slide 7 of 12

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Separation of Interface from Implementations
Interfaces are specifications for many possible implementations. Interfaces are used to define a
contract for how you interact with an object, independent of the underlying implementation. The
objective of an object-oriented programmer is to separate the specification of the interface from the
hidden details of the implementation.
Consider the specification of a common LIFO stack.
public interface StackInterface {
boolean empty();
void push(Object x);
Object pop() throws EmptyStackException;
Object peek() throws EmptyStackException;
}
Note that the methods in this interface are defined to operate on objects of type Object. Since a stack
is a “container type”, it is very common to use the base class for all objects, namely Object, as the
type of the arguments and results to a container type. Since all objects ultimately inherit from class
Object, we can always generically refer to any object in the system using an Object reference.
However, when we pop from a stack, we have to explicitly type case from the very general type
Object to a concrete type, so that we can manipulate the object concretely.
Q: How many different ways are there to implement a stack? If we are using just using a Stack object
(as opposed to implementing it ourselves) should we care?

Greg Lavender

Slide 8 of 12

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Stack implementation of the StackInterface
public class Stack implements StackInterface {
private Vector v = new Vector();

// use java.util.Vector class

public boolean empty() { return v.size() == 0; }
public void push(Object item) { v.addElement(item); }
public Object pop() {
Object obj = peek();
v.removeElementAt(v.size() - 1);
return obj;
}
public Object peek() throws EmptyStackException {
if (v.size() == 0)
throw new EmptyStackException();
return v.elementAt(v.size() - 1);
}
}

Greg Lavender

Slide 9 of 12

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Should a stack use or inherit from a vector?
The java.util.Stack class is deﬁned as a subclass of the java.util.Vector class, rather than using a
Vector object as in the previous example. This sort of inheritance is not subtype inheritance, because
the interface of a Stack object can be violated because a Vector has a “wider” interface than a Stack,
i.e., a vector allows insertion into the front and the rear, so it is possible to violate the stack contract
by treating a stack object as a vector, and violating the LIFO speciﬁcation of a stack.
public class Stack extends Vector {
public Object push(Object item) {addElement(item); return item;}
public Object pop() {
Object obj;
int len = size();
obj = peek();
removeElementAt(len - 1);
return obj;
}
public Object peek() {
int len = size();
if (len == 0) throw new EmptyStackException();
return elementAt(len - 1);
}
public boolean empty() { return size() == 0;}
}
Vector v = new Stack(); // legal - base class reference to subclass object
v.insertElementAt(x, 2); // insert object x into Vector slot 2!!

Greg Lavender

Slide 10 of 12

6/15/99

Java Tutorial


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When to use an Interface vs when to use an abstract class
Having reviewed their basic properties, there are two primary differences between interfaces and
abstract classes:
• an abstract class can have a mix of abstract and non-abstract methods, so some default
implementations can be defined in the abstract base class. An abstract class can also have static
methods, static data, private and protected methods, etc. In other words, a class is a class, so it
can contain features inherent to a class. The downside to an abstract base class, is that since their
is only single inheritance in Java, you can only inherit from one class.
• an interface has a very restricted use, namely, to declare a set of public abstract method
singatures that a subclass is required to implement. An interfaces defines a set of type
constraints, in the form of type signatures, that impose a requirement on a subclass to implement
the methods of the interface. Since you can inherit multiple interfaces, they are often a very
useful mechanism to allow a class to have different behaviors in different situations of usage by
implementing multiple interfaces.
It is usually a good idea to implement an interface when you need to define methods that are to be
explicitly overridden by some subclass. If you then want some of the methods implemented with
default implementations that will be inherited by a subclass, then create an implementation class
for the interface, and have other class inherit (extend) that class, or just use an abstract base class
instead of an interface.

Greg Lavender

Slide 11 of 12

6/15/99

Java Tutorial


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Example of default interface implementations
interface X {
void f();
int g();
}
class XImpl implements X {
void g() { return -1; } // default implementation for g()
}
class Y extends XImpl implements X {
void f() { ... } // provide implementation for f()
}
Note that when you invoke an abtract method using a reference of the type of an abstract class or an
interface, the method call is dynamically dispatched.
X x = new Y();
x.f();
The call x.f() causes a run-time determination of the actual type of object that ‘x’ refers to, then a
method lookup to determine which implementation of f() to invoke. In this case, Y.f(x) is called, but
the type of x is ﬁrst converted to Y so that the ‘this’ reference is initialized to a reference of type Y
inside of f(), since the implicit type signature of Y.f() is really Y.f(ﬁnal Y this);

Greg Lavender

Slide 12 of 12

6/15/99

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