OCP Java SE 8 Exam - Sample Questions - Lambda Expressions

OCP Java SE 8 Exam
Sample Questions
Lambda Expressions
Hari Kiran & S G Ganesh

Ques8on
Which of these are valid lambda expressions (select ALL
that apply):
A. (int x) -> x + x
B. x -> x % x
C. -> 7
D. (arg1, int arg2) -> arg1 / arg2

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Answer
Which of these are valid lambda expressions (select ALL that apply):
A. (int x) -> x + x
B. x -> x % x
C. -> 7

Explana8on
A. & B. are correct lambda expressions.

Why other op8ons are wrong:

C. -> 7. if no parameters, then empty parenthesis () must be
provided i.e., ()->7

if argument types are provided, then it should be provided
for all the arguments, or none of them


Ques8on
Determine the behaviour of the following program:

class BlockLambda {
interface LambdaFunction {
String intKind(int a);
}
public static void main(String []args) {
LambdaFunction lambdaFunction =
(int i) -> { //#1
if((i % 2) == 0) return "even";
else return "odd";
};
System.out.println(lambdaFunction.intKind(10));
}
}

A. Compiler error at #1
B. Prints even
C. Prints odd
D. RunKme error (throws excepKon)

Answer
Determine the behaviour of the following program:

class BlockLambda {
String intKind(int a);
}
LambdaFunction lambdaFunction =
(int i) -> { //#1
if((i % 2) == 0) return "even";
else return "odd";
};
System.out.println(lambdaFunction.intKind(10));
}
}

A. Compiler error at #1
B. Prints even
C. Prints odd
D. RunKme error (throws excepKon)

Explana8on
B. is the correct answer as the expression evaluates input
value as even


A. There is no compilaKon error, this is correct way of
deﬁning block lambda

C. Input value passed is 10 so the expression returns even

D. This program doesn’t thrown any runKme excepKons


Ques8on
Predict the output of below program:
interface SuffixFunction {
void call();
}
class Latin {
String word = "hello";
SuffixFunction suffixFunc = () -> System.out.println(word + "ay");
word = "e";
suffixFunc.call();
}
}
Choose the correct op8on
A. Prints helloay
B. Prints helloe
C. Prints eay
D. Compiler error

Answer
interface SuffixFunction {
void call();
}
class Latin {
SuffixFunction suffixFunc = () -> System.out.println(word + "ay");
word = "e";
suffixFunc.call();
}
}
Choose the correct op8on
A. Prints helloay
B. Prints helloe
C. Prints eay
D. Compiler error
LaKn.java:7: error: local variables referenced from a lambda expression must be
final or effecKvely final

Explana8on
Inside the lambda expression, we are using the local variable
word. Because it is used in a lambda expression, this variable
is considered to be ﬁnal (though it is not explicitly declared
ﬁnal). Hence A, B and C are incorrect opKons

Snippet

SuffixFunction suffixFunc = () -> System.out.println(word +
"ay");
word = "e";

Ques8on
Which of the following has correct usage of
func8onal interfaces and doesn’t result in compila8on error
(select all that apply):

A. @FunctionalInterface
public abstract class AnnotationTest {
abstract int foo();
}
B. @FunctionalInterface
public interface AnnotationTest {
default int foo() {};
}
C. @FunctionalInterface
public interface AnnotationTest { /* no methods provided */ }
D. @FunctionalInterface
public interface Comparator<T> {
int compare(T o1, T o2);
boolean equals(Object obj);
}

Answer
Which of the following has correct usage of
func8onal interfaces and doesn’t result in compila8on error
(select all that apply):

A. @FunctionalInterface
public abstract class AnnotationTest {
abstract int foo();
}
B. @FunctionalInterface
public interface AnnotationTest {
default int foo() {};
}
C. @FunctionalInterface
public interface AnnotationTest { /* no methods provided */ }
D. @FunctionalInterface
public interface Comparator<T> {
int compare(T o1, T o2);
boolean equals(Object obj);
}

Explana8on
D. This interface is a funcKonal interface though it declares
two abstract methods: compare() and equals() methods.
How is it a funcKonal interface when it has two abstract
methods? Because equals() method signature matches
from Object , and the compare() method is the only
remaining abstract method, and hence the Comparator
interface is a funcKonal interface.


A. An abstract class cannot be declared with annotaKon
@FuncKonalInterface

C. and B. doesn’t have abstract methods. Hence they do no
qualify to be declared as funcKonal interfaces

Ques8on

class LambdaFunctionTest {
@FunctionalInterface
int apply(int j);
boolean equals(java.lang.Object arg0);
}
LambdaFunction lambdaFunction = i -> i * i; // #1
System.out.println(lambdaFunction.apply(10));
}
}
A. This program results in a compiler error: interfaces cannot be deﬁned inside
classes
B. This program results in a compiler error: @FuncKonalInterface used for
LambdaFuncKon that deﬁnes two abstract methods
C. This program results in a compiler error in code marked with #1: syntax error
D. This program compiles without errors, and when run, it prints 100 in console

Answer

class LambdaFunctionTest {
int apply(int j);
boolean equals(java.lang.Object arg0);
}
LambdaFunction lambdaFunction = i -> i * i; // #1
System.out.println(lambdaFunction.apply(10));
}
}
A. This program results in a compiler error: interfaces cannot be deﬁned inside
classes
B. This program results in a compiler error: @FuncKonalInterface used for
LambdaFuncKon that deﬁnes two abstract methods
C. This program results in a compiler error in code marked with #1: syntax error
D. This program compiles without errors, and when run, it prints 100 in console

Explana8on
D. is the correct answer as this program compiles without
errors, and when run, it prints 100 in console.
A.  An interface can be deﬁned inside a class
B.  The signature of the equals method matches that of the
equal method in Object class; hence it is not counted as
an abstract method in the funcKonal interface
C.  It is acceptable to omit the parameter type when there
is only one parameter and the parameter and return
type are inferred from the LambdaFuncKon abstract
method declaraKon int apply(int j)

Ques8on
interface DoNothing {
default void doNothing() { System.out.println("doNothing"); }
}
interface DontDoAnything extends DoNothing {
@Override
abstract void doNothing();
}
class LambdaTest {
DontDoAnything beIdle = () -> System.out.println("be idle");
beIdle.doNothing();
}
}
A. This program results in a compiler error for DontDoAnything interface: cannot
override default method to be an abstract method
B. This program prints: be idle
C. This program prints: doNothing
D. This program results in a compiler error: DontDoAnything is not a funcKonal
interface

Answer
interface DoNothing {
default void doNothing() { System.out.println("doNothing"); }
}
interface DontDoAnything extends DoNothing {
@Override
abstract void doNothing();
}
class LambdaTest {
DontDoAnything beIdle = () -> System.out.println("be idle");
beIdle.doNothing();
}
}
A. This program results in a compiler error for DontDoAnything interface: cannot
override default method to be an abstract method
B. This program prints: be idle
C. This program prints: doNothing
D. This program results in a compiler error: DontDoAnything is not a funcKonal
interface

Explana8on
B. is the correct answer as the call beIdle.doNothing() calls the
System.out.println given in the lambda expression and hence it
prints “be idle” on the console


A.  A default method can be overridden in a derived interface
and can be made abstract
C. DoNothing.doNothing() will not be called
D. DontDoNothing is a funcKonal interface because it has an
abstract method

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