Chapter 5: Class design

Question 1

Methods of an interface always have to be …. and one of the following…

Answer 1

public and either abstract, default or static. if you don’t specify a modifier, they are automatically made public.

Question 2

Can top level classes be made private?

Answer 2

No, they can only be public or default

Question 3

what 3 things can be inherited by extending a class?

Answer 3

state - instance fields behaviour - instance methods type - the class its self

Question 4

what type of methods can’t be overriden by a subclass?

Answer 4

static and private methods can’t be overriden. you can only hide static methods.

Question 5

What are the rules concerning overriding methods

Answer 5

To override a method correctly: 1) the overriding method must have the same signature 2) the return type has to either be the same or a subtype 3) overriding method can’t be less accessible than overriden method. 4)The overriding method can’t declare more exceptions than declared by the overriden method unless the exceptions are a subtype or are runtime exceptions as it doesn’t matter how many of those you declare.

Question 6

What modifiers are you not allowed to use on an abstract method?

Answer 6

private, static, final

Question 7

What kind of variables are interfaces permitted to have?

Answer 7

all variables defined in an interface are implicitly public static & final

Question 8

Which elements of a class cannot be inherited

Answer 8

constructors, static & instance initialisers aren’t considered members of the class thus are not inherited

Question 9

What is the difference in the way subclasses inherit instance members vs static members

Answer 9

subclasses can inherit their own copy of instance members this is shown by overriding and how subclasses can replace the behaviour of the methods with their own implementation. However subclasses only get access rights static members and can’t not change their behaviour, only hide it

Question 10

What are the benefits of inheritance?

Answer 10

1) code reuse 2) information hiding 3) polymorphism

Question 11

Does this code compile if new Animal( ) is called? abstract class Animal{}

Answer 11

No, you can’t instantiate abstract classes.

Question 12

What is a concrete class?

Answer 12

A class that extends an abstract class

Question 13

Which of the following are valid declarations inside an interface independent of each other? void compute(); // 1 public void compute(); // 2 public final void compute(); // 3 static void compute(); // 4 protected void compute(); // 5

Answer 13

1) All interface methods have to be public. No access control keyword in the method declaration also means public in an interface. 2) correct 3) final is not allowed. 4)An interface can have a static method but the method must have a body in that case. 5) all methods must be public

Question 14

What happens to child classes as a result of adding an abstract method to the parent

Answer 14

adding an abstract method to the parent class means that the child classes are obligated to provide an implementation for the abstract method.

Question 15

in what case does a subclass not have to implement its parents abstract method

Answer 15

when the subclass is also declared abstract

Question 16

Can a final class have an abstract method?

Answer 16

no, final means that a subclass can’t be extended, thus its abstract method would never get implemented which defeats the purpose of abstract

Question 17

A final method is a …

Answer 17

method that can’t get overriden

Question 18

which classes can contain final methods?

Answer 18

Any classes whether abstract or final can have final methods.

Question 19

Will this code compile? class Sofa { private final void recline(){ } // 1 } abstract class Sofa { private abstract void recline(); // 2 } abstract class Bed extends Furniture{ static abstract void getWidth(); }

Answer 19

1) yes! you can have a mark a final method as private, its a bit redundant as they basically mean the same thing 2) doesn’t compile, a private member can’t be inherited 3) static methods can’t be overriden thus this will not compile

Question 20

Does an abstract class need to have an abstract method?

Answer 20

no

Question 21

how would you prevent a subclass from hiding the super’s static method?

Answer 21

by using static & final together

Question 22

What are the rules about the application of access modifiers, final, abstract and static

Answer 22

1)An abstract class doesn’t necessarily have to have an abstract method but if a class has an abstract method, it must be declared abstract. 2)A final class or a final method cannot be abstract. 3)A final class cannot contain an abstract method but an abstract class may contain a final method . 4)A private method is always final . 5)A private method can never be abstract . 6)A static method can be final but can never be abstract

Question 23

What kind of methods can an interface have?

Answer 23

abstract, static and default methods

Question 24

what are interface methods by default?

Answer 24

abstract

Question 25

Can you mark methods of an interface as final?

Answer 25

no

Question 26

What keyword do you use if an interface wants to inherit from another interface?

Answer 26

extends

Question 27

What methods of an interface ca’t be inherited?

Answer 27

static methods

Question 28

can you override a static method with a non static method and vice versa?

Answer 28

no, it will result in compilation error

Question 29

Can a subinterface override a default method with a static method?

Answer 29

no, it will result in a compilation error

Question 30

Is this legal?

interface I {

public default void invalid(){ }

public static void valid(){ } // 1

}

interface I2 extends I{

public static void invalid(){ } //2

public default void valid(){ } //3

}

Answer 30

1) Can be called only using I.valid(); 2) will not compile 3) this is legal and will compile you can have a default method in a subinterface with the same signature as a static method in the super

Question 31

Complete the code with the correct option below: public class Exam { void method(){} } public class OCAJP extends Exam{ …………. void method(){ } } A. abstract B. final C. private D. default E. int

Answer 31

B is the correct answer Explanation: B. we can use final since the method is not abstract A. can’t use abstract keyword with non-abstract method ( also abstract method has no body) C. overriding method use a more restrictive modifier than parent method D. default methods are only allowed in interfaces

Question 32

Given code of Test.java file:

package com.udayankhattry.oca;

class Lock {

public void open() {

System.out.println(“LOCK-OPEN”);

}

}

class Padlock extends Lock {

public void open() {

System.out.println(“PADLOCK-OPEN”);

}

}

class DigitalPadlock extends Padlock {

public void open() {

/*INSERT*/

}

}

public class Test {

public static void main(String[] args) {

Lock lock = new DigitalPadlock();

lock.open();

}

}

Which of the following options, if used to replace /*INSERT*/, will compile successfully and on execution will print LOCK-OPEN on to the console?

1. super.super.open();
2. (Lock)super.open();
3. ((Lock)super).open();
4. super.open();
5. None of the other options

Answer 32

5. None of the other options is correct

1. super.super.open() is illegal in java and won’t compile
2. (Lock)super.open() => compilation error, super.open() is evaluated first as (.) has higher precedence than cast. The call returns void and can’t be casted to Lock as they are incompatible types
3. ((Lock)super).open(); illegal and results in compilation error, can’t cast to super keyword
4. this option will case Padlocks method to be invoked instead of Digital padlock

its not possibe to directly access 2 levels above the child. super allows access to 1 level above.

Question 33

Given code of Test.java file:

package com.udayankhattry.oca;

class Car {

void speed(Byte val) { //Line n1

System.out.println(“DARK”); //Line n2

} //Line n3

void speed(byte… vals) {

System.out.println(“LIGHT”);

}

}

public class Test {

public static void main(String[] args) {

byte b = 10; //Line n4

new Car().speed(b); //Line n5

}

}

Which of the following needs to be done so that LIGHT is printed on to the console?

1. No changes are required as the given code prints LIGHT
2. Delete line n1, n2 & n3
3. Replace Line n4 with byte…b = 10;
4. Replace Line n4 with new Car().speed(byte…) = 10;

Answer 33

§

Question 34

What are the rules for throwing unchecked/runtime exceptions with regards to inheritance?

Answer 34

If an overridden method throws any unchecked exception or Error, then the overriding method must not throw any checked exceptions.

Question 35

Consider below code of Test.java file, what is printed?

public class Test {

public static void main(String[] args) {

P p = new R(); //Line 1

System.out.println(p.compute(“Go”)); //Line 2

}

}

class P {

String compute(String str) {

return str + str + str;

}

}

class Q extends P {

String compute(String str) {

return super.compute(str.toLowerCase());

}

}

class R extends Q {

String compute(String str) {

return super.compute(str.replace(‘o’, ‘O’)); //2nd argument is uppercase O

}

}

Answer 35

gogogo is printed

1. At Line n1, ‘p’ refers to an instance of class R, thus p.compute(“Go”) invokes the compute(String) method of R class.
2. inside this method, there is a call to super(class Q), super.compute(str.replace(‘o’, ‘O’)); which replaces o with O and calls super passing in the sting “GO” –> super.compute(“GO”);
3. Inside Q, It again invokes the compute(String) method of its Parent class(P) but first changes the string to lowercase. “go”
4. Finally P’s method is invoked which changes the sring to “gogogo”
5. Control goes back down to compute(String) method of Q and then to the compute(String) method of R, which returns “gogogo”.