Ch12 Java Fundmentals

Question 1

Is the following code valid?

private void doSomething() {
final int x;
}

Answer 1

Yes.

we do not need to assign a value when a final variable is declared. The rule is that it must be assigned a value before it can be used.

Question 2

Is the following code valid?

final StringBuilder s = new StringBuilder();
s.append(“test”);

Answer 2

Yes.

The object reference is constant and never changes. The state of the object, however, can still change.

Question 3

Is the following code valid?

public class Bear {
static final int age;
{ age = 8; }
}

Answer 3

The code does not compile because the age variable is static, while the initializer is not.

Question 4

Given the following enum

public enum Season {
WINTER, SPRING, SUMMER, FALL
}

Does the following code throw an exception?

Season s = Season.valueOf(“summer”);

Answer 4

Yes, it will throw an IllegalArgumentException.

It is case sensitive.

Question 5

Given the following enum

public enum Season {
WINTER, SPRING, SUMMER, FALL
}

How can we loop over the set of enums and print the names?

Answer 5

Using Season.values() method, which returns an array of all the values, and the name() method.

for (Season season : Season.values()) {
System.out.println(season.name());
}

In addition we can print each order number using ordinal().

Question 6

Does the following code compile?

Season summer = Season.SUMMER;
switch(summer) {
case Season.SUMMER:
// Do something
break;
default:
// Do something
break;
}

Answer 6

It doesn’t because the enum type (Season) is implicitely added by Java in the case statement. For that reason we only have to write the value (SUMMER) instead of Season.SUMMER.

Question 7

Which of the following are allowed in an Enum object?

constructor
field
method

Answer 7

All of them.

Question 8

Does the following code compile?

public enum ConstructorEnum {
public ConstructorEnum() { }
}

Answer 8

No.

The constructor of an enum is implicitely private. This is reasonable since an enum can’t be extended and the constructors can be called only within the enum itself

Question 9

Given the following enum: Season, the value: SUMMER, and the method: print().

How do we call the print method from outside the enum?

Answer 9

Season.SUMMER.print();

Question 10

When does Java call the constructor of an enum?

Answer 10

When the enum is first called.

Question 11

A nested class can come in what flavours?

Answer 11

• Inner class
• Static nested class
• Local class
• Anonymous class

Question 12

What is an inner class?

Answer 12

An inner class is a non-static type defined at the member level of a class.

Question 13

What is a static nested class?

Answer 13

A static type defined at the member level of a class.

Question 14

What is a local class?

Answer 14

A class defined within a method body.

Question 15

What is an anonymous class?

Answer 15

A special case of a local class that does not have a name.

Question 16

What are the properties of an inner class?

Answer 16

• Can be declared public, protected, package-private or private
• Can extend any class and implement interfaces
• Can be marked abstract or final
• Cannot declare static fields or methods, except for static final fields
• Can access members of the outer class including private members

Question 17

Given top level class Outer and nested class Inner, how do you instantiate a nested class from outside the Outer class?

Answer 17

Outer.Inner inner = new Outer().new Inner();

or:

Outer outer = new Outer();
Inner inner = outer.new Inner();

Question 18

If Inner and Outer both define a variable x, how does inner acces both variables?

Answer 18

• For x in Inner: x or this.x
• For x in Outer: Outer.this.x

Question 19

What is the difference between a nested class and a static nested class?

Answer 19

For a static nested class, you do not need an instance of the enclosing class to access it (it acts like a static member field).

Question 20

Which access modifiers are allowed for a local class?

Answer 20

None.

Question 21

What is this type of class called?

SaleTodayOnly sale = new SaleTodayOnly() {
int dollarsOff() { return 3; }
};

Answer 21

An anonymous class

Question 22

What were anonymous classes frequently used for?

Answer 22

asynchronous tasks and event handlers.

Question 23

Which access modifiers are allowed for an anonymous class?

Answer 23

None.

Question 24

Which access modifiers are allowed for a static nested class?

Answer 24

All.

Question 25

Are static nested classes allowed to be abstract?

Answer 25

Yes.

Question 26

Are local classes allowed to be abstract?

Answer 26

Yes.

Question 27

Are inner classes allowed to be final?

Answer 27

Yes.

Question 28

Are static nested classes allowed to be final?

Answer 28

Yes.

Question 29

Are local classes allowed to be final?

Answer 29

Yes.

Question 30

Are anonymous classes allowed to be final?

Answer 30

No.

Question 31

Which inner class type is allowed to have static methods?

Answer 31

Static nested class

Question 32

Which inner class type is allowed to have static variables that are not final?

Answer 32

Static nested class

Question 33

Which inner class type can extend or implement exactly one class/interface?

Answer 33

Anonymous class

Question 34

What are the Default interface definition rules?

Answer 34

• A default method may be declared only within an interface
• A default method must be marked with the default keyword and include a method body.
• A default method is assumed to be public.
• A default method cannot be marked abstract, final, or static.
• A default method may be overridden by a class that implements the interface.
• If a class inherits two or more default methods with the same method signature, then the class must override the method.

Question 35

Can default methods be marked static?

Answer 35

No. They are associated with the instance of the class implementing the interface.

Question 36

Given the following code, how do we print the default getSpeed() in the Walk interface, from the Cat class?

public interface Walk {
default int getSpeed() { return 1; }
}

public interface Run {
default int getSpeed() { return 3; }
}

public class Cat implements Walk, Run {
public int getSpeed() { return 2; }
// …
}

Answer 36

using Walk.super.getSpeed();

Question 37

Are interface methods allowed to be static?

Answer 37

Yes.

Question 38

What are the Static interface Method Definition rules?

Answer 38

1. A static method must be marked with the static keyword and include a method body.
2. A static method without an access modifier is assumed to be public
3. A static method cannot be marked abstract or final
4. A static method is not inherited and cannot be accessed in a class implementing the interface without a reference to the interface name.

Question 39

Are interface methods allowed to be private?

Answer 39

Since Java 9, yes.

Question 40

What are the Private Interface Method Definition rules?

Answer 40

1. A private interface method must be marked with the private modifier and include a method body.
2. A private interface method may be called only by default and private (non-static) methods within the interface definition.

Question 41

What are the Private Static Interface Method Definition rules?

Answer 41

1. A private static method must be marked with the private and static modifiers and include a body.
2. A private static interface method may be called only by other methods within the interface definition.

Question 42

What is the one exception to the single abstract method rule (functional interface)?

Answer 42

If a functional interface includes an abstract method with the same signature as a public method found in Object, then those methods do not count toward the single abstract method test.

This is because any class that implements the interface will inherit from Object.

Question 43

Is the following lambda valid?

(var num) -> 1

Answer 43

Yes

Question 44

Is the following lambda valid?

var num -> 1

Answer 44

No. When the type is added (like var in this case) you must use parenthesis.

Question 45

Is the following lambda valid?

(String x, var y, Integer z) -> true

Answer 45

No. In lambda expressions, when one of the parameters is of type var, all parameters must be var too.

Question 46

Does the following anonymous class compile?

new Runnable() {
static final int x = 0;
static int y = 0;
@Override
public void run() {…} };

Answer 46

Anonymous classes cannot have any static members except for those that are constant.

This code won’t compile

Question 47

Write an anonymous class at the dots (…) and call the display method to print something.

interface Polygon {
public void display();
}

class AnonymousDemo {
public void createClass() {
…
}
}
class Main {
public static void main(String[] args) {
AnonymousDemo an = new AnonymousDemo();
an.createClass();
}
}

Answer 47

Polygon p1 = new Polygon() {
public void display() {
System.out.println(“Inside an anonymous class.”);
}
};
p1.display();

Question 48

Which statements concerning the relation between a non-static inner class and its outer class instances are true?

A. Member variables of the outer instance are always accessible to inner instances, regardless of their accessibility modifiers.
B. Member variables of the outer instance can always be referred to using only the variable name within the inner instance.
C. More than one inner instance can be associated with the same outer instance.
D. An inner class can extend its outer class.
E. A final outer class cannot have any inner classes.

Answer 48

A, C, D

B: This is possible only if that variable is not shadowed by another variable in inner class.

E: There is no such rule.

Question 49

Which of the following options can be a part of a correct inner class declaration or a combined declaration and instance initialization ?

A. private class C { }
B. new SimpleInterface() { }
C. new ComplexInterface(x) { }
D. private final abstract class C { }
E. new ComplexClass() implements SimpleInterface { }

Answer 49

A, B

C: You cannot pass parameters when you implement an interface by an anonymous class.
D: A final class can never be abstract.
E: ‘implements’ part comes only in class definition not in instantiation.