Chapter 12: Java fundamentals

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

What does final mean when applied to a method?

Answer 4

The method cannot be overridden.

Question 5

What does final mean when applied to a reference variable?

Answer 5

The variable cannot be assigned more than once. The object associated with it can still be modified.

Question 6

What does final mean when applied to a class?

Answer 6

The class cannot be extended.

Question 7

Given the following enum

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

Does the following code throw an exception?

Season s = Season.valueOf(“summer”);

Answer 7

Yes, it will throw an IllegalArgumentException.

It is case sensitive.

Question 8

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 8

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 9

Is it possible to extend an enum?

Answer 9

No, an enum cannot be extended.

Question 10

Does the following code compile?

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

Answer 10

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 11

Which of the following are allowed in an Enum object?

• constructor
• field
• method

Answer 11

All of them.

page 503

Question 12

Does the following code compile?

public enum ConstructorEnum {
    public ConstructorEnum() { }
}

Answer 12

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 13

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

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

Answer 13

Season.SUMMER.print();

Question 14

When does Java call the constructor of an enum?

Answer 14

When the enum is first called.

Question 15

A nested class can come in what flavours?

Answer 15

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

Question 16

What is an inner class?

Answer 16

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

Question 17

What is a static nested class?

Answer 17

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

Question 18

What is a local class?

Answer 18

A class defined within a method body.

Question 19

What is an anonymous class?

Answer 19

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

Question 20

Wat are the properties of an inner class?

Answer 20

• 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 21

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

Answer 21

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

or:

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

Question 22

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

Answer 22

• For x in Inner: x or this.x

- For x in Outer: Outer.this.x

Question 23

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

Answer 23

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 24

Can local classes access all local variables?

Answer 24

No, they can only access final or effectively final variables.

Question 25

Which access modifiers are allowed for a local class?

Answer 25

None.

Question 26

What is this type of class called?

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

Answer 26

An anonymous class

Question 27

What were anonymous classes frequently used for?

Answer 27

asynchronous tasks and event handlers.

Question 28

Which access modifiers are allowed for an anonymous class?

Answer 28

None.

Question 29

Which access modifiers are allowed for an Inner class?

Answer 29

All.

Question 30

Which access modifiers are allowed for a static nested class?

Answer 30

All.

Question 31

Are inner classes allowed to be abstract?

Answer 31

Yes.

Question 32

Are static nested classes allowed to be abstract?

Answer 32

Yes.

Question 33

Are local classes allowed to be abstract?

Answer 33

Yes.

Question 34

Are anonymous classes allowed to be abstract?

Answer 34

No.

Question 35

Are inner classes allowed to be final?

Answer 35

Yes.

Question 36

Are static nested classes allowed to be final?

Answer 36

Yes.

Question 37

Are local classes allowed to be final?

Answer 37

Yes.

Question 38

Are anonymous classes allowed to be final?

Answer 38

No.

Question 39

Which inner class type is allowed to have static methods?

Answer 39

Static nested class

Question 40

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

Answer 40

Static nested class

Question 41

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

Answer 41

Anonymous class

Question 42

What is a default method?

Answer 42

A default method is a method defined in an interface with the default keyword and includes a method body.

Question 43

What are the Default interface definition rules?

Answer 43

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

Question 44

Where can default methods be defined?

Answer 44

In interfaces only.

Question 45

Can default methods be marked static?

Answer 45

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

Question 46

Given the following code, what will be the output?

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

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

public class Cat implements Walk, Run {
    public static void main(String[] args) {
        System.out.println(new Cat().getSpeed());
    }
}

Answer 46

The code will not compile because Java doesn’t know which default method to call.

Question 47

Given the following code, what will be the output?

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; }

    public static void main(String[] args) {
        System.out.println(new Cat().getSpeed());
    }
}

Answer 47

It will print 2 as the method in Cat overrides both default methods.

Question 48

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 48

using Walk.super.getSpeed();

Question 49

Are interface methods allowed to be static?

Answer 49

Yes.

Question 50

What are the Static interface Method Definition rules?

Answer 50

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 51

Are interface methods allowed to be private?

Answer 51

Since Java 9, yes.

Question 52

What are the Private Interface Method Definition rules?

Answer 52

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 53

What are the Private Static Interface Method Definition rules?

Answer 53

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 54

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

Answer 54

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 55

Is the following code a functional interface?

public interface Soar {
abstract String toString();
}

Answer 55

It is not because toString is a public method in Object and therefore doesn’t count towards the single abstract method test;

Question 56

Is the following code a functional interface?

public interface Soar {
abstract String toString();
}

Answer 56

It is not because toString is a public method in Object and therefore doesn’t count towards the single abstract method test;

Question 57

Is the following code a functional interface?

public interface Soar {
    String toString();
    void dive();
}

Answer 57

Yes, because void is the only abstract method which is not defined in Object.

Question 58

What is the type of the lambda parameter?

Predicate< Integer> p = x -> true;

Answer 58

Integer

Question 59

Is the following lambda valid?

(var num) -> 1

Answer 59

Yes

Question 60

Is the following lambda valid?

var num -> 1

Answer 60

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

Question 61

Is the following lambda valid?

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

Answer 61

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

Question 62

Is the following lambda valid?

(var x, var y, var z) -> { int c = 0; return 5; }

Answer 62

Yes, we can use local variables in a code block.

Question 63

Is the following lambda valid?

(a, b) -> { int a = 0; return 5; }

Answer 63

No. Variable a was already defined and is redeclared in the code block.

Question 64

Does the following anonymous class compile?

new Runnable() {
    static final int x = 0;
    static int y = 0;

@Override
public void run() {...} };

Answer 64

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

This code won’t compile

Question 65

Does the following code compile?

class Polygon {
   public void display() {
      System.out.println("Inside the Polygon class");
   }
}

class AnonymousDemo {
   public void createClass() {

      Polygon p1 = new Polygon() {
         public void display() {
            super.display();
         }
      };
      p1.display();
   }
}

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

Question 66

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 66

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