Chapter 8

Question 1

How do we prevent a class to be extended?

Answer 1

Using the final modifier on the class.

Question 2

Given the following code, how do we print the variable in the parent class?

class Mammal {
    String type = "mammal";
}

public class Bat extends Mammal {
    String type = "bat";

public String printParentType(){
    return ... //code here
} }

Answer 2

super.type

Question 3

What is the difference between super and this?

Answer 3

The super reference is similar to the this reference, except that it excludes any members found in the current class.

Question 4

Can we call a parent variable using this keyword?

Answer 4

yes, as long as the variable does not exist in the current class.

Question 5

What does the this keyword do?

example: this.label

Answer 5

Look for label references including the current class. If it cant find any in the current class, it will try and look in inherited classes.

Question 6

What does the super keyword do?

example: super.label

Answer 6

Look for label references excluding the current class. It will only look at inherited classes.

Question 7

What are the constraints of a constructor?

Answer 7

• The name must be the same of the class (case sensitive)
• It cannot have a return type.
• It cannot have a parameter of type var.

Question 8

Can a class have multiple constructors?

Answer 8

yes, as long as each constructor has a unique signature (paremeters must be different).

Question 9

Can we call a constructor without the new keyword from outside the class?

Answer 9

no. You need to use the new keyword for a constructor to be called.

Question 10

What does Java do when the new keyword is used?

Answer 10

It allocated memory for the new object and then looks for a constructor with a matching signature and calls it.

Question 11

What is the default constructor?

Answer 11

Every class in Java has a constructor wether you code one or not. This Java-created constructor is called a default constructor.

Question 12

What does a default constructor look like?

Answer 12

A default constructor is a Java created constructor without any parameters or body.

Question 13

Multiple constructors in a class is called ..

Answer 13

constructor overloading.

Question 14

How do we call an overloaded constructor from a constructor within the same class?

Answer 14

using this().

When this() is used with paranthesis, Java calls another constructor on the same instance of the class.

Question 15

What are the rules of using this() inside a constructor to call another constructor?

Answer 15

• There can only be one this() call
• It must be the first statement in the method
• It cannot refer to its own constructor (loop)

Question 16

What is the difference between this and this()?

Answer 16

The this keyword refers to an instance of the class, while the this() keyword refers to a constructor call within the class.

Question 17

What is the difference between super and super()?

Answer 17

The super keyword refers to members of the parent class, while the super() keyword calls a parent constructor.

Question 18

Can a class that doesn’t extend another class use the super() method?

Answer 18

Yes. It will call the constructor of the class Object.

Question 19

Will the following code compile?

public Zoo() {
    System.out.println("Zoo created");
    super();
}

Answer 19

No. The super() (and this()) method can only be called in the first line of the method.

Question 20

True or false

The first line of every constructor MUST always be either this() or super().

Answer 20

True.

However, it is not required to code this. Java will automatically insert a call to the no-argument constructor super().

Question 21

Does the following code compile?

public class Mammal {
    public Mammal(int age) {}
}

public class Elephant extends Mammal {
}

Answer 21

No.

Since Elephant does not define any constructors, the Java compiler will attempt to insert a default no-argument constructor. It will also insert a call to super() as the first time on the constructor.

Since Mammal does not have a no-argument constructor and no default no-arg constructor is inserted by Java, it does not compile.

Question 22

A class may have multiple ancestors via inheritance. In the case of Mammal -> Elephant -> AfricanElephant, which class is referred when using super() in AfricanElephant?

Answer 22

The Elephant class.

super() always refers to the most direct parent.

Question 23

Can final variables be assigned after the constructor?

Answer 23

No. All final variables must be assigned by the time the constructor completes.

Question 24

What is the order of initialization of a class?

Answer 24

Initialize class X

1. If there is a superclass Y of X, then initialize class Y first.
2. Process all static variable declarations in the order they appear in the class.
3. Process all static initializers in the order they appear in the class.

Question 25

What is the order of initialization of an instance?

Answer 25

Initialize instance of class X

1. If there is a superclass Y of X, then initialize the instance of class Y first.
2. Process all instance variable declarations in the order they appear in the class.
3. Process all instance initializers in the order they appear in the class.
4. Initialize the constructor including any overloaded contructors referenced with this().

Question 26

What are the constructor rules?

Answer 26

• The first statement of every constructor is a call to an overloaded constructor via this(), or a direct parent constructor via super().
• If the first statement of a constructor is not a call to this() or super(), then the compiler will insert a no-argument super() as the first statement of the constructor.
• Calling this() and super() after the first statement of a constructor results in a compiler error
• If the parent class doesn’t have a no-argument constructor, then every constructor in the child class must start with an explicit this() or super() constructor call.
• If the parent class doesn’t have a no-argument constructor and the child doesn’t define any constructors, then the child class will not compile.
• If a class only defines private constructors, then it cannot be extended by a top-level class.
• All final instance variables must be assigned a value exactly once by the end of the contstructor. Any final instance variables not assigned a value will be reported as a compiler error on the line the constructor is declared.

Question 27

What are the rules of method overriding?

Answer 27

1. The method in the child class must have the same signature of the method in the parent class.
2. The method in the child class must be at least as accessible as the method in the parent class.
3. The method in the child class may not declare a checked exception that is new or broader than the class of any exception declared in the parent class method.
4. If the method returns a value, it must be the same or a subtype of the method in the parent class, known as covariant return types.

Question 28

What will happen if super is not used in method overriding when it is neccessary to call the super method?

Answer 28

It will produce a StackOverflowError at runtime.

Question 29

Can a method in a parent class be overridden with a more restrictive access modifier?

Answer 29

No.

Example: If the method in the parent class is package-private, it can only be package-private or public.

Question 30

Can a method in a parent class be overridden with a method that has a narrowed checked exception?

Answer 30

Yes.

Question 31

Can a method in a parent class be overridden with a method that returns a child object of the object that is returned in the parent method?

Answer 31

Yes.

Question 32

Does the following code compile?

public class LongTailAnimal {
    protected void chew(List input) {}
}

public class Anteater extends LongTailAnimal {
    protected void chew(ArrayList input) {}
}

Answer 32

Yes. They are considered overloaded methods (not overridden) because the signature is not the same).

Question 33

Can generic methods be overloaded by just changing the generic parameter type?

Answer 33

No.

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Question 34

What is the difference between method overriding and method hiding?

Answer 34

it is method hiding if the two methods are marked static and method overriding if they are not marked static.

If one is marked static and the other isn’t, it will not compile.

Question 35

What is the output?

public class Bear {
   public static void eat() {
      System.out.println("Bear is eating");
   }
}

public class Panda extends Bear {
   public static void main(String... hey) {
      eat();
   }
}

Answer 35

Bear is eating.

Question 36

What is the output?

public class Bear {
   public static void eat() {
      System.out.println("Bear is eating");
   }
}

public class Panda extends Bear {
   public static void eat() {
      System.out.println("Panda is chewing");
   }
   public static void main(String... hey) {
      eat();
   }
}

Answer 36

Panda is chewing

Question 37

What is the output?

public class Bear {
   public void eat() {
      System.out.println("Bear is eating");
   }
}

public class Panda extends Bear {
   public static void eat() {
      System.out.println("Panda is chewing");
   }
   public static void main(String... hey) {
      eat();
   }
}

Answer 37

Compile error

Cannot overload a method (eat()) where one is static and the other isn’t.

Question 38

What is the output?

public class Bear {
   public static void eat() {
      System.out.println("Bear is eating");
   }
}

public class Panda extends Bear {
   protected static void eat() {
      System.out.println("Panda is chewing");
   }
   public static void main(String... hey) {
      eat();
   }
}

Answer 38

Compile error

Cannot override a method where the access modifier in the child class is more restrictive than the one it inherits.

Question 39

What does a final method mean?

Answer 39

The method cannot be replaced by one in a child class.

Question 40

WHat does polymorphism mean?

Answer 40

The property of an object to take on many different forms.

Question 41

What are the rules of casting objects?

Answer 41

1. Casting a reference from a subtype to a supertype doesn’t require an explicit cast.
2. Casting a reference from a supertype to a subtype requires an explicit cast.
3. The compiler disallows casts to an unrelated class.
4. At runtime, in invalid cast of a reference to an unrelated type results in a ClassCastException being thrown.

Question 42

What does the following code output?

public class Rodent {}

public class Capybara extends Rodent {
   public static void main(String[] args) {
      Rodent rodent = new Rodent();
      Capybara capybara = (Capybara) rodent;
   }
}

Answer 42

It will throw a ClassCastException.

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Question 43

Can you use the this keyword to access a static variable or method?

Answer 43

Yes but it is not encouraged.