Defining constructors

Question 1

How do we call the superclass constructor from the subclass constructor ?

Answer 1

We call the super class constructor from the subclass constructor using the keyword ‘super()’

Question 2

What is the characteristic of that keyword ?

Answer 2

The keyword super() can only be the first non commented statement of a constructor

Question 3

Give an example where the constructor of the super class has parameters !

Answer 3

public class Animal {
private int age;
public Animal(int age) {
super();
this.age = age;
}
}

public class Zebra extends Animal {
public Zebra(int age) {
super(age);
}
public Zebra() {
this(4);
}
}

Question 4

What happens if the parent class has more than one constructor ?

Answer 4

If the parent class has more than one constructor then the child class can use any valid parent constructor in its definition

Question 5

Describe how the java compiler handle a class declared with no constructor !

Answer 5

When a class doesn’t have a constructor, the java compiler inserts automatically a default no argument constructor that contains in its body a call to the non-argument constructor of the super class using the ‘super’ keyword

Question 6

Describe what happens if the parent class doesn’t have a no-argument constructor !

Answer 6

If a class doesn’t have a constructor and at the same time, extends a class that doesn’t have a no-argument constructor, this combination will produce a compilation error

Question 7

Give an example that compiles as well as one that doesn’t compile !

Answer 7

EXAMPLE THAT COMPILES :

public class Mammal {
public Mammal(int age) {
}
}
public class Elephant extends Mammal {
public Elephant() {
super(10);
}
}

EXAMPLE THAT DOESN’T COMPILE :

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

public class Elephant extends
Mammal { // DOES NOT COMPILE
}

Question 8

Give the constructor rules with an example for each one ! (IMPORTANT)

Answer 8

1. The first statement of every constructor is a call to another constructor within the class using this(), or a call to a constructor in the direct parent class using super().
2. The super() call can not be used after the first statement of the constructor
3. If no super() call is declared in a constructor, Java will insert a no-argument super() as the first statement of the constructor.
4. If the parent doesn’t have a no-argument constructor and the child doesn’t define any constructors, the compiler will throw an error and try to insert a default no-argument constructor into the child class.
5. If the parent doesn’t have a no-argument constructor, the compiler requires an explicit call to a parent constructor in each child constructor

Question 9

Give an important rule related to Parent and Child constructors with an example !

Answer 9

In Java, the parent constructor always runs before the child constructor because it is the topmost class in the hierarchy.