Java Object Oriented Design - Java Inheritance and Constructors

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Constructors are not members of a class and they are not inherited by subclasses.

They are used to initialize instance variables.

class CSuper {/* ww w  . j  a v  a  2s .co m*/
  public CSuper() {
    System.out.println("Inside CSuper() constructor.");
  }
}

class CSub extends CSuper {
  public CSub() {
    System.out.println("Inside CSub()  constructor.");
  }
}

public class Main {
  public static void main(String[] args) {
    CSub cs = new CSub();
  }
}

The code above generates the following result.

Example

The following shows how does Compiler Injection of a super() Call to Call the Immediate Ancestor's no-args Constructor

class CSuper {//  w  ww.ja  v  a  2  s.  co m
  public CSuper() {
    super(); // Injected by the compiler
    System.out.println("Inside CSuper() constructor.");
  }
}

class CSub extends CSuper {
  public CSub() {
    super(); // Injected by the compiler
    System.out.println("Inside CSub()  constructor.");
  }
}

public class Main {
  public static void main(String[] args) {
    CSub cs = new CSub();
  }
}

The code above generates the following result.

The keyword super refers to the immediate ancestor of a class.

We can call superclass constructors using the super keyword only as the first statement inside a constructor.

no-args constructor

We can call the no-args constructor or any other constructors of the superclass explicitly as the first statement inside constructors of a class.

The compiler injects the no-args constructor call only if you have not added one explicitly.

class Employee {//  w ww  .j a  va2s .  co  m
  private String name = "Unknown";

  public Employee(String name) {
    this.name = name;
  }

  public void setName(String name) {
    this.name = name;
  }

  public String getName() {
    return name;
  }
}

class Manager extends Employee {
  public Manager(String name) {
    super(name);
  }
}

public class Main {
  public static void main(String[] args) {
    Manager mgr = new Manager("Tom");
    String name = mgr.getName();
    System.out.println("Manager name:  " + name);
  }
}

The code above generates the following result.

Every class must call the constructor of its superclass from its constructors directly or indirectly.

If the superclass does not have a no-args constructor, we must call any other constructors of the superclass explicitly.

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