Java Tutorials

• Constructor order dependencies
• Constructor phases
• Constructor phase example

Constructors in extended classes

• When you extend a class, the new class must choose one of it's superclass's constructors to invoke.
• If you do not invoke a superclass constructor as your constructors first executable statement, the superclass no-arg constructor is automatically invoked before any statements in your new constructor are executed.
• If the superclass does not have a no-arg constructor, you must explicitly invoke one of the superclass's other constructors, or invoke another of your own constructors using the this construct.
• If you use super(), it must be the first executable statement of the constructor.
• The language provides a default no-arg constructor for you.

Constructor order dependencies

• When an object is created, all it's fields are set to default initial values.
• Then the constructor is invoked.

Constructor phases

• Invoke a superclass's constructor.
• Initialize the fields using their initializers and any initialization blocks.
• Execute the body of the constructor.

Constructor phase example

• An example of how parent/child constructors work:

  public class ParentClass {
    public ParentClass() {
      System.out.println( "ParentClass constructor was called" );
    }
  }
  public class ChildClass extends ParentClass {
    public ChildClass() {
      System.out.println( "ChildClass constructor was called" );
    }
  }
  public class Main {
    public static void main(String[] args) {
      ChildClass cc = new ChildClass();
    }
  }