How to call a method from Constructor

Question

public abstract class A
{
  public A()
  {
    this.load();
  }

  protected void load()
  {

  }
}

public class B extends A
{
  private String testString = null; 

  public B()
  {
    super();
  }

  @Override
  protected void load()
  {
    testString = "test";
  }
}

The application is doing this when creating an instance of the class B using a load class by name method:

• Calls overridden load() in class B
• Initializes variables (calls "private string testString = null" according to debugger), nulling them out.

Is this expected Java behavior? What could cause this? It's a Java 1.6 application running on the 1.7 JDK.

Answer 1

Have a glance at below steps:

• You create an instance of B.
• B() calls superclass constructor - A(), to initialize the superclass members.
• A() now invokes a non-final method which is overridden in B class, as a part of initialization.
• Since the instance in the context is of B class, the method load() invoked is of B class.
• load() initializes the B class instance field - testString.
• The superclass constructor finishes a job, and returns (Assuming chaining of constructor till Object class have been finished)
• The B() constructor starts executing further, initializing its own member.
• Now, as a part of an initialization process, B overwrites the previously written value in testString and re-initializes it to null.