How to implement constructor wrapping in Java?

Question

This is what I'm trying to do (in Java 1.6):

public class Foo {
  public Foo() {
    Bar b = new Bar();
    b.setSomeData();
    b.doSomethingElse();
    this(b);
  }
  public Foo(Bar b) {
    // ...
  }
}

Compiler says:

call to this must be first statement in constructor

Is there any workaround?

Answer 1

You could implement it like this:

public class Foo {
  public Foo() {
    this(makeBar());
  }
  public Foo(Bar b) {
    // ...
  }
  private static Bar makeBar() {
    Bar b = new Bar();
    b.setSomeData();
    b.doSomethingElse();
    return b;
  }
}

The makeBar method should be static, since the object corresponding to this is not available at the point you are calling the method.

By the way, this approach has the advantage that it does pass a fully initialized Bar object to the Foo(Bar). (@RonU notes that his approach does not. That of course means that his Foo(Bar) constructor cannot assume that its Foo argument is in its final state. This can be problematical.)

Finally, I agree that a static factory method is a good alternative to this approach.

Answer 2

You can implement the "default constructor" as a static factory method:

public class Foo {
  public static Foo createFooWithDefaultBar() {
    Bar b = new Bar();
    b.setSomeData();
    b.doSomethingElse();
    return new Foo(b);
  }
  public Foo(Bar b) {
    // ...
  }
}