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How do I call one constructor from another in Java?

785

Is it possible to call a constructor from another (within the same class, not from a subclass)? If yes how? And what could be the best way to call another constructor (if there are several ways to do it)?

2Answer


0

Yes, it is possible:

public class Foo
{
    private int x;

    public Foo()
    {
        this(1);
    }

    public Foo(int x)
    {
        this.x = x;
    }
}

To chain to a particular superclass constructor instead of one in the same class, use super instead of this. Note that you can only chain to one constructor, and it has to be the first statement in your constructor body.

See also this related question, which is about C# but where the same principles apply.

  • answered 2 years ago
  • Sandy Hook

0

In Java another constructor of the same class can be called from a constructor via this(). Note however that this has to be on the first line.

public class MyClass {

  public MyClass(double argument1, double argument2) {
    this(argument1, argument2, 0.0);
  }

  public MyClass(double argument1, double argument2, double argument3) {
    this.argument1 = argument1;
    this.argument2 = argument2;
    this.argument3 = argument3;
  }
}

That this has to appear on the first line looks like a big limitation, but you can construct the arguments of other constructors via static methods. For example:

public class MyClass {

  public MyClass(double argument1, double argument2) {
    this(argument1, argument2, getDefaultArg3(argument1, argument2));
  }

  public MyClass(double argument1, double argument2, double argument3) {
    this.argument1 = argument1;
    this.argument2 = argument2;
    this.argument3 = argument3;
  }

  private static double getDefaultArg3(double argument1, double argument2) {
    double argument3 = 0;

    // Calculate argument3 here if you like.

    return argument3;

  }

}
  • answered 2 years ago
  • Sandy Hook

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