Can object method call be done simultaneously with object instantiation?

Question

I'm trying to use java class BitSet as a field for a customized class. And I want the class to use a default BitSet with all bits set.

import java.util.BitSet;

public class MyClass {
    private BitSet mask;

    public MyClass() {
        this(new BitSet(4));
        // want to set all bits first
        // something like 
        // this( new BitSet(4).set(0,3) );
    }

    public MyClass(BitSet mask) {
        this.mask = mask;
    }    
}

By default BitSet constructor unsets all bits. So before I send it as an anonymous object, I would like call set(int, int) method to set all bits. I know that I could simply initialize the field mask to a new BitSet and then call set(int, int) method from there.

However, in general I'm wondering is it possible to access an instance method at time of object instantiation?

Answer 1

Why not writing a separate constructor that allows for the BitSet initialization? Using Java 8, this could look like something like this:

public class MyClass {
  private BitSet mask;
  public MyClass() {
    this(new BitSet(4),(m)->m.set(0,3));
  }
  public MyClass(BitSet mask,Consumer<BitSet> initializer) {
    initializer.accept(mask);
    this.mask = mask;
  }
}

You can even make that more generic by introducing a static method with type parameters:

public static <T> T initialize(T t,Consumer<T> initializer) {
  initializer.accept(t);
  return t;
} 

In that case, the earlier MyClass would look as follows:

public class MyClass {
  private BitSet mask;
  public MyClass() {
    this(initialize(new BitSet(4),(m)->m.set(0,3)));
  }
  public MyClass(BitSet mask) {
    this.mask = mask;
  }
}

UPDATE

And there is one more way, without introducing new methods or constructors:

public class MyClass {
  private BitSet mask;
  public MyClass() {
    this(new BitSet(4) {{ set(0,3); }});
  }
  public MyClass(BitSet mask) {
    this.mask = mask;
  }
}

An anonymous class is being instantiated by extending BitSet and adding an instance initialization block, hence the double curly braces.