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javainheritancesubtyping

Dynamic constructor calling in java?

Assume I have 4 classes: A, B, SA and SB where B extends A and SB extends SA.

Class A has the following constructor:

private SA a;
public A() {
   a = new SA();
}

Obviously when I'm calling the contructor for class B and since B extends A constructor of class A is also called. But in such a case I would like the constructor of A to do a = new SB(); instead of a = new SA();.

Is there an easy way to do this without changing the public interfaces of both A and B?

Solution

  • Just have a public constructor and a protected constructor:

    private SA a;
public A() {
   this(new SA());
}
protected A(final SA a) {
   this.a = a;
}

    Then in B:

    public B() {
   super(new SB());
}