Question

Can some one explain to me the private constructor capture idiom with an example and point out where we actually need / do not need this kind of design?

Answer 1

The aim is to capture a temporary value during construction.

An example is given in Solution 53 of Java Puzzlers:

public class MyThing extends Thing {
    private final int arg;

    public MyThing() {
        this(SomeOtherClass.func());
    }

    private MyThing(int i) {
        super(i);
        arg = i;
   }
}

In this case, we want to capture and store the parameter that we wish to pass to the superclass constructor. In order to do so, we create a private helper constructor, which our public constructor then calls.