How to call a different constructor conditionally in Java?

Question

Let\'s say someone gives you a class, Super, with the following constructors:

public class Super
{
    public Super();
    public Super(int arg)         


        

Answer 1

You can't do that, but you can do this from the code that calls your class:

        if (some_condition_1)
            new Super();
        else if (some_condition_2)
            new Super("Hi!");
        else if (some_condition_3)
            new Super(new int[] { 5 });
        else
            new Super(arg);

Answer 2

Can't be done as such as super must be first statement in a constructor.

The proper alternative is a builder class and to have one constructor in the derived class for each constructor in the super class.

eg.

Derived d = new DerivedBuilder().setArg(1).createInstance();

---

public class DerivedBuilder {

    private int arg;

    // constructor, getters and setters for all needed parameters

    public Derived createInstance() {
        // use appropriate constructor based on parameters
        // calling additional setters if need be
    }
}

Answer 3

Yeah, what @Johan Sjöberg said.

Also looks like your example is highly contrived. There's no magical answer which would clear this mess :)

Usually, if you have such a bunch of constructors it would be a good idea to refactor them as four separate classes (a class should be only responsible for one type of thing).

Answer 4

Use static factories, and four private constructors.

class Foo {
 public static Foo makeFoo(arguments) {
    if (whatever) {
      return new Foo(args1);
    } else if (something else) {
      return new Foo(args2);
    }
    etc...
  }
  private Foo(constructor1) { 
    ...
  }
  ...
}

Answer 5

super must be the first statement in a constructor, hence the logic in your sample is not valid.

The proper way is to create the same 4 constructors in your extending class. If you need validation logic you can use e.g., the builder pattern. You can also as suggested in the comments by @davidfrancis make all constructs private and supply a static factory method. E.g.,

public static Derived newInstance(int arg) {
      if (some condition) {
         return new Derived(arg);
      }
      // etc
}