Question

I’m having some trouble using multiple constructors in java.

what I want to do is something like this:

public class MyClass {

 // first constructor
 public MyClass(arg1, arg2, arg3) {
  // do some construction
 }

 // second constructor
 public MyClass(arg1) {
      // do some stuff to calculate arg2 and arg3
      this(arg1, arg2, arg3);
    }
}

but I can’t, since the second constructor cannot call another constructor, unless it is the first line.

What is the common solution for such situation?
I can’t calculate arg2 and arg3 “in line”. I thought maybe creating a construction helper method, that will do the actual construction, but I’m not sure that’s so “pretty”…

EDIT: Using a helper method is also problematic since some of my fields are final, and I can’t set them using a helper method.

Answer 1

Typically use another common method – a “construction helper” as you’ve suggested.

public class MyClass { 

    // first constructor 
    public MyClass(arg1, arg2, arg3) { 
      init(arg1, arg2, arg3); 
    } 

    // second constructor 
    public MyClass(int arg1) { 
      // do some stuff to calculate arg2 and arg3 
      init(arg1, arg2, arg3); 
    } 

    private init(int arg1, int arg2, int arg3) {
      // do some construction 
    }
} 

The alternative is a Factory-style approach in which you have a MyClassFactory that gives you MyClass instances, and MyClass has only the one constructor:

public class MyClass { 

    // constructor 
    public MyClass(arg1, arg2, arg3) { 
      // do some construction 
    } 
} 

public class MyClassFactory { 

    public static MyClass MakeMyClass(arg1, arg2, arg3) { 
      return new MyClass(arg1, arg2, arg3);
    } 

    public static MyClass MakeMyClass(arg1) { 
      // do some stuff to calculate arg2 and arg3 
      return new MyClass(arg1, arg2, arg3);
    } 
} 

I definitely prefer the first option.