How do I call one constructor from another in Java?

Question

Is it possible to call a constructor from another (within the same class, not from a subclass)? If yes how? And what could be the best way to call another constructor (if th

Answer 1

[Note: I just want to add one aspect, which I did not see in the other answers: how to overcome limitations of the requirement that this() has to be on the first line).]

In Java another constructor of the same class can be called from a constructor via this(). Note however that this has to be on the first line.

public class MyClass {

  public MyClass(double argument1, double argument2) {
    this(argument1, argument2, 0.0);
  }

  public MyClass(double argument1, double argument2, double argument3) {
    this.argument1 = argument1;
    this.argument2 = argument2;
    this.argument3 = argument3;
  }
}

That this has to appear on the first line looks like a big limitation, but you can construct the arguments of other constructors via static methods. For example:

public class MyClass {

  public MyClass(double argument1, double argument2) {
    this(argument1, argument2, getDefaultArg3(argument1, argument2));
  }

  public MyClass(double argument1, double argument2, double argument3) {
    this.argument1 = argument1;
    this.argument2 = argument2;
    this.argument3 = argument3;
  }

  private static double getDefaultArg3(double argument1, double argument2) {
    double argument3 = 0;

    // Calculate argument3 here if you like.

    return argument3;

  }

}

Answer 2

There are design patterns that cover the need for complex construction - if it can't be done succinctly, create a factory method or a factory class.

With the latest java and the addition of lambdas, it is easy to create a constructor which can accept any initialization code you desire.

class LambdaInitedClass {

   public LamdaInitedClass(Consumer<LambdaInitedClass> init) {
       init.accept(this);
   }
}

Call it with...

 new LambdaInitedClass(l -> { // init l any way you want });

Answer 3

Yes, you can call constructors from another constructor. For example:

public class Animal {
    private int animalType;

    public Animal() {
        this(1); //here this(1) internally make call to Animal(1);
    }

    public Animal(int animalType) {
        this.animalType = animalType;
    }
}

you can also read in details from Constructor Chaining in Java

Answer 4

Yes it is possible to call one constructor from another. But there is a rule to it. If a call is made from one constructor to another, then

that new constructor call must be the first statement in the current constructor

public class Product {
     private int productId;
     private String productName;
     private double productPrice;
     private String category;

    public Product(int id, String name) {
        this(id,name,1.0);
    }

    public Product(int id, String name, double price) {
        this(id,name,price,"DEFAULT");
    }

    public Product(int id,String name,double price, String category){
        this.productId=id;
        this.productName=name;
        this.productPrice=price;
        this.category=category;
    }
}

So, something like below will not work.

public Product(int id, String name, double price) {
    System.out.println("Calling constructor with price");
    this(id,name,price,"DEFAULT");
}

Also, in the case of inheritance, when sub-class's object is created, the super class constructor is first called.

public class SuperClass {
    public SuperClass() {
       System.out.println("Inside super class constructor");
    }
}
public class SubClass extends SuperClass {
    public SubClass () {
       //Even if we do not add, Java adds the call to super class's constructor like 
       // super();
       System.out.println("Inside sub class constructor");
    }
}

Thus, in this case also another constructor call is first declared before any other statements.

Answer 5

You can call another constructor via the this(...) keyword (when you need to call a constructor from the same class) or the super(...) keyword (when you need to call a constructor from a superclass).

However, such a call must be the first statement of your constructor. To overcome this limitation, use this answer.

Answer 6

Calling constructor from another constructor

class MyConstructorDemo extends ConstructorDemo
{
    MyConstructorDemo()
    {
        this("calling another constructor");
    }
    MyConstructorDemo(String arg)
    {
        System.out.print("This is passed String by another constructor :"+arg);
    }
}

Also you can call parent constructor by using super() call