Java – When to call the constructor and when to call the method in Java

constructorsjavamethods

I am struggling to fully understand the usage of constructors in Java.

What I have learned so far about constructors is the following:

  • same name as class
  • abbreviation ctor
  • overloading
  • no return type
  • create an object of a class
  • every class has a default constructor

When for example a string has to be returned from a class that needs to be called then a method could be created, i.e. a constructor will not be sufficient as this will not return anything.

Attempt to answer the question

In order to explain what I mean, I have created a class with two constructors and two methods that return a string.

public class HelloWorldConstructor {
    public HelloWorldConstructor() { }

    public HelloWorldConstructor(String a) {
        saySomething(a);
    }

    public HelloWorldConstructor(String a, String b) {
        saySomething(a, b);
    }

    void saySomething(String a) {
        System.out.println(a);
    }

    void saySomething(String a, String b) {
        System.out.println(a + ", " + b);
    }
}

Option 1

It is possible to return a string by calling the method that resides in the class.

public class CallConstructor {
    public static void main(String[] args) {
        HelloWorldConstructor hwc = new HelloWorldConstructor();

        hwc.saySomething("allo");
        hwc.saySomething("allo", "allo");
    }
}

returns:

allo
allo, allo

Option 2

It is also possible to return a string by calling the constructor directly.

public class CallConstructor2 {
    public static void main(String[] args) {
        new HelloWorldConstructor("allo");
        new HelloWorldConstructor("allo", "allo");
    }
}

returns the same as option 1.

Discussion

When option 2 is chosen, then two objects have to be created instead of one as depicted by option 1, but when to choose option 2 and when option 1? In this case I think it is better to choose option 1 as one object will be created, but option 2 could be suitable when other circumstances are applicable.

Using Constructors in Java

The constructor in the example just gives an initial value to class
members.

https://stackoverflow.com/a/19941847/2777965

Constructors are used to initialize the instances of your classes. You
use a constructor to create new objects often with parameters
specifying the initial state or other important information about the
object

After reading about theory and Q&As about constructors I am struggling to fully understand them. I know how to call a constructor and how to call a method, but I cannot rationalize this.

  • A constructor has to be called directly when …
  • Constructor overloading will be done when …
  • Methods will be called directly by calling the default constructor when …

Best Answer

You call a constructor when you want to create a new object instance. For example if you have a class Button, then you call the constructor if you want to create a new Button instance.

If you don't need a new object, then don't use a constructor. In your example you just use the constructor as a convoluted way of calling a method, but you don't actually use the created object (the result of the new-expression) for anything. This is a clear hint that you don't need to call a constructor. If you made saySomething static you would not need to call any constructors.

The reason for your confusion is probably that the HelloWorldConstructor class does not really have any purpose beyond writing output. Therefore it is not obvious whether you should have one or two instances of it. In reality you don't need any instance at all.

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