java this(null)

Question

I want to know what does this means?

public Settings() {
    this(null);
}

The above code is the constructor of a class "Settings". What does this(null) means here?

possible duplicate of [what type of java constructors are these? Constructor chaining?](http://stackoverflow.com/questions/9204560/what-type-of-java-constructors-are-these-constructor-chaining) — Paul Bellora, May 18 '12 at 18:28
possible duplicate of [How do I call one constructor from another in Java?](http://stackoverflow.com/questions/285177/how-do-i-call-one-constructor-from-another-in-java) — Ciro Santilli OurBigBook.com, Mar 02 '15 at 12:27

porfiriopartida · Accepted Answer · 2012-05-18T18:34:26.753

15

public Settings() {
    this(null); //this is calling the next constructor
}
public Settings(Object o) {
//  this one
}

This is often used to pass default values so you can decide to use one constructor or another..

public Person() {
    this("Name"); 
}
public Person(String name) {
    this(name,20)
}
public Person(String name, int age) {
    //...
}

edited May 18 '12 at 18:34

answered May 18 '12 at 18:27

porfiriopartida

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score 10 · Answer 2 · answered May 18 '12 at 18:27

10

It means you are calling an overloaded constructor which takes an Object of some sort but you do not pass an object, but a plain null.

answered May 18 '12 at 18:27

Mechkov

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Could that be calling to an inherited constructor, or would that look different? – ametren May 18 '12 at 18:30
3

No it can't @ametren, if you want to do that you have to do it with `super(this)` – Mattias Isegran Bergander May 18 '12 at 18:31
well you can call the parent constructor with super, if Settings extends a class that receives String as an argument, you can use super("somestring") – porfiriopartida May 18 '12 at 18:33
Thanks, I was just wondering if that might have been causing his confusion. :) – ametren May 18 '12 at 18:33

score 5 · Answer 3 · answered May 18 '12 at 18:27

It's a constructor that is calling another constructor in the same class.

You presumably have something like this:

public class Settings {
    public Settings() {
        this(null);  // <-- This is calling the constructor below
    }

    public Settings(object someValue) {
    }
}

Often this pattern is used so that you can offer a constructor with fewer parameters (for ease of use by the callers) but still keep the logic contained in one place (the constructor being called).

score 4 · Answer 4 · answered May 18 '12 at 18:27

4

It's calling a different constructor inside the Settings class. Look for another constructor that accepts a single parameter.

answered May 18 '12 at 18:27

domvoyt

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score 1 · Answer 5 · answered May 18 '12 at 18:27

1

It call the default constructor passing null as argument ...

answered May 18 '12 at 18:27

aleroot

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Samir Mangroliya · Answer 6 · 2012-05-18T19:37:17.667

1

Try to read Overloaded constructor in java and you call constructor who has only one parameter..

.

    public Settings() {
        this(null);
    }

   public Settings(Object obj){

}

edited May 18 '12 at 19:37

answered May 18 '12 at 18:30

Samir Mangroliya

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score 1 · Answer 7 · answered May 18 '12 at 18:50

This is called Constructor Chaining in Java. By this call you actually invoke overloaded constructor of your class object. For Example

class Employee extends Person {
    public Employee() {
        this("2")  //Invoke Employee's overloaded constructor";
    }

    public Employee(String s) {
        System.out.println(s);
    }
}

score 0 · Answer 8 · answered Mar 01 '16 at 20:59

This basically calls another parameterised constructor in the same class as every one mentioned.

One thing to note here, it will produce an error, if there is no parameterised constructor available.

I do not se any use of passing this() with null value. But can be a tricky question to ask some one. :)

java this(null)

8 Answers8