Does a constructor call create a new instance?

Question

I learned two things:

The new-operator creates a new instance and then the stated connstructor is executed to initialise that new instance
A constructor call (this()) creates a new instance.

For my understanding these statements object each other.

For example wouldn't new Example() create two instances then, because the new-operator creates one and the constructor calls this() and creates another? Of course it doesn't but what exactly creates an instance now...?

class Example
{
    private boolean _b;

    public Example()
    {
        this(false);
    }

    public Beispiel(boolean b)
    {
        _b = b;
    } 
}

11

Your second point is false. – SLaks Jul 06 '14 at 14:32

score 6 · Accepted Answer · answered Jul 06 '14 at 14:41

6

Your second point is incorrect: Invoking this() doesn't "create a new instance". Rather, it calls a (usually different) constructor than the one called by new.

Calling new is what creates the new instance.

answered Jul 06 '14 at 14:41

Bohemian

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score 0 · Answer 2 · answered Jul 06 '14 at 14:36

You can use this inside a constructor to call constructors of the same class with different number of arguments for example:

class Example{
    private boolean b;
    public Example(){
        this(false) // you now call public Example(boolean b) to save code istead of    this.b=false
    }
    public Example(boolean b){
        this.b = b;
    }
}

paisanco · Answer 3 · 2014-07-29T03:29:39.080

0

No. Invoking this doesn't create a new instance. Only invoking new creates a new instance of the object.

Your syntax this(argument) is explicit invocation of a different constructor within the same class.

Such invocation can be done , for instance, to initialize some variables in the object.

See from the Oracle Java tutorials Using the this keyword

edited Jul 29 '14 at 03:29

answered Jul 06 '14 at 14:36

paisanco

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score 0 · Answer 4 · edited May 23 '17 at 11:50

By calling this(false) you do not create a new instance. You just call a constructor within a constructor(the one matching the number of the arguments you are passing), for which i cannot think of any efficient practical use right now, but nevertheless is perfectly valid. Note that, to chain constructors like that, you have to make a new constructor call in the first line of the "parent" constructor.If that makes sense. Bottomline; you create one object. Maybe adding print statements for each different constructor call can help you grasp this thing better.

Also, take a look here: How do I call one constructor from another in Java?

Does a constructor call create a new instance?

4 Answers4