Java - Passing a parameter to a class constructor, when an array of that class is being created

Question

I know I can instantiate an object of another class, with an argument in its constructor like below:

public class A{

    B myB;
    myB = new B(this);

}

public class B{

    A instanceThatCreatedMe;

    B(A myA){
        instanceThatCreatedMe = myA;
    }
}

I want to do the same thing, but when B is created in a 2D array. In other words, to create a 2D array of B objects, with (this) as parameter in their constructor Something like this:

public class A{

    B[][] myBArr;
    myBArr = new B[][](this); //<--- That isn't allowed! Neither is myBArr = new B(this)[][]
}

public class B{
    //No change
    A instanceThatCreatedMe;

    B(A myA){
        instanceThatCreatedMe = myA;
    }
}

Is there a way to do it without having to go through the whole array and setting the instanceThatCreatedMe in each object?

Thank you everyone!

new B[][] doesn't create any B instances, so there's no where to pass the A argument. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Feb 12, 2014 at 22:39
Ok thanks! So I have to populate the array by going through it and creating objects like in the first example is that correct? — Nicolas Maltais
– Nicolas Maltais, Commented Feb 12, 2014 at 22:45
Yes, that is correct. Or use an array initialization expression. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Feb 12, 2014 at 22:45

Dolda2000 · Accepted Answer · 2014-02-12 22:40:54Z

3

Sorry, but Java doesn't do that for you. Array creation only creates the arrays themselves, but cannot fill them with anything but null. You have to loop through the created arrays manually and create instances to fill them.

Furthermore, however, your example code wouldn't compile to begin with, since you don't specify the size of the array.

What you want, then, is probably something akin to this:

int d1 = 5, d2 = 6; /* Or however large you want the arrays to be. */
myBArr = new B[d1][d2];
for(int i1 = 0; i1 < d1; i1++) {
    for(int i2 = 0; i2 < d2; i2++) {
        myBArr[i1][i2] = new B(this);
    }
}

answered Feb 12, 2014 at 22:40

Dolda2000

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3 Comments

Nicolas Maltais Over a year ago

Oh I thought that by declaring the type of array as a class (in this case the class B), an Array of B objects was created?

Dolda2000 Over a year ago

No, it only creates an array to hold references to B objects. You still have to create the objects themselves manually.

Thorn G Over a year ago

It's deceptive, because if you create an array of primitives, the array objects are indeed "constructed"...but only because primitives are defined to have default values.

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