CODEWITHSUNDEEP
javaarraysclassobjectconstructor
chandank
chandank

Reputation: 1011

Instantiating different class in constructor in Java

I am very new to Java have just started working on simple example programs.

How can I create class A's instance inside class B's constructor. For example, I want to create an array of objects of class A , in class B's constuctor. The psudo code would look like

class B {

public static A myarray;
B (int number){
  myarray = new A [number];
}

Edited:

public class TestClassA {

    public static int []  ArrayA = new int [6];
    TestClassA () {
        for (int i=0; i < 6; i++){
            ArrayA[i]=i;
            System.out.print("TestClassA ");
        }
    }
}

public class TestClassB {

    public TestClassA [] A;
    TestClassB (int num) {
        A = new TestClassA[num];
    }
}

public class Exec {

    public static void main (String[] args) {

        TestClassB B;
        B = new TestClassB(2);

    }
}

When I execute this, I don't see any messages as "TestClassA ". I expect it to create 2 instances of TestClassA array , hence I should see the TestClassA 12 times. Not sure where am I doing wrong.

Upvotes: 1

Views: 2324

Answers (5)

Ravi K Thapliyal
Ravi K Thapliyal

Reputation: 51721

Couple of pointers

  • Don't use static, unless you want to share the array of A objects with every instance of class B.
  • You do however need to use [] while declaring the reference to indicate that it's an Array.
  • Make your member fields private as well. Then control access to them through public or protected getter/setter methods.

Your code should look like

public class B {

    private A[] arrayOfAobjects;

    B (int number) {
         arrayOfAobjects = new A[number];
    }

    public A[] getArrayOfAobjects() {
        return arrayOfAobjects;
    }
}

EDIT: (to elaborate on @MikeStrobel's comments below)
When you create an array, it gets initialized with default values as per the type of the Array. For example, every array element is set to 0 for an int [], 0.0 for a double [], null for all types of Object [] object arrays.

new int[100]; // creates an Array with 100 zeroes
new A[number]; // creates an Array of size "number"
               // but filled with nulls (instead of A objects)

So, you need to populate the arrays with correct values yourself. In your case, something like

B (int number) {
     arrayOfAobjects = new A[number];
     for (int i=0; i < number; i++) {
         arrayOfAobjects[i] = new A(); // initialize the A[] array
     }
}

EDIT 2 :

public class TestClassB {

    public TestClassA [] A;

    TestClassB (int num) {
        A = new TestClassA[num];
        for (int i=0; i < num; i++) {
            A[i] = new TestClassA(); // You need to INITIALIZE your Array
        }
    }
}

Upvotes: 4

Shreyos Adikari
Shreyos Adikari

Reputation: 12764

If you want to create an object of class A inside the constructor of class B you can simply do it like this:

class B { 
    public A object;
    B (int number){
      object= new A();
    }

class A{

    }

If you want to create an array of A class then do not make the variable as static.

class B { 
        public A[] myarray;
        int number = 5;
        B (int number){
          myArray = new A[number];
        }

class A{

        }

EDIT: Syntax of array of objects(You need array of 4 objects).

 A[] a = new A[4]; // Create the array of size 4.
 A a1 = new A(); //Create an object
 ............    //Similarly create other three objects
 a[0] = a1; //Add the object to the array
 ............   //Similarly add other three objects

Upvotes: 1

Hunter McMillen
Hunter McMillen

Reputation: 61550

public class B {

  private A[] arrayOfAs;

  public B (int number) {
     this.arrayOfAs = new A[number];
  }

  public getAs() {
     return this.arrayOfAs;
  }
}

Not above that arrayofAs is not static, because you don't want to share your array among all instances of B. I also made it private because that is considered good practice for instance variables.

You can return this array to a different class using the getAs() accessor method above.

Upvotes: 0

Guanxi
Guanxi

Reputation: 3131

This is what you should be writing.

class B{
public A[] myArray;

B(int number){
  myArray = new A[number];
   }
  }

Upvotes: 0

Kris
Kris

Reputation: 8873

class B {

public A[] myarray;
B (int number){
  myarray = new A [number];
}

it would be good to practice to use instance variables in private or protected mode, and using getter and setter methods to access it. This code will just create a un-initialized array of A objects only. If you want to use those, you need to init them seperately like myarra[i]=new A(); where i being any number 0<=i<number

Upvotes: 0

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