Thread: Re: Please help me to find out the error in the following code

I'm also getting a runtime null pointer exception with this version when MyClass.set(int) is called, and I suspect it has to do with the creation of the array elements.

public class Main {public class Main {
  public static void main(String[] args){
 
    // class
    class MyClass{
      int x;
      public int get(){
        return this.x;
      };
      public void set(int newX){
        this.x = newX;
      }
    }
    // create array
    MyClass[] mcArray = new MyClass[10];
    System.out.println(String.format("array is %s elements long", mcArray.length));
 
    // attempt to set array elements
    for(MyClass thisOne : mcArray){
      thisOne.set(2);
    }
  } // main
} // Main

Moved to own thread.

getting a runtime null pointer exception

What variable has the null value?
Where in the code is that variable given a value?

has to do with the creation of the array elements.

Declaring an array does not give any value to the array's elements. Each element of the array needs to be assigned a value before it is used.

  MyObject[] anArray = new MyObject[2];   // creates an array with 2 elements containing null
  anArray[0] = new MyObject();                 // assign a value to first element of array

Is there a line numbering feature for this forum's code boxes?

I don't think there is.

I notice this behavior of array of myclass objects doesn't seem identical to the behavior of, say an array of booleans

  boolean[] destinationBag;
 
  // load a destination bag with second bag's contents
  destinationBag = new boolean[secondBag.length + 1];
  for( int index = 0; index < secondBag.length; index++){
    destinationBag[index] = secondBag[index];
  }

  public static void main(String[] args){
 
    // class
    class MyClass{
      int x;
      public int get(){
        return this.x;
      };
      public void set(int newX){
        this.x = newX;
      }
    }
    // create array
    MyClass[] mcArray;
    mcArray = new MyClass[10]; // space allocated, but elements still null
    System.out.println(String.format("array is %s elements long", mcArray.length));
 
    // attempt to set array elements
    for(MyClass thisOne : mcArray){
      // thisOne = new MyClass(); // uncomment to create array element
      thisOne.set(2);
    }
  } // main
} // Main

array of myclass objects doesn't seem identical to the behavior of, say an array of booleans

Yes, primitives have a default value. An array of a class does not and is initially null.

override this behavior through the use of a default constructor

I don't know that arrays have any constructors.

import java.util.Arrays;
 
public class Main {
  public static void main(String[] args){
 
    // simple class 
    class MyClass{
      Integer x = 100;
      // initialization constructor
      public MyClass(Integer starting_x){
        this.x = starting_x;
      }
      public void set(Integer new_x){
        this.x = new_x;
      }
      // string representation
      public String toString(){
        return "x:" + x.toString();
      }
    } // MyClass
 
    // create array
    MyClass[] mcArray;
    mcArray = new MyClass[10]; // space allocated, but elements still null
    System.out.println(
      String.format("array is %s elements long", mcArray.length)
    );
 
    // populate array
    for(Integer index = 0; index < mcArray.length; index++){
      mcArray[index] = new MyClass( 10 * (index+1) ); 
    }
    mcArray[4].set(5000);
 
    System.out.println(Arrays.toString(mcArray));
  } // main
} // Main

typical run output

array is 10 elements long
[x:10, x:20, x:30, x:40, x:5000, x:60, x:70, x:80, x:90, x:100]