how to add elements to an Arraylist of an object in Java?

So, I have a class with some vectors and I’m trying to add elements to those vectors. But, when I try to push elements with add method, it throws NullPointerException. I’m using eclipse as editor.

package com.vogella.eclipse.ide.first;  import java.util.Vector;  public class Qr {       // The class   int x,y;   Vector<String> at = new Vector<String>(); }   public class cls {   public static void main(String[] args) {     Qr[] var = new Qr[5];     String q = new String("ggg");      var[1].at.add(q);     System.out.print("gg");   } } 

It throws an error

 Exception in thread "main" java.lang.NullPointerException  at sf.eclipse.ide.first.nfd.main(nfd.java:89) 

Is this correct or am I doing something wrong?

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1 Answer(s)

You need to make following changes to your code to make it work :

  1. Create a constructor to initialize the variable of the class Qr.

  2. Once you have created the Qr array, you need to initialize , individual element of the Qr array.

       class Qr {       // The class     int x,y;     Vector<String> at;      Qr(){          //Step1        this.x=0;        this.y=0;        this.at = new Vector<String>();     }  } 

Main method:

public static void main(String[] args) {     Qr[] var = new Qr[5];     var[1] = new Qr(); // Step 2     String q = new String("ggg");      var[1].at.add(q);     System.out.print("gg"); } 
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