arraylist java compiling errors

Question

ok i try to play with arraylist in java awhile to experiment some thing that related my project..so i come up with a simple code like this

having 3 file...DataStruc.java , DataStrucHand.java , testcase1.java

DataStruc.java


public class DataStruc {
private String testString;

public DataStruc(String s){
    this.testString = s;
}

public String getTestString() {
    return testString;
}

public void setTestString(String testString) {
    this.testString = testString;
}

public String toString(){
    return String.format("%s",testString);
}
}


DataStrucHand.java

 import java.util.ArrayList;

  public class DataStrucHand {
private ArrayList<DataStruc> ds;

public void addData(String ss){
    ds.add(new DataStruc(ss));
}

public ArrayList<DataStruc> getData(){
    return ds;
}
}


testcase1.java
 import java.util.*; 
 public class testcase1 {
public static void main(String args []){
    DataStrucHand dsh = new DataStrucHand();

    String gdata = "test";

    dsh.addData(gdata);


  }
   }

i tried to compile it and having this error

 Exception in thread "main" java.lang.NullPointerException
at DataStrucHand.addData(DataStrucHand.java:7)
at testcase1.main(testcase1.java:8)

can i know what is wrong actually? i cant even add the data...i am trying to add the data and retrieve it back by creating another testcase2.java...but than i having problems in adding now to the arraylist...my purpose is to create a temp storage to keep a specific string that can be obtain by 1 program but runs with 2 different classes..

nicholas.hauschild · Accepted Answer · 2012-02-27 19:48:18Z

2

You never assign anything to the ds field.

DataStrucHand.java

import java.util.ArrayList;

public class DataStrucHand {
    private ArrayList<DataStruc> ds; //I am null because nothing is ever new'd up here...

    public void addData(String ss){
        ds.add(new DataStruc(ss));
    }

    public ArrayList<DataStruc> getData(){
        return ds;
    }
}

Try it with this line:

private ArrayList<DataStruc> ds = new ArrayList<DataStruc>();

Or, you can have a constructor that will new it up if you prefer that method:

public DataStrucHand() {
     ds = new ArrayList<DataStruc>();
}

answered Feb 27, 2012 at 19:48

nicholas.hauschild

42.9k9 gold badges129 silver badges120 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user1217609 Over a year ago

oh my god, thanks for the help...is like 3.50AM here and i really need some sleep

Hovercraft Full Of Eels Over a year ago

@user1217609: You need sleep and to learn the steps on how to debug a NPE. The key is to check the variables on the line throwing the exception then trace back in the code to see why it's null.

SLaks · Accepted Answer · 2012-02-27 19:48:24Z

0

You need to put an ArrayList<DataStruc> instance in ds.

answered Feb 27, 2012 at 19:48

SLaks

891k182 gold badges1.9k silver badges2k bronze badges

Comments

sharakan · Accepted Answer · 2012-02-27 19:48:38Z

0

You haven't instantiated the ArrayList. Write this:

public class DataStrucHand {
    private ArrayList<DataStruc> ds = new ArrayList<DataStruc>();

    public void addData(String ss){
       ds.add(new DataStruc(ss));
    }

answered Feb 27, 2012 at 19:48

sharakan

6,9212 gold badges38 silver badges63 bronze badges

Comments

Cloudy · Accepted Answer · 2012-02-27 19:48:54Z

0

You've never initialized ds, so it is null when you call ds.add(new DataStruc(ss)); Add a constructor to DataStrucHand that initializes ds, such as ds = new ArrayList<DataStruc>();.

answered Feb 27, 2012 at 19:48

Cloudy

7,5297 gold badges64 silver badges98 bronze badges

Comments

Adam Zalcman · Accepted Answer · 2012-02-27 19:54:50Z

0

The problem is that your DataStrucHand class never initializes its private field ds, so when you try to call ds.add(...) it fails with NullPointerException.

In fact, the way the class looks right now, there is no way ds can be anything else than null.

Shortest way to fix this is to initialize ds properly:

private final List<DataStruc> ds = new ArrayList<DataStruc>();

This way each DataStrucHand instance is constructed with an ArrayList inside and ds is never null.

edited Feb 27, 2012 at 19:54

answered Feb 27, 2012 at 19:49

Adam Zalcman

27.3k4 gold badges75 silver badges95 bronze badges

Collectives™ on Stack Overflow

arraylist java compiling errors

5 Answers 5

2 Comments

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related