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..
You never assign anything to the ds field.
ReplyDeleteDataStrucHand.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>();
}
You need to put an ArrayList<DataStruc> instance in ds.
ReplyDeleteYou haven't instantiated the ArrayList. Write this:
ReplyDeletepublic class DataStrucHand {
private ArrayList<DataStruc> ds = new ArrayList<DataStruc>();
public void addData(String ss){
ds.add(new DataStruc(ss));
}
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>();.
ReplyDeleteThe problem is that your DataStrucHand class never initializes its private field ds, so when you try to call ds.add(...) it fails with NullPointerException.
ReplyDeleteIn 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.