```
I have an expandable listview with checkboxes in group and child. 

```

What I want is when I check on group checkbox , all its child items should get checked. 
But I'm getting null pointer exeption when I'm running program. Here is my code......thanks 

```
//ExpandableListAdapter.java:

import java.util.ArrayList;
import java.util.HashMap;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseExpandableListAdapter;
import android.widget.CheckBox;
import android.widget.TextView;
import android.view.View.OnClickListener;

public class ExpandableListAdapter extends BaseExpandableListAdapter {

    LayoutInflater inflater;

    /*list of group */
    private ArrayList<Group> groups;
    private ArrayList<Boolean> isCheckedHeader=new ArrayList<Boolean>();
    private HashMap<String, ArrayList<Boolean>> IsCheckedChild;

public ExpandableListAdapter(Context context,ArrayList<Group> groups,ArrayList<Boolean> isCheckedHeader,HashMap<String, ArrayList<Boolean>> IsCheckedChild) {
    super();
        this.groups=groups;
        this.isCheckedHeader=isCheckedHeader;
        this.IsCheckedChild=IsCheckedChild;
        inflater= (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }
    /**
     * @param child
     * @param group
     *  use for adding item to list view
     */
    public void addItem(Child child,Group group) {
        if(!groups.contains(group)) {
            groups.add(group);
        }
        int index=groups.indexOf(group);
        ArrayList<Child> ch=groups.get(index).getChildrens();
        ch.add(child);
        groups.get(index).setChildrens(ch);
    }

    public Child getChild(int groupPosition, int childPosition) {
        ArrayList<Child> ch=groups.get(groupPosition).getChildrens();

        return ch.get(childPosition);
    }

    public long getChildId(int groupPosition, int childPosition) {
        return childPosition;
    }

    @Override
    public int getChildrenCount(int groupPosition) {
        ArrayList<Child> ch=groups.get(groupPosition).getChildrens();
        return ch.size();
    }

    public View getChildView(int groupPosition, int childPosition, boolean isLastChild,
            View convertView, ViewGroup parent) {
        Child child= (Child) getChild(groupPosition,childPosition);
        TextView childName=null;
        if(convertView==null) {
          convertView=inflater.inflate(R.layout.child_view, null);
        }

        final CheckBox chkChild=(CheckBox)convertView.findViewById(R.id.checkBoxC);
        childName=(TextView) convertView.findViewById(R.id.textViewChildName);
        childName.setText(child.getChildName());
        chkChild.setChecked(IsCheckedChild.get(this.groups.get(groupPosition)).get(childPosition));

        final int iGroupPos = groupPosition;
        final int iChildPos = childPosition;

        chkChild.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                if(chkChild.isChecked()) //the child was enable (checked)
                {
                    IsCheckedChild.get(groups.get(iGroupPos)).set(iChildPos, true);
                }
                else //the child was disable (non-checked)
                {
                    IsCheckedChild.get(groups.get(iGroupPos)).set(iChildPos, false);
                }
            }
        });

        return convertView;
    }
    public Group getGroup(int groupPosition) {
        return groups.get(groupPosition);
    }

    public int getGroupCount() {
        return groups.size();
    }

    public long getGroupId(int groupPosition) {
        return groupPosition;
    }

    public View getGroupView(int groupPosition, boolean isExpanded, View convertView,
            ViewGroup parent) {
        TextView groupName = null;
        Group group=(Group) getGroup(groupPosition);
        if(convertView==null) {
          convertView=inflater.inflate(R.layout.group_view, null);         
        }   

        groupName=(TextView) convertView.findViewById(R.id.textViewGroupName);
        groupName.setText(group.getGroupName());
        final CheckBox chkParent=(CheckBox)convertView.findViewById(R.id.checkBoxH);
        chkParent.setChecked(isCheckedHeader.get(groupPosition));
        final int iPos = groupPosition;
        chkParent.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                if(chkParent.isChecked()) {
                    isCheckedHeader.set(iPos, true);
                    //Checking all SubItem(Childs) of this
                    int iSizeChilds = getChildrenCount(iPos);
                    for(int i=0;i<iSizeChilds;i++) {
                        IsCheckedChild.get(groups.get(iPos)).set(i, true);
                    }
                }
                notifyDataSetChanged();
            }
        });
        return convertView;
    }

    public boolean isChildSelectable(int groupPosition, int childPosition) {
        return true;
    }

    public boolean hasStableIds() {
        return true;
    }
}

```

//Group.java:

```
import java.util.ArrayList;

public class Group {
    public String groupId;
    public String groupName;
    public ArrayList<Child> childrens;
    public Group(String groupId, String groupName,
            ArrayList<Child> childrens) {
        super();
        this.groupId = groupId;
        this.groupName = groupName;
        this.childrens = childrens;
    }
    public String getGroupId() {
        return groupId;
    }
    public void setGroupId(String groupId) {
        this.groupId = groupId;
    }
    public String getGroupName() {
        return groupName;
    }
    public void setGroupName(String groupName) {
        this.groupName = groupName;
    }
    public ArrayList<Child> getChildrens() {
        return childrens;
    }
    public void setChildrens(ArrayList<Child> childrens) {
        this.childrens = childrens;
    }

}

//Child.java:

public class Child {
    public String chiledId;
    public String childName;
    public Child(String chiledId, String childName) {
        super();
        this.chiledId = chiledId;
        this.childName = childName;
    }
    public String getChiledId() {
        return chiledId;
    }
    public void setChiledId(String chiledId) {
        this.chiledId = chiledId;
    }
    public String getChildName() {
        return childName;
    }
    public void setChildName(String childName) {
        this.childName = childName;
    }
}

```

//In MainActivity items are added dynamically.

```
    MainActivity.java:

   public class MainActivity extends Activity {
    /*our expandable adapter */
    ExpandableListAdapter expandableListAdapter;
    /*expandable list*/
    ExpandableListView expandableListView;
    /*list items*/
    ArrayList<Group> groups=new ArrayList<Group>();
    ArrayList<Boolean> isCheckedHeader;
    HashMap<String, ArrayList<Boolean>> IsCheckedChild;
        @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        isCheckedHeader=new ArrayList<Boolean>();
        IsCheckedChild=new HashMap<String, ArrayList<Boolean>>();
        /*genarate data for list view*/
        genarateData();
        /*instantiate adapter with our item list*/
        expandableListAdapter=new ExpandableListAdapter(this, groups,isCheckedHeader,IsCheckedChild);
        /*we get list view*/
        expandableListView=(ExpandableListView) findViewById(R.id.expandableListView);
        /*set adapter to list view*/
        expandableListView.setAdapter(expandableListAdapter);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }
    //Generate dummy data for list view
    public void genarateData() {
        Group group;

        for (int i = 0; i < 10; i++) {

            ArrayList<Child> childrens=new ArrayList<Child>();
            childrens.clear();
            Child child;

            for (int j = 0; j < 5; j++) {
                child=new Child(""+j, "Child "+j);
                childrens.add(child);   
            }
            group=new Group(""+i, " Group "+i, childrens);
            groups.add(group);
            isCheckedHeader.add(false);
        }
    }
}

```

Here is my logcat.

```
09-16 03:24:19.341: I/Process(1863): Sending signal. PID: 1863 SIG: 9
09-16 03:24:27.871: D/dalvikvm(1903): GC_FOR_ALLOC freed 59K, 5% free 2949K/3092K, paused 31ms, total 32ms
09-16 03:24:27.871: I/dalvikvm-heap(1903): Grow heap (frag case) to 3.562MB for 635812-byte allocation
09-16 03:24:27.921: D/dalvikvm(1903): GC_FOR_ALLOC freed 2K, 4% free 3567K/3716K, paused 40ms, total 40ms
09-16 03:24:28.441: D/AndroidRuntime(1903): Shutting down VM
09-16 03:24:28.441: W/dalvikvm(1903): threadid=1: thread exiting with uncaught exception (group=0xb3a12ba8)
09-16 03:24:28.451: E/AndroidRuntime(1903): FATAL EXCEPTION: main
09-16 03:24:28.451: E/AndroidRuntime(1903): Process: com.android.guide.expandablelistview, PID: 1903
09-16 03:24:28.451: E/AndroidRuntime(1903): java.lang.NullPointerException
09-16 03:24:28.451: E/AndroidRuntime(1903):     at com.android.guide.expandablelistview.ExpandableListAdapter.getGroupView(ExpandableListAdapter.java:117)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.ExpandableListConnector.getView(ExpandableListConnector.java:446)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.AbsListView.obtainView(AbsListView.java:2263)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.ListView.makeAndAddView(ListView.java:1790)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.ListView.fillDown(ListView.java:691)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.ListView.fillFromTop(ListView.java:752)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.ListView.layoutChildren(ListView.java:1630)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.AbsListView.onLayout(AbsListView.java:2091)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.View.layout(View.java:14817)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.ViewGroup.layout(ViewGroup.java:4631)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1525)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.LinearLayout.onLayout(LinearLayout.java:1434)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.View.layout(View.java:14817)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.ViewGroup.layout(ViewGroup.java:4631)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.View.layout(View.java:14817)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.ViewGroup.layout(ViewGroup.java:4631)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at com.android.internal.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:374)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.View.layout(View.java:14817)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.ViewGroup.layout(ViewGroup.java:4631)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.View.layout(View.java:14817)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.ViewGroup.layout(ViewGroup.java:4631)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:1987)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1744)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5670)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.Choreographer.doCallbacks(Choreographer.java:574)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.Choreographer.doFrame(Choreographer.java:544)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.os.Handler.handleCallback(Handler.java:733)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.os.Handler.dispatchMessage(Handler.java:95)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.os.Looper.loop(Looper.java:136)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at android.app.ActivityThread.main(ActivityThread.java:5017)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at java.lang.reflect.Method.invokeNative(Native Method)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at java.lang.reflect.Method.invoke(Method.java:515)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
09-16 03:24:28.451: E/AndroidRuntime(1903):     at dalvik.system.NativeStart.main(Native Method)
09-16 03:25:11.961: I/Process(1903): Sending signal. PID: 1903 SIG: 9

How can I correct this?
PS:I'm a beginner. Thanks :)

```