How to get multiple selected items from a ListActivity in Android

Question

I am basically trying to get which items have been selected within a multiple choice enabled ListActivity. I am sure that there are many beginning Android programmers that would love this to be clarified. This app runs fine until it gets to the FOR loop and tries to assign the values of the multi-choice items to ArrayList 'items'. I'm simply trying to use Toast to see if the multi-choice items are being assigned to the Arraylist 'items'. The app crashes when the button is clicked. testing is done in android 2.2 emulator

Here is my Main.xml layout

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >

<TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/hello" />

<Button android:id="@+id/button"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="Selected Items" />    

<ListView android:id="@android:id/list" android:layout_width="fill_parent"
    android:layout_height="wrap_content" android:choiceMode="multipleChoice"
    android:textFilterEnabled="true" android:fastScrollEnabled="true" >     
</ListView>

</LinearLayout>

and here is the code

package com.tests.TestCode;

import android.app.ListActivity;
import android.os.Bundle;
import java.util.ArrayList;
import android.util.SparseBooleanArray;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.Toast;

public class MainActivity extends ListActivity {

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

   final String[] list = {"wrenches","hammers","drills","screwdrivers","saws","chisels","fasteners"};

   // Initializing An ArrayAdapter Object for the ListActivity
   final ArrayAdapter<String> adapter = new ArrayAdapter<String> (this, android.R.layout.simple_list_item_multiple_choice, list);
   setListAdapter(adapter);

   Button button = (Button) findViewById(R.id.button);
   button.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {

            ListView thelistview = (ListView) findViewById(android.R.id.list);
            ArrayList<String> items = null;

            SparseBooleanArray checkedItems = thelistview.getCheckedItemPositions();

            if (checkedItems != null) {

                for (int i=0; i<checkedItems.size(); i++) {
                    if (checkedItems.valueAt(i)) {
                        String item = adapter.getItem(checkedItems.keyAt(i)).toString();
                        items.add(item);
                    }
                }
                Toast.makeText(getBaseContext(), items.toString(), Toast.LENGTH_LONG).show();

            } else {
                Toast.makeText(getBaseContext(), "checkedItems == null", Toast.LENGTH_LONG).show();
            }
        }

     });        
}
}

Much obliged to anyone that can provide a good answer as to the best way to go about accomplishing this ListActivity scenario, or anyone that can explain why the app crashes at the point of assignment to the ArrayList. Thanks!

Answer 1

Your items list is null and you try to use it before initializing it.

You need to use items = new ArrayList<String>(); somewhere before trying to use it, or you will keep getting NullPointerExceptions.