Java Data Structures

There are a number of commonly used Java data structures. Here are links to additional documentation and highlights of structural aspects: 

  • Array: fixed size, best for primitives 
  • List: fixed size, interface, ordered, allows duplicate
  • ArrayList: resizable , add to top, preferred unless need the capabilities of another structure
  • Vector: resizable, synchronized, add to top
  • Stack: LIFO, push/pop
  • Queue: FIFO, add to bottom/remove from top, interface
  • LinkedList: doubly-linked, push/pop, many other methods
  • Set: no duplicates, interface
  • HashMap: random access
  • TreeMap: sorted, efficient search
  • Binary Tree: each node has <=2 children
  • Binary Search Tree: sorted, fast lookup/add/remove
  • k-d Tree: k dimensional, space-partitioning, nearest neighbor search
  • Red-Black Tree: self-balancing, binary search tree, extra bit per node
  • Iterator: move forward and backward through data
  • Collections: perform operations on collections of data
  • Working with Java Lists: methods, iterators, collections
  • Data Structure Decisions: change, access, indexing, sizing
  • Common Java Array Manipulation Constructs: split, sort, binary search, iteration
  • Common Java ArrayList Manipulation Constructs: add, indexOf, remove, sort, binary search, interator
  • B-tree and Binary Search Tree Data Structures: like TreeMap
  • Sorting Algorithms: Collections sort, exchange, hybrid, insertion, merge
  • Big O Notation: O(1), O(log (n)), O(n), O(n log(n))
  • Update and Access Overview: The diagram below indicates how elements and values are updated and accessed. This is a broad overview of commonly used data structure classes, interfaces and objects ... please see the links above and below for more details. Some general observations are:
    • Arrays are different than the other structures in that they are accessed using bracketed index [n] references, such as myStringArray[3] instead of methods and can only contain primitives, not objects.
    • ArrayLists are generally preferred over Lists as they allow resizing. It's a very commonly used structure.
    • Vectors are much like ArrayLists with the additional characteristic that they are synchronized.
    • Queues and Stacks are specialized for LIFO and FIFO access.
    • LinkedLists are powerful structures that require more space for storage and time to execute some operations but are efficient for inserting and deleting objects in the existing order.
    • Sets are simple structures that don't allow duplicates.
    • HashMaps are commonly used when rapid, direct access to elements is the primary requirement.
    • TreeMaps are preferred when rapid search is the primary requirement.
    • Iterators and Collections operate on whole data structures.
    • ( ) empty parameter methods generally operate on the ends of a structure.
    • (element | object) only parameter methods generally operate on sets or the ends of structures.
    • (index | key) and (index | key, element | object) parameter methods generally operate on specific places within a structure.

Working with Java Lists

As shown in the diagram below, there are three major Java class groups used for working with lists: Lists (such as List, ArrayList, LinkedList, AbstractList, Stack and Vector), Iterators (Iterator and ListIterator) and Collections. Together, these classes and their methods provide a wide spectrum options for list construction, examination and manipulation. For an example of using these classes, look here.

Common Java Array Manipulation Constructs

Working with array data structures is one of the most frequently used programming functions. Arrays are implemented in a number of Java classes, such as String, Integer, and Boolean. Those arrays can be manipulated using the Java Arrays class (note that the Arrays class name is a plural). Below are some common Java array manipulation constructs.

split( )

The String split() method separates string characters into array entries using a supplied regular expression. For example, the following statement would create entries in the String array wordsArray using a blank character as a regular expression separator for processing the String wordString:

String wordString = "this is a string of words";
String[] wordsArray = wordString.split(" ");

sort( )

The Arrays sort() methods provide a variety of options for sorting array entries. Note that the array is sorted in place without returning a new array. This can be done because the array parameter is a value pointing to the array of objects. For example, the following statement would sort the words array from the split() example above using a Quicksort process in an average O(n log(n)) time performance:


binarySearch( )

The Arrays binarySearch() methods provide an efficient way to find an entry in a sorted array. For example, the following statement would find the index of the array wordsArray entry for the String entryValue in an average of O(log(n)) time performance:

int entryIndex = Arrays.binarySearch(wordsArray, entryValue);

for( )

The for() construct can be used to iterate over the entries in an array. For example, the following statement would iterate over and print every entry in wordsArray:

for (String word : wordsArray) {System.out.print(word);}

If you want to know and use the index of each entry, then this construct can be used:

for (int i=0; i<wordsArray.length; i++) {System.out.print(wordsArray(i);}

Android Data View Adapters Overview

Android APIs provide classes and methods to facilitate displaying data in various forms, such as lists, arrays and custom layouts. The graphic below shows how the various classes and methods interact to produce the desired result.

Some examples of coding and use are:


The data to be displayed can be in such forms as an Array, List, File or SQLite database. If a database is used, a Cursor object is also employed. A simple array of String values might be created and converted to an ArrayList like this:

String[] mStringArray = new String[] { "blue", "green", "yellow",
        "red", "orange", "black", "white", "purple"};
List<String> mList = Arrays.asList(mStringArray);
ArrayList<String> mArrayList = new ArrayList<String>(mList);


Cursor is an interface that provides random read-write access to the result set returned by a database query. Setting up a Cursor for an SQLite database might look like this:

MySQLiteOpenHelper mDatabaseHelper = new MySQLiteOpenHelper(this);
Cursor mCursor = mDatabaseHelper.getDataIntoCursor();
ListView mListView = (ListView) findViewById(;
MyCursorAdapter mCursorAdapter = new MyCursorAdapter(this, mCursor);

class MyCursorAdapter extends CursorAdapter {

    public MyCursorAdapter(Context context, Cursor c) {
        super(context, c);
    // Implement CursorAdapter bindView method.
    public void bindView(View view, Context context, Cursor cursor) {
        String name = cursor.getString(1);
        TextView textView = (TextView) view;
    // Implement CursorAdapter newView method.
    public View newView(Context context, 
                         Cursor cursor, ViewGroup parent) {
        TextView view = new TextView(context);
        return view;
private final class MySQLiteOpenHelper extends SQLiteOpenHelper {

    private static final String DATABASE_NAME = "myDatabase";
    private static final int DATABASE_VERSION = 1;
    private static final String CREATE_TABLE_TIMELINE = 
        "CREATE TABLE IF NOT EXISTS table_name 
        (_id INTEGER PRIMARY KEY AUTOINCREMENT, name varchar);";

    public DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("INSERT INTO ddd (name) VALUES ('Name1')");
        db.execSQL("INSERT INTO ddd (name) VALUES ('Name2')");
        db.execSQL("INSERT INTO ddd (name) VALUES ('Name3')");
    public void onUpgrade(SQLiteDatabase db, 
                          int oldVersion, int newVersion) {
    public Cursor getDataIntoCursor() {
        String selectQuery = "SELECT  * FROM table_name;
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);
        return cursor;


An Adapter acts as a bridge between an AdapterView and the underlying data for that view. A custom array adapter defined in Java might look like this:

class MyArrayAdapter extends ArrayAdapter<String> {
    HashMap<String, Integer> mHashMap = new HashMap<String, Integer>();
    public MyArrayAdapter(Context context, int textViewResourceId,
        List<String> objects) {
      super(context, textViewResourceId, objects);
      for (int i = 0; i < objects.size(); ++i) {
        mHashMap.put(objects.get(i), i);
    public long getItemId(int position) {
      String item = getItem(position);
      return mHashMap.get(item);
    public boolean hasStableIds() {
      return true;

The custom array adapter mArrayAdapter would be instantiated and linked to a ListView mListView with Java code like this:

final ListView mListView = (ListView) findViewById(;
final MyArrayAdapter mArrayAdapter = new MyArrayAdapter(this,
        android.R.layout.simple_list_item_1, mArrayList);


An AdapterView is a view whose children are determined by an Adapter. You might use an AdapterView like this:

private OnItemClickListener mMessageClickedHandler = 
  new OnItemClickListener() {
    public void onItemClick(AdapterView parent, 
                            View v, int position, long id) {
        // Do something in response to the click


View is the basic building block for user interface components. A view occupies a rectangular area on the screen and is responsible for drawing and event handling. A ListView contained in a layout might look like this:

<LinearLayout xmlns:android=""
    android:orientation="vertical" >
        android:layout_height="wrap_content" >


ViewGroup is a special view that can contain other views called children. A ViewGroup defined in XML might look like this:

<ViewGroup xmlns:android=""
    android:layout_width="fill_parent" >


A layout defines the visual structure for a user interface. A layout might look like this:

<LinearLayout xmlns:android=""
    android:orientation="vertical" >
        android:layout_height="wrap_content" >


An Activity provides a screen with which users can interact.

public class MyActivity extends Activity {
  protected void onCreate(Bundle savedInstanceState) {
    . . .


A Fragment represents a behavior or a portion of user interface an an Activity. Fragment code might look like this:

public class ArticleFragment extends Fragment {
    public View onCreateView(LayoutInflater inflater, 
                             ViewGroup container,
        Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.article_view, 
                                container, false);

Android Job Interview Topics

Because Android contains an extremely large number of Classes and Methods, it's impossible to hold all of the syntax and details in your head. For this reason, I've found that Android interview questions tend to be more conceptual in nature. For example, you might be asked how you'd approach a given problem using Android and Java constructs.

As you study the commonly used interview topics below, try to retain the main ideas and some relevant details. 

Android Fragments Overview

Android Fragments provide a mechanism to create modular sections of an Activity. You can see how Fragments fit into the overall Android structure in the blog post Android App Major Elements. You can see an example of Fragment use in the blog post Android Fragments.

Fragments provide a couple of significant benefits for Apps:

  • Fragments facilitate App adaptation to various screen sizes and dimensions. 
  • Fragments have their own life cycle that can be used to implement unique behavior for different App Fragments.

Below is an overview of Fragments and the Android elements they interact with for Fragment control.

Activity (Main)

The main Activity of an app uses the setContentView() method to establish a View.

Activity (for Fragment)

Activities can start and be started by Fragments.


The View class represents the basic building block for user interface components. A View occupies a rectangular area on the screen and is responsible for drawing and event handling. View is the base class for widgets, which are used to create interactive UI components (buttons, text fields, etc.). Views can be accessed via their ids or from the Root View.

Root View

The Root View is the base of the tree for all the Views for an app. It can be referenced using  

Layout (XML)

Layout XML can be used to define Views. When an app is compiled, the XML definitions are used to create View details. Note that this can include Fragment properties. If a Layout that includes a Fragment specification is used by the Activity setContentView() method, that Fragment will be automatically started by the Android operating system.


A Fragment is a piece of an application's user interface or behavior that can be placed in an Activity. Sub classes of Fragment include: PreferenceFragment, DialogFragment and ListFragment.

Fragment Manager

The FragmentManager class is an Interface for interacting with Fragment objects.

Fragment Transaction

The FragmentTransaction class is used for performing Fragment operations.

Android App Major Elements

There are many elements that can be included in an Android app. It's helpful to see how these are clustered and interact, as shown in the chart below. Only major elements are shown. The Android Developers Website provides details on the full set of app elements.

The minimum required elements are shown with a red dot. An app with just these elements wouldn't be very useful, but you can see that there are many optional elements that can be added for a variety of functions. 

Intent Messaging

Intents are used to request an action from an app component. Intents are detected and acted upon by Broadcast Receivers.


Activities manage the Displays that app users see. Fragments are modular sections of an Activity that, among other uses, facilitate app adaptation to various screen sizes and dimensions.

User Interface

The User Interface includes a variety of pre-built elements that facilitate constructing screen displays, including: View, Control, Dialog, Notification, Setting, Action Bar, Menu and Toast.


Services perform long-running operations in the background and do not provide a User Interface.

Asynchronous Operations

These elements facilitate asynchronous operations that protect the UI thread from undesirable delays. Loaders make it easy to asynchronously load data in an Activity or Fragment. AsyncTask allows performing background operations and publishing results on the UI thread without having to manipulate threads and/or handlers.

Data, Devices and Sensors

These elements facilitate the access to Data, Devices and Sensors. Connectivity provides access to devices and the Internet.  Content Providers can optionally be used to access databases and internal/external files. Sensors include: GPS, network location, accelerometer, gyroscope and more. 

Reference Elements

These elements are essential for app operation. Libraries provide access to compiled code from Google and other sources. Access to the user Display is built into the operating system. The Manifest include essential information about the app and its elements. Resources include: Layouts, Values and Drawables. Shared Preferences are use to store key-value pairs that act as global variables for the app.

Java final, abstract and static Modifiers

The meaning of the final, abstract and static Java modifiers can be confusing and hard to remember. Here's a brief summary:


Generally means "can't be changed".

  • final class: can't be extended (sub-classed)
  • final method: can't be overridden or hidden by sub-classes
  • final variable: can only be initialized once 


Generally means "can't be instantiated".

  • abstract class: can't be instantiated, can be extended, must be declared abstract if it contains one or more abstract methods
  • abstract method: has only a declaration with no body
  • abstract variable: not allowed


Generally means "not associated with an instance".

  • static class: only nested inner classes can be static, they are not associated with any instance of the enclosing class
  • static method: doesn't use instance variables
  • static variable: is the same across all instances of it's containing class

additional notes

  • constants: use static final as variable modifiers
  • abstract static methods are not allowed

Android Listeners and Toasts

This video, part of my Android Quick Code Access series of posts, explains Android Listeners and Toasts. It's from my course Learning Android App Programming published by InfiniteSkills.

The Java code for the example is below...

import java.util.Random;
import android.content.Context;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;

public class ListenerToastMain extends Activity {
    // onCreate method.
	protected void onCreate(Bundle savedInstanceState) {
	    // Initialize display.
           // Declare button variable for Get Answer.
	    Button getAnswer =
	    // Set listener for button.
    // Define OnClickListener for getAnswer button.
    private OnClickListener getAnswerListener = new OnClickListener() {
        public void onClick(View v){
            // Generate answer number.
            Random toastGen = new Random();
            int toastNum = toastGen.nextInt(19);
            // Get answer text.
            CharSequence text = "";
            switch (toastNum) {
                case 1:  text = getString(R.string.answer1);  break;
                case 2:  text = getString(R.string.answer2);  break;
                case 3:  text = getString(R.string.answer3);  break;
                case 4:  text = getString(R.string.answer4);  break;
                case 5:  text = getString(R.string.answer5);  break;
                case 6:  text = getString(R.string.answer6);  break;
                case 7:  text = getString(R.string.answer7);  break;
                case 8:  text = getString(R.string.answer8);  break; 
                case 9:  text = getString(R.string.answer9);  break;
                case 10: text = getString(R.string.answer10); break;
                case 11: text = getString(R.string.answer11); break;
                case 12: text = getString(R.string.answer12); break;
                case 13: text = getString(R.string.answer13); break;
                case 14: text = getString(R.string.answer14); break;
                case 15: text = getString(R.string.answer15); break;
                case 16: text = getString(R.string.answer16); break;
                case 17: text = getString(R.string.answer17); break;
                case 18: text = getString(R.string.answer18); break;
                case 19: text = getString(R.string.answer19); break;
                case 20: text = getString(R.string.answer20); break;       		    
            // Toast answer.
            Context context = getApplicationContext();
            int duration = Toast.LENGTH_LONG;
            Toast toast = Toast.makeText(context, text, duration);

Android Broadcast Receiver Concepts and Sample App

This video, part of my Android Quick Code Access series of posts, explains Android Broadcast Receivers.  It's from my course Learning Android App Programming published by InfiniteSkills.

Here's the Java code for the example...

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.Bundle;
import android.widget.Toast;

// Declare main activity for the application.
public class Main extends Activity {

    // Instantiate BroadcastReceiver.
    private BroadcastReceiver receiver = new BroadcastReceiver(){

        // Define actions to be taken when broadcast is received.
        public void onReceive(Context c, Intent i) {
            // Do the work of the BroadcastReceiver.
            // In this example, a message is toasted to the user.
                           "ACTION_TIME_TICK intent received.", 
    // Initialize the user interface in the onCreate method
    // of the main activity.
    public void onCreate(Bundle savedInstanceState) {  
    // Register the receiver with its filter.
    protected void onResume() {  
        this.registerReceiver(receiver, filter);  
    // Unregister the receiver when the application is paused.
    protected void onPause() {  

Android Database Query Display

This is part of the Android Quick Code Access series of posts. The code below displays the results of a Database Query. You'll need to run the Query code in a background thread in your production version.

// Database Display Activity.
public class DatabaseDisplayActivity extends ListActivity {

   // Database display columns.
   private static final String[] PROJECTION = new String[] {
    	COLUMN_NAME_1,   // 0 Position
   // Activity onCreate method.
   protected void onCreate(Bundle savedInstanceState) {
       // Database Query. 
       Cursor cursor = managedQuery(
       // Cursor columns to display.
       String[] dataColumns = {     		
       } ;
       // View IDs for formatting.
       int[] viewIDs = { 	
       // Adapter for the ListView.
       SimpleCursorAdapter adapter = new SimpleCursorAdapter(       		
       // Binder to display row results.
       adapter.setViewBinder(new SimpleCursorAdapter.ViewBinder() {
           public boolean setViewValue(
                View view, Cursor cursor, int columnIndex) {
                // Get row of cursor.
                String displayText = 

                // Set text in display.
                ((TextView) view).setText(displayText);
                return true;
       // List Adapter.

Android Eclipse Waiting for Debugger

This is part of the Android Quick Code Access series of posts. Depending on the specifics of your Android code, you may need to add code to force a wait for the Eclipse debugger. This is especially true for asynchronous code functions like background threads. Without the code below, you may see Eclipse break points not being triggered. You might think that your code is broken, when in fact you just need to add a code line to force it to wait for the debugger to catch up.


Android Allowing Main Thread I/O

An important aspect of Android App design is to never block the main UI thread with long running tasks. Doing so can result in an Application Not Responding (ANR) condition that can cause your App to be stopped. To prevent ANR conditions, always perform long running tasks such as I/O in background threads.

If you need to test I/O access from the main UI thread, you'll need to change the Thread Policy as shown below. Only do this for testing. In your production App, make sure you perform I/O operation in a background thread.

StrictMode.ThreadPolicy policy = 
    new  StrictMode.ThreadPolicy.Builder().permitAll().build(); 

Android Image Button Listener

This is part of the Android Quick Code Access series of posts. For more information on handling Buttons, see the Android Button class.

// Listener for Button.
ImageButton imageButton = (ImageButton) findViewById(;
imageButton.setOnClickListener(new OnClickListener() {
    // Button onClick method.
    public void onClick(View v) {
        // Take action here.
         . . .


Android Content Provider/SQLite Methods

There are a number of key methods for accessing data via an Android Content Provider or directly on the underlying SQLite database.  The table below summarizes the method parameters and return values. The graphic is from the video training course Learning Android App Programming.

Android Release and Device Support Summary

Below is a summary of Android releases and device support.  It shows that by using the  Android Support Libraries you can code your apps to run on a large percentage of Android devices.

For more information on the compatibility Support Libraries see: 

For more information on Device Support see:

Android WebView Combines Native and HTML5 Code

A battle is raging over whether native mobile code apps, such as those for Android and iOS, are a better approach than using HTML5.  Each has its advantages.

There is, though, a way to blend the two in order to take advantages of the benefits each has to offer.  The graphic below shows the anatomy of an app that displays an HTML5 Canvas on an Android screen display.

This app is from the Learning Android App Programming video training course.

The HTML5 Canvas animation displayed is from HTML5 Canvas for Dummies.

You can see the HTML5 Canvas animation by clicking here. To see the JavaScript code that generates the animation, right click on the display page and select View Source Code.

Android Bound Services Using Inter Process Communications

Android Service components can be implemented using a number of mechanisms.  If you want your Service to be able to communicate with other applications running in separate processes, using Inter Process Communications (IPC) is one approach.

The diagram below shows the major classes and methods needed in the binding activity and bound service.  The details of this implementation can be found on the Android Developers Web site here on the Bound Services page

In summary, the major classes involved are:

  • Service - Application component for longer-running operations that do not interface directly with the user.
  • ServiceConnection - Interface for monitoring a service.
  • Messenger - Creates a Messenger pointing to a Handler in one process, and handing that Messenger to another process.
  • Message - Description and arbitrary data object that can be sent to a Handler.
  • IBinder - Interface for a remotable object.
  • Handler - allows you to send and process Message and Runnable objects associated with a thread's MessageQueue.