Useful
Java Stuff

Making a grid-based game?  Follow the instructions on this page.

Making a simple graphical project that is not based on a grid?  Download this starter code.

Making a project with lots of moving sprites, not based on a grid?  Start from the Sprite Lab.
(And here are some notes on inheritance.)

Making a project where the user will draw on the screen? Download this starter code.

Want a simple example where the computer will draw on the screen? Download this file.

Using 3-D graphics? Read these instructions.

Making a networking project? Download this starter code.
(Later, you can use this FakeServer.jar to test your client-side code
and this FakeClient.jar to test your server-side code.)

Looking for other useful stuff in Java?  Here's what you'll find on this page:

Main method
Random int
Exceptions
Convert between int and String
Test if a String represents an integer
User input
Wait for user to press enter
Load from a text file
Save to a text file
Wait
Time something
Threads
Exit the program
GUI (graphical user interface)
Pop-Up Dialogs
Custom Dialogs
Drawing
Draw Text
Draw an image
Rotate an image
Get pixel color from an image
Animation
Gravity:  jumping and falling
Mouse Input
Keyboard Input
Smooth movement with keyboard input
Sound
Networking:  Client
Networking:  Server
Executable JAR File

Main method

public static void main(String[] args)

{

}

Random int

int n = (int)(Math.random() * howManyPossibleValues) + lowestPossibleValue;

Exceptions

There are 2 kinds of exceptions in Java:  run-time and compile-time.  Run-time exceptions occur when a program crashes.  For example:

int n = 3 / 0;

This code compiles successfully, but crashes with an ArithmeticException.

You can use the throw instruction to force your program to crash.  (Forcing your program to crash immediately in the event of an error condition is much better than ignoring the error condition and allowing the program to continue running.)  This code will compile successfully, but will crash at run-time.

if (numTimes < 0)

  throw new RuntimeException("cannot be negative");

You can prevent your code from crashing by detecting when an exception occurs.

try

{

  doStuff();

  System.out.println("It worked!");

}

catch(SomeKindOfException e)

{

  System.out.println("It failed.");

}

Compile-time exceptions (such as IOException) must be handled by your program or your code won't even compile.  There are two ways to handle a compile-time exception.  One way is to catch it, as shown above.  The other is to declare that your method throws the exception.

public static void save() throws IOException

{ ... }

Of course, now whoever calls save will need to handle this exception.

Convert between int and String

String s = 3 + "";

int n = Integer.parseInt("3");

Test if a String represents an integer

try

{

  int n = Integer.parseInt(s);

  System.out.println("integer");

}

catch(NumberFormatException e)

{

  System.out.println("not an integer");

}

User input

import java.util.*;  // for Scanner

...

Scanner in = new Scanner(System.in);  // only once per program

System.out.print("Enter first name:  ");

String first = in.nextLine();

System.out.print("Enter last name:  ");

String last = in.nextLine();

Wait for user to press enter

new java.util.Scanner(System.in).nextLine();

Load from a text file

import java.io.*;  // for BufferedReader, FileReader

...

BufferedReader in = new BufferedReader(new FileReader("file.txt"));

String line = in.readLine();

while (line != null)

{

  System.out.println(line);

  line = in.readLine();

}

in.close();

Save to a text file

import java.io.*;  // for PrintWriter, FileWriter

...

PrintWriter out = new PrintWriter(new FileWriter("file.txt"), true);

out.println("first line");

out.println("second line");

out.close();

Wait

// before waiting

try { Thread.sleep(500); } catch(Exception e) { }

// half a second later

Time something

long start = System.currentTimeMillis();

...

long millisecondsElapsed = System.currentTimeMillis() - start;

Threads

This class extends Thread and overrides its run method to print forever.

public class MyThread extends Thread

{

  public void run()

  {

    while (true)

    System.out.println("running");

  }

}

The following code creates the thread and runs it, and then prints at the same time.

Thread t = new MyThread();

t.start();

while (true)

  System.out.println("at the same time");

Consider the following code segment, which appears in some method.

a();

b();

Here's a quick and dirty way to rewrite this code so that methods a and b run at the same time:

new Thread()

{

    public void run()

    {

        a();

    }

}.start();

b();

Exit the program

System.exit(0);

GUI (graphical user interface, pronounced "gooey")

import java.awt.event.*;  // for ActionListener, ActionEvent

import javax.swing.*;  // for JFrame, BoxLayout, JLabel, JTextField, JButton

public class GUI implements ActionListener

{

  private JTextField field;

  public GUI()

  {

    // make a window

    JFrame frame = new JFrame();

    frame.setTitle("My Window");

    // tell window to place each new component under the previous ones

    frame.getContentPane().setLayout(new BoxLayout(frame.getContentPane(), BoxLayout.PAGE_AXIS));

    // add some text

    frame.getContentPane().add(new JLabel("some text"));

    // add a text field

    field = new JTextField(25);

    frame.getContentPane().add(field);

    // add a button

    JButton button = new JButton("a button");

    frame.getContentPane().add(button);

    // tell button to call this object's actionPerformed method when pressed

    button.addActionListener(this);

    // store text in button that can be retrieved in actionPerformed method

    button.setActionCommand("button1");

    frame.pack();  // determine best size for window

    frame.setVisible(true);  // show the window

  }

  public void actionPerformed(ActionEvent e)

  {

    // button was pressed

    if (e.getActionCommand().equals("button1"))

    {

      // we now know which button was pressed

      System.out.println("text field contains:  " + field.getText());

    }

  }

}

Pop-Up Dialogs

You will need:

import javax.swing.*;  // for JOptionPane

Dialog with OK button:

JOptionPane.showMessageDialog(parentComponent, "Hard drive erased");

Dialog with text field:

String name = JOptionPane.showInputDialog(parentComponent, "Enter your name");

Dialog with Yes/No/OK/Cancel buttons:

int selected = JOptionPane.showConfirmDialog(parentComponent,

  "Are you sure?", "Confirm", JOptionPane.YES_NO_OPTION);

if (selected == JOptionPane.YES_OPTION)

  ...

Dialog with drop-down:

Object[] choices = {"blue", "green", "red", "yellow"};

Object selected = JOptionPane.showInputDialog(parentComponent, "What is your favorite color?",

  "Color", JOptionPane.PLAIN_MESSAGE, null, choices, choices[0]);

Custom Dialogs

Here is a complete example of a custom dialog.

public class SimpleDialog extends JDialog implements ActionListener

{

    private JTextField field;

    public SimpleDialog(Frame frame)

    {

        super(frame, true);

        field = new JTextField(25);

        field.addActionListener(this);

        getContentPane().add(field);

        pack();

        setVisible(true);

    }

    public void actionPerformed(ActionEvent e)

    {

        setVisible(false);

    }

    public String getResult()

    {

        return field.getText();

    }    

}

This following code segment shows how to use the custom dialog.

SimpleDialog dialog = new SimpleDialog(frame);

String result = dialog.getResult();

Drawing

import java.awt.*;  // for Graphics

import javax.swing.*;  // for JComponent, JFrame

public class Draw extends JComponent

{

  public Draw()

  {

    JFrame frame = new JFrame();

    setPreferredSize(new Dimension(400, 400));

    frame.getContentPane().add(this);

    frame.pack();

    frame.setVisible(true);

  }

  public void paintComponent(Graphics g)

  {

    g.setColor(Color.WHITE);

    g.fillRect(0, 0, getWidth(), getHeight());

    g.setColor(Color.RED);

    g.drawOval(100, 100, 200, 200);

  }

}

Draw Text

In the paintComponent method:

g.setColor(Color.BLUE);  // optional code to choose color before drawing text
g.setFont(new Font(null, Font.PLAIN, 24));  // optional code to change the font size
g.drawString("Hello", 10, 200);  // places the bottom-left of the string at position (10, 200).

Draw an image

You will need:

import java.awt.image.*;  // for BufferedImage

import java.net.*;  // for URL

import javax.imageio.*;  // for ImageIO

Create a fields for storing images:

private BufferedImage image;

Load images:

String fileName = "somefile.png";

URL url = getClass().getResource(fileName);

if (url == null)

  throw new RuntimeException("file not found:  " + fileName);

image = ImageIO.read(url);

In paintComponent method:

g.drawImage(image, x, y, null);  // to draw image in its original size

g.drawImage(image, x, y, width, height, null);  // to draw image in the given dimensions

Rotate an Image

Graphics2D g2 = (Graphics2D)g;  // g is a Graphics object

AffineTransform old = g2.getTransform();

g2.rotate(angleInRadians, centerX, centerY);  // rotate around center of image at (centerX, centerY)

g2.drawImage(image, leftX, topY, null);  // (leftX, topY) is top left of image (before rotating)

g2.setTransform(old);

Get pixel color from an image

Color c = new Color(image.getRGB(x, y));  // where image is a BufferedImage

Animation (automatic graphical changes not triggered by user input)

Create a class that extends JComponent.  Store state in fields:

private int x;

In paintComponent method, what you draw should depend on state:

g.fillOval(x, 100, 10, 10);

Write a loop to change state and redraw:

while (true)

{

  x++;  // change state

  repaint();  // ask JComponent to redraw when it has time

  try { Thread.sleep(100); } catch(Exception e) { }  // provide time to redraw

}

Gravity:  jumping and falling

Declare fields to keep track of both position and velocity:

private double y;

private double vely;

Repeatedly change position and velocity, and redraw:

while (true)

{

  y = y + vely;  // change y coordinate by amount of velocity

  vely = vely + 0.01;  // change velocity by constant acceleration due to gravity

  repaint();

  try { Thread.sleep(10); } catch(Exception e) { }

}

To begin falling, set velocity to 0.  To begin jumping, set velocity to a negative value:

vely = -10;

Mouse input

You will need:

import java.awt.event.*;  // for MouseListener, MouseEvent

Create a class that extends JComponent and implements MouseListener. In the constructor, tell the component to notify itself of mouse events:

addMouseListener(this);

Implement each of the methods in MouseListener, such as:

public void mousePressed(MouseEvent e)

{

  int x = e.getX();

  int y = e.getY();

  System.out.println("Pressed mouse button at:  " + x + ", " + y);

}

(If you want to be notified every time the mouse moves, then you will need to implement MouseMotionListener.)

Keyboard input

You will need:

import java.awt.event.*;  // for KeyListener, KeyEvent

Create a class that extends JComponent and implements KeyListener. In the constructor, tell the component to notify itself of keyboard events:

setFocusable(true);

addKeyListener(this);

Implement each of the methods in KeyListener, such as:

public void keyPressed(KeyEvent e)

{

  int key = e.getKeyCode();

  System.out.println("Pressed key with code:  " + key);

}

Smooth movement with keyboard input

Create a field to remember whether a key is down, and initialize it to false.

private boolean rightKeyDown;

Set the field to true when the key is pressed and false when the key is released.

public void keyPressed(KeyEvent e)

{

  int key = e.getKeyCode();

  if (key == 39)  //right arrow key

    rightKeyDown = true;

}

public void keyReleased(KeyEvent e)

{

  int key = e.getKeyCode();

  if (key == 39)  //right arrow key

    rightKeyDown = false;

}

Repeatedly change coordinate values when the field is true.

while (true)

{

  if (rightKeyDown)

    x++;

  repaint();

  try { Thread.sleep(10); } catch(Exception e) { }

}

Play Sound

import java.io.*;
import javax.sound.sampled.*;
...

//load sound
Clip clip = (Clip)AudioSystem.getLine(new Line.Info(Clip.class));
clip.open(AudioSystem.getAudioInputStream(new File("beep.wav")));

...

//play sound
clip.setFramePosition(0);
clip.start();

This code throws 3 compile-time exceptions: LineUnavailableException, IOException, UnsupportedAudioFileException.

Networking:  Client

import java.io.*;  // for BufferedReader, InputStreamReader, PrintWriter

import java.net.*;  // for Socket

...

Socket socket = new Socket("localhost", 9000); // connect to server on port 9000

BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));

PrintWriter out = new PrintWriter(socket.getOutputStream(), true);

out.println("lowercase");  // send message to server

String line = in.readLine();  // wait for message from server

System.out.println(line);  // print message from server

...

// disconnect from server

in.close();

out.close();

socket.close();

Networking:  Server

import java.io.*;  // for BufferedReader, InputStreamReader, PrintWriter

import java.net.*;  // for ServerSocket, Socket

...

ServerSocket server = new ServerSocket(9000);  // start server on port 9000

while (true)

{

  Socket socket = server.accept();  // wait for client to connect

  BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));

  PrintWriter out = new PrintWriter(socket.getOutputStream(), true);

  String line = in.readLine();  // wait for message from client

  out.println(line.toUpperCase());  // send message to client

  // disconnect from client

  in.close();

  out.close();

  socket.close();

}

Executable JAR File

From terminal/command, navigate to the root folder of your project, and write:

jar cvfe nameofjarfile.jar NameOfClassWithMainMethod *.class *.otherfileextensiontoinclude