space invaders game

In this and following tutorials you will learn about Applets, Threads, Graphics and a few other things. By the end of this tutorial you should have the skills to make basic games in Java. For this tutorial you will be making a simple ‘space invaders’ type game.

I assume that you have basic knowledge of Java as I wont be going into the basic details of how some things work.

First we will be starting by creating an applet and drawing a circle to the applet area.

1. Create a file called ‘Game.java’.
2. Open the file.

The next step is to import the necessary packages. For now we will only be requiring 2 packages:
import java.applet.*;
import java.awt.*;

Now that the importing has been taken care of we will need to set up the Java applet by the following:

public class Game extends Applet implements Runnable
{
}

This basically gives access to the Applet class and the ‘Runnable’ makes it so we can implement threads.

The variables come next as we wish to make these ones global:

Thread gameThread;
int width=400, height=400, MAX=1;
int currentX[] = new int[MAX];
int currentY[] = new int[MAX];

I have decided to use arrays for the X and Y cords now because they will be used at a later stage. It makes it easier to set it up now rather than changing it later.
Next comes the methods. I have included methods that are currently not used at this stage but they are used later.

Start() is used for starting a new thread for the class.

public void start()
{
Thread gameThread = new Thread(this);
gameThread.start();
}


init() is used for setting the initial values

public void init()
{
currentX[0]=0;
currentY[0]=0;
}

run() is the main method we will use later. It is initialised after a new thread is started.

public void run()
{
}


paint() calls update().

public void paint(Graphics g)
{
update(g);
}

update () is where all the actual drawing is done.

public void update(Graphics g)
{
Graphics2D g2 = (Graphics2D)g;

// Set the background color.
g2.setBackground(Color.black);

// Clear the applet.
g2.clearRect(0, 0, width, height);

// Set the drawing color to green.
g2.setColor(Color.green);

//(X pos, Y pos, Width, Height)
g2.fillOval(currentX[0], currentY[0], 20,20);
}


* Note this is an applet which means you must run it from a HTML file. The HTML code to run this applet is as follows and I will use the same code throughout this series of tutorials.

Set Column name

File: Professor.java


import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
//@Table(name="EMP", schema="HR")
@Table(name="EMP")
public class Professor {
@Id
@Column(name = "EMP_ID")
private int id;
@Column(name = "COMM")
private String name;
@Column(name = "SAL")
private long salary;

public Professor() {
}

public Professor(int id) {
this.id = id;
}

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public long getSalary() {
return salary;
}

public void setSalary(long salary) {
this.salary = salary;
}

public String toString() {
return "Professor id: " + getId() + " name: " + getName() + " salary: " + getSalary();
}
}


File: ProfessorService.java

import java.util.Collection;

import javax.persistence.EntityManager;
import javax.persistence.Query;

public class ProfessorService {
protected EntityManager em;

public ProfessorService(EntityManager em) {
this.em = em;
}

public Professor createProfessor(int id, String name, long salary) {
Professor emp = new Professor(id);
emp.setName(name);
emp.setSalary(salary);
em.persist(emp);
return emp;
}

public void removeProfessor(int id) {
Professor emp = findProfessor(id);
if (emp != null) {
em.remove(emp);
}
}

public Professor raiseProfessorSalary(int id, long raise) {
Professor emp = em.find(Professor.class, id);
if (emp != null) {
emp.setSalary(emp.getSalary() + raise);
}
return emp;
}

public Professor findProfessor(int id) {
return em.find(Professor.class, id);
}

public Collection findAllProfessors() {
Query query = em.createQuery("SELECT e FROM Professor e");
return (Collection) query.getResultList();
}
}


File: JPAUtil.java

import java.io.Reader;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.Statement;

public class JPAUtil {
Statement st;

public JPAUtil() throws Exception{
Class.forName("org.hsqldb.jdbcDriver");
System.out.println("Driver Loaded.");
String url = "jdbc:hsqldb:data/tutorial";

Connection conn = DriverManager.getConnection(url, "sa", "");
System.out.println("Got Connection.");
st = conn.createStatement();
}
public void executeSQLCommand(String sql) throws Exception {
st.executeUpdate(sql);
}
public void checkData(String sql) throws Exception {
ResultSet rs = st.executeQuery(sql);
ResultSetMetaData metadata = rs.getMetaData();

for (int i = 0; i < metadata.getColumnCount(); i++) { System.out.print("\t"+ metadata.getColumnLabel(i + 1)); } System.out.println("\n----------------------------------"); while (rs.next()) { for (int i = 0; i < metadata.getColumnCount(); i++) { Object value = rs.getObject(i + 1); if (value == null) { System.out.print("\t "); } else { System.out.print("\t"+value.toString().trim()); } } System.out.println(""); } } } File: Main.java import javax.persistence.EntityManager; import javax.persistence.EntityManagerFactory; import javax.persistence.Persistence; public class Main { public static void main(String[] a) throws Exception { JPAUtil util = new JPAUtil(); EntityManagerFactory emf = Persistence.createEntityManagerFactory("ProfessorService"); EntityManager em = emf.createEntityManager(); ProfessorService service = new ProfessorService(em); em.getTransaction().begin(); Professor emp = service.createProfessor(158, "AAA", 45000); em.getTransaction().commit(); System.out.println("Persisted " + emp); util.checkData("select * from EMP"); // remove an employee em.getTransaction().begin(); service.removeProfessor(158); em.getTransaction().commit(); System.out.println("Removed Professor 158"); util.checkData("select * from EMP"); em.close(); emf.close();
}
}
File: persistence.xml