public class Contact implements ContactListener {
#Override
public void beginContact(Contact contact) {
Fixture fa = contact.getFixtureA();
Fixture fb = contact.getFixtureB();
if (fa.getFilterData().categoryBits==16){
((Gamescreen)fa.getUserData()).starttouch(fa,fb);
}
#Override
public void endContact(Contact contact) {
Fixture fa = contact.getFixtureA();
Fixture fb = contact.getFixtureB();
if (fa.getFilterData().categoryBits==16)
{
((Gamescreen)fa.getUserData()).endtouch();
}
This code works fine when there is just one object to touch but some time i need to make like tow object beside of each others.
Like when the player walk on 2 objects (without jumping) beside each others the second method (endcontact) called but the first method (begincontact) does not get call again.
If I understood correctly, then this link may be what you're looking for:
http://www.iforce2d.net/b2dtut/collision-callbacks
It's C++, but you can see example implementation of collision callback for a pair of objects:
void BeginContact(b2Contact* contact) {
//check if fixture A was a ball
void* bodyUserData = contact->GetFixtureA()->GetBody()->GetUserData();
if ( bodyUserData )
static_cast<Ball*>( bodyUserData )->startContact();
//check if fixture B was a ball
bodyUserData = contact->GetFixtureB()->GetBody()->GetUserData();
if ( bodyUserData )
static_cast<Ball*>( bodyUserData )->startContact();
}
Additionally, you don't save true/false if contact started, but number of contacts:
//new implementation for contact state change
void startContact() { m_numContacts++; }
void endContact() { m_numContacts--; }
I've been working on a cell evolution simulation in Java. Just so everyone knows, I'm a beginner/intermediate Java programmer. I know pretty much all the basics and then a little bit, but I don't quite have the skills to write code from scratch. The code I have here is roughly based on a source I found online, I added my own touches and some other pieces I found online as well. It seems to work just fine, except the screen flickers. It seems every time the repaint() is called it flickers, probably clearing and redrawing. It creates something that is practically impossible to look at. There are no errors in my code. I am new to using applets, so if there is a better way to do this, please let me know. How can I stop the screen from flickering? Is there an easy way to buffer the images to prevent that? Here's the class that draws the applet
/* <!-- Defines the applet element used by the appletviewer. -->
<applet code='CellLife.java' width='1920' height='1080'></applet> */
import java.applet.Applet;
import java.awt.Event;
import java.awt.Graphics;
import java.util.Enumeration;
import java.util.Vector;
public class CellLife extends Applet implements Runnable {
// ========================================================================
// VARIABLES
// ========================================================================
// Data
/** Thread object for CellLife applet */
private Thread m_cellLife = null;
// Static constants
/**
* the maximum number of creatures in the world. When the number of
* creatures alive drops below half this, a new one is created to bring the
* numbers back up.
*/
protected static final int MAX_CREATURES = 60;
// Data
/**
* A list of the creatures currently alive. Stores CLCreature references.
*/
protected Vector creatures;
/** The world is a rectangle from (0,0) to (limit.x,limit,y) */
protected CL2dVector limit;
/**
* The number of creatures that have been born since the simulation started
*/
protected long generations;
/** A test creature controllable by the user to allow response testing */
private CLCreature testCreature;
/** space-partitioning structure to speed collision detection */
protected CLBuckets buckets;
// ========================================================================
// METHODS
// ========================================================================
public CellLife() {
creatures = new Vector();
limit = new CL2dVector(500.0F, 500.0F);
generations = 0;
// initilaize our bucket structure
float bucketScale = CLCell.RADIUS; // could stretch to root-two times
// this
buckets = new CLBuckets(bucketScale, (int) Math.ceil(limit.x / bucketScale), (int) Math.ceil(limit.y / bucketScale));
}
public String getAppletInfo() {
return "Name: Cell Evolution\r\n" + "Author: Josh Marchand\r\n" + "Made in Eclipse";
}
// first time initialazion
public void init() {
resize((int) limit.x, (int) limit.y);
for (int i = 0; i < MAX_CREATURES; i++) {
CLCreature new_creature = new CLCreature();
new_creature.InitSimple(limit, buckets);
creatures.addElement(new_creature);
}
}
public void destroy() {
// TODO: Place applet cleanup code here
}
public void paint(Graphics g) {
g.drawString("No. creatures: " + creatures.size(), 0, 11);
g.drawString("Births: " + generations, 0, 22);
// draw cells
for (int i = 0; i < creatures.size(); i++) {
((CLCreature) creatures.elementAt(i)).Draw(g);
}
// DEBUG: also indicate the contents of the buckets
// buckets.Draw(g);
// get all creatures to do their stuff
CLCreature creature;
for (int i = 0; i < creatures.size(); i++) {
creature = (CLCreature) creatures.elementAt(i);
if (creature.DoTimeStep(g, buckets, limit) && creatures.size() < MAX_CREATURES) {
// inherit new creature from current
CLCreature newCreature = new CLCreature();
newCreature.InheritFrom(creature, buckets, limit);
creatures.addElement(newCreature);
generations++;
}
}
// delete the ones that died doing it
for (Enumeration e = creatures.elements(); e.hasMoreElements();) {
creature = (CLCreature) e.nextElement();
if (creature.hasDied) creatures.removeElement(creature);
}
// breed nwe creatures if pop. is low
if (creatures.size() < MAX_CREATURES / 2) {
// just add one for now,fix later
CLCreature newCreature = new CLCreature();
newCreature.InheritFrom((CLCreature) creatures.elementAt((int) Math.random() * creatures.size()), buckets, limit);
creatures.addElement(newCreature);
generations++;
}
}
public void start() {
if (m_cellLife == null) {
m_cellLife = new Thread(this);
m_cellLife.start();
}
// TODO: place any additional startup code here
}
public void stop() {
if (m_cellLife != null) {
m_cellLife.stop();
m_cellLife = null;
}
}
public void run() {
while (true) {
try {
repaint();
// quick nap here to allow user interface to catch up
Thread.sleep(100);
} catch (InterruptedException e) {
stop();
}
}
}
public boolean mouseDown(Event e, int x, int y) {
// create a single celled creature at specific loc
testCreature = new CLCreature();
testCreature.rootCell.location.x = x;
testCreature.rootCell.location.y = y;
testCreature.rootCell.type = CLGene.RED;
creatures.addElement(testCreature);
buckets.PutCell(testCreature.rootCell);
return true;
}
public boolean mouseDrag(Event e, int x, int y) {
testCreature.rootCell.location.x = x;
testCreature.rootCell.location.y = y;
return true;
}
public boolean mouseUp(Event evt, int x, int y) {
creatures.removeElement(testCreature);
buckets.RemoveCell(testCreature.rootCell);
return true;
}
}
Thank you all so much for the help, and I'm very sorry about my "noobiness", I am doing my best to teach myself!
I would consider using technique called double buffering, where you create an offscreen Graphics object bound to and Image, perform all the drawing on that then paint the result to screen.
You can find a handy tutorial on creating graphics from image here. More complete sample can be found here.
So I've read about the pass-by-value nature of Java and I've tried to change my variable after passing it to a function by having the function return the variable again. I didn't succeed in that.
My code
public class Logic {
private int position;
public class Logic(){
position = 1;
}
public void appendPosition(){
position = calculatePosition(position);
}
}
This is the barebones code. I call this method from an instance of Logic which is instantiated in another class:
public class MainLogic {
ILogic L;
public MainLogic(ILogic L){
this.L = L;
}
public void start(){
L.appendPosition();
}
}
Through repeated debugging I find to my dismay that the position variable does not change at all. The position variable passed to calculatePosition changes fine, as expected. What am I missing? I've tried making the variable public and static.
calculatePosition
private int calculatePosition(int position){
position += 6;
if(snakeLocations[position]>0) {
position -= 6;
}
else if(ladderLocations[position]>0) {
position += 6;
}
return position;
}
private final int[] snakeLocations = new int[] {0,0,0,0,0,0,0,0,0,9,0,0,0,13,0,0,0,0,0,19,0,0,0,0,0};
private final int[] ladderLocations = new int[] {0,0,0,0,0,0,0,0,0,0,0,11,0,0,0,15,0,0,0,0,20,0,0,0,0};
public class Logic {
private int position;
public Logic(){
position = 1;
}
//to get position.....................
public int getPosition(){
return this.position;
}
public void appendPosition(){
position = calculatePosition(position);
}
private int calculatePosition(int position){
position += 6;
if(snakeLocations[position]>0) {
position -= 6;
}
else if(ladderLocations[position]>0) {
position += 6;
}
return position;
}
private final int[] snakeLocations =
new int[] {0,0,0,0,0,0,0,0,0,9,0,0,0,13,0,0,0,0,0,19,0,0,0,0,0};
private final int[] ladderLocations =
new int[] {0,0,0,0,0,0,0,0,0,0,0,11,0,0,0,15,0,0,0,0,20,0,0,0,0};
}
//next class
public class MainLogic {
Logic L;
public MainLogic(Logic L){
this.L = L;
}
public void start(){
L.appendPosition();
}
public static void main(String[] args) {
Logic L = new Logic();
MainLogic ml = new MainLogic(L);
System.out.println(ml.L.getPosition());
ml.start();
System.out.println(ml.L.getPosition());
}
}
Let's read your code.
// There is a logic class.
public class Logic {
// So Logic has a position. it starts with zero.
private int position;
// Logic also has an inner class, also called Logic.
public class Logic(){
position = 1;
// When a new Logic().Logic() is created, the instance
// variable of the parent instance gets set to one. WHAT?!?
}
// ... lots of code
}
So, It's a bit like that Yo Dawg! meme - I heard you like Logic, so we've put A logic instance into your Logic instance so you can Logic while your Logic.
You probably want a constructor.
// So this is Logic.
public class Logic {
private int position;
// When an Logic instance is created, position starts with 1.
public Logic(){
this.position = 1;
}
Ok. So we're not talking about Yo Dawg Memes.
So Let's talk pass by value.
Pass-by-value means classes are like very egotistic children: You can't play with their toys, unless they tell you so.
This is a good thing® since this means only the owning instance is allowed to change their private state. Trust me, it prevents quite a bit of havoc.
One way to allow the outside world to actually change the state is by using query and mutator methods. Back in the day, we used to call them getter and setter, but that sounds too simple, so software architects will usually use the fancier term.
But... all of this doesn't really apply since calculatePosition() is defined at Logic. Oops.
Let's try this:
Renaming some instance variables
Be a bit verbose to help the debugger. baby steps.
(Also, dropping a few jokes to fish for upvotes).
public class Logic {
private int position = 1;
public void appendPosition(){
// When debugging strange stuff,
// keep each step simple.
// Is calculatePosition working as it should?
int newPosition = calculatePosition(this.position);
this.position = newPosition;
}
// Always use parameters as final. It's good karma.
// You don't NEED to declare them as final,
// but let's try to be EXTRA clear.
private int calculatePosition(final int targetPosition){
// Yes, make as much as you can immutable
// You'll save a ton of mental bandwidth.
final int localCopy = targetPosition +6;
if(snakeLocations[localCopy]>0) {
return (localCopy -6);
// Don't force the maintenance programmer to
// read all your stuff. Return often, return early.
// This isn't Cc++, where you need to
// actually free your reference/pointers,
// so there's no point enforcing a single return.
}
if(ladderLocations[localCopy]>0) {
return (localCopy+6);
}
return localCopy;
}
}
So... Did this worked as it should?
I found the answer. This is often my mistake in posting here. I try to strip down my code as much as possible to ease the work for you guys, but sometimes the problem lies outside of the scope of what I provide due to a lack of understanding, or oversight, on my part.
I was actually calling MainLogic from two levels above:
public mainFrame() {
initComponents();
logic = Factory.getMainLogic();
}
where
public static class Factory {
public MainLogic getMainLogic(){
PlayerLogic pL = new PlayerLogic();
ImageLogic iL = new ImageLogic();
DieLogic dL = new DieLogic();
MainLogic mainLogic = new MainLogic(pL,iL,dL);
return mainLogic;
}
}
I forgot I had accidentally put Factory as static. My sincerest apologies for wasting your time.
I'm still a relative newbie when it comes to Java, coming mainly from a C# background.
I was discussing the lack of 'out' parameters in Java methods with a colleague and how to work around this. He suggested creating a structure/class to hold the various parameters and passing it back.
Sometimes this feels 'wrong' to me - especially if I have a special method that I want to use to return a subset of parameters from a larger class.
So I wondered about using anonymous inline classes instead to achieve this. Code sample below.
Is this a sensible approach? Just wondering what the perceived wisdom is on this.
public class MyClass {
Patient myPatient = null;
// An interface to enable us to return these variables in a single call
public interface VitalStatsResponse { public void returnStats(int bloodPressure, int heartRate); }
public class Patient {
int bloodPressure = 100;
int heartRate = 280;
// Lots of other variables here
public void calculateVitalStats(VitalStatsResponse response)
{
response.returnStats((bloodPressure * 2), (heartRate / 10) ;
}
}
public void doWork()
{
// We want the patient's blood pressure and heart rate returned by a single method call, so use an anonymous inline class
myPatient.calculateVitalStats(new VitalStatsResponse() {
#Override
public void returnStats(int bloodPressure, int heartRate) {
// Handle returned variables here
}
});
}
}
I would go for the simple solution of creating a VitalStats object. If you need the VitalStatus of a patient, then VitalStats is a concept in your application that can be represented as an Object.
public class VitalStatus {
final int bloodPressure;
final int heartRate;
public VitalStats(int bloodPressure, int heartRate) {
this.bloodPressure = bloodPressure;
this.heartRate = heartRate;
}
}
public class Patient {
int bloodPressure = 100;
int heartRate = 280;
// Other variables
public VitalStatus getVitalStatus() {
return new VitalStats(bloodPressured * 2, heartRate / 2);
}
}
Out params is a procedural solution for return times. Java primarily fits the Object Oriented paradigm of programming and as such don't be afraid to make objects. This fits with the S in SOLID if your class is doing a lot of complex things see if you can break it down into smaller more manageable pieces.
I would also use "class to hold the parameters" over "inline anonymous inner class"
public class MyClass implements VitalStatsResponse{
Patient myPatient = null;
private ArrayList<VitalStatsResponse> response;
void MyClass(ArrayList<VitalStatsResponse> response) {
this.response = response;
}
public class Patient {
int bloodPressure = 100;
int heartRate = 280;
// Lots of other variables here
public void calculateVitalStats()
{
for(int i = 0; i < response.length; i++) {
// call returnStats method of every registered callback
response.get(i).returnStats((bloodPressure * 2), (heartRate / 10) ;
}
}
}
// any client can register/unregister callback via these methods
void registerResponse(VitalStatsResponse response) {
this.response.add(response);
}
void unRegisterResponse(VitalStatsResponse response) {
this.response.remove(response);
}
public void doWork()
{
// We want the patient's blood pressure and heart rate returned by a single method call, so use an anonymous inline class
myPatient.calculateVitalStats();
}
public void returnStats(int bloodPressure, int heartRate) {
// implement the body according to this class requirement
}
}
I am currently making a terrain generator, everything works fine in one class but I am going to be expanding my application.
Currently I have a JFrame class which holds everything, generating the terrain, painting the terrain, finding locations etc.
I want to add another class that will generate the terrain but when I create this class I need to access fields from the main JFrame class and when I do I get a stack overflow error - here is my code.
public class Simulator extends Applet
{
//fields
public Simulator()
{
grid = new int[100][100];
inhabGrid = new boolean[grid.length][grid.length];
gridSize = grid.length - 1;
dist = grid.length;
TerrainGenerator gen = new TerrainGenerator();
setSize(dist,dist);
seedGrid();
findInhabLocation();
printGridToConsole();
}
public void paint(Graphics g)
{
//panting the grid
}
public void seedGrid()
{
//seeding
}
public boolean generateTerrain(int x1,int y1, int x2, int y2)
{
//terrain generator
}
public boolean mouseUp(Event evt, int x, int y)
{
seedGrid(); //Create a new map
findInhabLocation();
repaint();
printGridToConsole();
return true;
}
public boolean keyEvents(Event evt, int x, int y)
{
seedGrid(); //Create a new map
findInhabLocation();
repaint();
printGridToConsole();
return true;
}
public void findInhabLocation()
{
//find best inhabitant location
}
public int locateWater(int x, int y)
{
//finding closest water
}
public int locateJungle(int x, int y)
{
//finding closest jungle
}
}
}
That works fine in its own class but when I create a class for example:
public class TerrainGenerator
{
Simulator sim = new Simulator();
}
I know this has something to do with the constructor and it's something silly I am doing, what would be the best way of splitting up this app into classes, for example terrain generator, inhabitants etc
For example I want to be able to call a method from the 'TerrainGenerator' class and call i.e. terrainGenerator.generateTerrain
Your TerrainGenerator creates a Simulator object and vice versa, hence you'll end up with infinitely many objects (but at some point the stack is full and a stack overflow exception is thrown instead...)
Instead of creating a new Simulator in your TerrainGenerator, you should pass a reference to your current Simulator (well, actually, that is not a great design either, but I'm not gonna confuse you with the problems of circular references).
Heuster answer is correct, furthermore, I think you could take look at MVC to help you organize your classes.
Depending which should be the parent, you can pass in the instantiated class to the other, ie;
private final TerrainGenerator gen; //if you need to save this.
public Simulator(TerrainGenerator terrainGenerator)
{
this.gen = terrainGenerator;
....etc
}
public class TerrainGenerator
{
Simulator sim = new Simulator(this);
}
or
private final TerrainGenerator gen; //if you need to save this.
public Simulator()
{
this.gen = new TerrainGenerator(this);
....etc
}
private final Simulator sim; //If you need to save it.
public class TerrainGenerator
{
public TerrainGenerator(Simulator simulator) {
this.sim = simulator;
}
}