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Getting a sprite to bounce off the walls/edges in android app

Most learned friends
I have a sprite that moves around on screen but at the moment it just moves diagonally from left to right and goes off screen and then comes back on the other side.
What I would like it to do is bounce off the edges of the screen in a random fashion but, not being all that clued up on maths, I'm struggling to figure out the coordinates to do this.
Below is what I have so far: (this is an updated code for the Sprite class:
public class Sprite {
//x,y position of sprite - initial position (0,50)
// int [] DIRECTION_TO_ANIMATION_MAP = {3, 1, 0, 2};
private static final int BMP_ROWS = 3;
private static final int BMP_COLUMNS = 4;
private int x = 0;
private int y = 0;
private int xSpeed = 5;//Horizontal increment of position (speed)
private int ySpeed;// Vertical increment of position (speed)
private int currentFrame = 0;
private GameView gameView;
private Bitmap spritebmp;
//Width and Height of the Sprite image
private int bmp_width;
private int bmp_height;
// Needed for new random coordinates.
//private Random random = new Random();
public Sprite(GameView gameView) {
this.gameView = gameView;
spritebmp = BitmapFactory.decodeResource(gameView.getResources(),
R.drawable.running_ninja_sprite);
this.bmp_width = spritebmp.getWidth() / BMP_COLUMNS;
this.bmp_height = spritebmp.getHeight() / BMP_ROWS;
/*Random rnd = new Random(System.currentTimeMillis());
xSpeed = rnd.nextInt(45)-5;
ySpeed = rnd.nextInt(25)-5;*/
}
//update the position of the sprite
public void update() {
//if (x < 0 || x > gameView.getWidth() ){ xSpeed = xSpeed * -1;}
//if (y < 0 || y > gameView.getHeight() ){ ySpeed = ySpeed * -1;}
if (x > gameView.getWidth() - bmp_width - xSpeed || x + xSpeed < 0) {
xSpeed = -xSpeed;
}
x = x + xSpeed;
if (y > gameView.getHeight() - bmp_height - ySpeed || y + ySpeed < 0) {
ySpeed = -ySpeed;
}
y = y + xSpeed;
currentFrame = ++currentFrame % BMP_COLUMNS;
//y = random.nextInt(gameView.getWidth());
//wrapAround(); //Adjust motion of sprite.
}
public void draw(Canvas canvas) {
update();
int srcX = currentFrame * bmp_width;
int srcY = 1 * bmp_height; //getAnimationRow()
Rect src = new Rect(srcX, srcY, srcX + bmp_width, srcY + bmp_height);
Rect dst = new Rect(x, y, x + bmp_width, y + bmp_height);
//Draw sprite image
canvas.drawBitmap(spritebmp, x, y, null);
}
/*private int getAnimationRow() {
double dirDouble = (Math.atan2(xSpeed, ySpeed) / (Math.PI / 2) + 2);
int direction = (int) Math.round(dirDouble) % BMP_ROWS;
return DIRECTION_TO_ANIMATION_MAP[direction];
}*/
public void wrapAround() {
//Code to wrap around
if (x < 0) x = x + gameView.getWidth(); //increment x whilst not off screen
if (x >= gameView.getWidth()) { //if gone of the right sides of screen
x = x - gameView.getWidth(); //Reset x
}
if (y < 0) y = y + gameView.getHeight();//increment y whilst not off screen
if (y >= gameView.getHeight()) {//if gone of the bottom of screen
y -= gameView.getHeight();//Reset y
}
}
// Checks if the sprite was touched
public boolean wasItTouched(float ex, float ey) {
boolean touched = false;
if ((x <= ex) && (ex < x + bmp_width) &&
(y <= ey) && (ey < y + bmp_height)) {
touched = true;
}
return touched;
}
}
It now does bounce off the edge but from top left, diagonally to the right and back up in the same direction and continues to do this back and forth. I'd like it to be random in its direction after hitting the edge. Any suggestions? As you can see from the code I have tried a lot of things but the sprite just keeps on doing this continuous back and forth diagonal motion and I'm at a loss as to what I can do.
Thanks

Just negate your speed every time you hit a wall
if (x < 0 || x > gameView.getWidth() ){ xSpeed = xSpeed * -1;}
if (y < 0 || y > gameView.getHeight() ){ ySpeed = ySpeed * -1;}
/////////////////////////////// Edit
It will fit something like this. You can also remove your wrap function as it is no longer applicable
//update the position of the sprite
public void update() {
x = x + xSpeed;
y = y + xSpeed;
bounce();
}
private void bounce(){
if (x <= 0 || x >= gameView.getWidth() ){ xSpeed = xSpeed * -1;}
if (y <= 0 || y >= gameView.getHeight() ){ ySpeed = ySpeed * -1;}
}

Freeze moving rectangles

I'm working on a project where we are supposed to have rectangles of random sizes bounce off a wall and change color each time they bounce.
When you click on them, they are supposed to freeze in place and turn red. I'm just having trouble having them stop and for some reason they slow when one is clicked.
import java.util.Random;
public class Main {
public static void main(String[] args) {
MovingRectangles[] rectangles = new MovingRectangles[5];
Random rng = new Random();
for (int i = 0; i < rectangles.length; i++) {
rectangles[i] = new MovingRectangles(rng.nextDouble(), rng.nextDouble(), rng.nextDouble(), rng.nextDouble(), rng.nextDouble(), rng.nextDouble());
}
for (int g = 0; g < rectangles.length; g++) {
rectangles[g] = new MovingRectangles(
rng.nextDouble(),
rng.nextDouble(),
rng.nextDouble() / 50 - 0.01,
rng.nextDouble() / 50 - 0.01,
rng.nextDouble() * 0.04 + 0.03,
rng.nextDouble() * 0.04 + 0.03
);
}
while (true) {
StdDraw.clear();
for (int h = 0; h < rectangles.length; h++) {
rectangles[h].draw();
rectangles[h].update();
}
int count = 0;
for (int i =0; i < rectangles.length; i++) {
rectangles[i].draw();
if (StdDraw.mousePressed() && rectangles[i].containsPoint(StdDraw.mouseX(), StdDraw.mouseY())) {
rectangles[i].freeze();
}
if (rectangles[i].isFrozen()) {
count++;
StdDraw.show(25);
}
}
}
}}
This is the class for moving rectangles. Stackoverflow says I need to add context to explain what this code is.
import java.util.Random;
public class MovingRectangles {
private double x;
private double y;
private double vx;
private double vy;
private double hx;
private double hy;
private boolean isFrozen;
private int red;
private int green;
private int blue;
public MovingRectangles(double x, double y, double vx, double vy, double hx, double hy) {
this.x = x;
this.y = y;
this.vx = vx;
this.vy = vy;
isFrozen = false;
this.hx = hx;
this.hy = hy;
randomColor();
}
public void randomColor() {
Random rng = new Random();
red = rng.nextInt(256);
blue = rng.nextInt(256);
green = rng.nextInt(256);
}
public void draw() {
if (isFrozen) {
StdDraw.setPenColor(StdDraw.RED);
} else {
StdDraw.setPenColor(red, green, blue);
}
StdDraw.filledRectangle(x, y, hx, hy);
}
public void update() {
x += vx;
y += vy;
if (x - hx < 0) {
vx *= -1;
x = 0 + hx;
randomColor();
}
if (x + hx > 1) {
vx *= -1;
x = 1 - hx;
randomColor();
}
if (y - hy < 0) {
vy *= -1;
y = 0 + hy;
randomColor();
}
if (y + hy > 1) {
vy *= -1;
y = 1 - hy;
randomColor();
}
}
public void freeze() {
isFrozen = true;
}
public boolean isFrozen() {
return isFrozen;
}
public boolean containsPoint(double a, double b) {
return
a > x - hx &&
a < x + hx &&
b > y - hy &&
b < y + hy;
}
}
The only other thing I need to add is for it to print "You Win" when all five of the boxes have been clicked. thanks for any help.

My thought is that you're not stopping the actual update of the rectangle.
In your MovingRectangles class...
public void update() {
if(!this.isFrozen) {
{...your code...}
}
}

add at the very beginning of update method line like
if(isFrozen) return;
it should stop your rectangle.
the another way (if you don't want to touch rectangle class).
after
for (int h = 0; h < rectangles.length; h++) {
rectangles[h].draw();
add
if(!rectangles[h].isFrozen()) rectangles[h].update();

This update to draw fixed the problem I was having.
public void draw() {
if (isFrozen) {
StdDraw.setPenColor(StdDraw.RED);
StdDraw.filledRectangle(x, y, hx, hy);
vx = 0;
vy = 0;
} else {
StdDraw.setPenColor(red, green, blue);
}
StdDraw.filledRectangle(x, y, hx, hy);
}

keeping sprites from colliding java

I am making a game in java and I have most of it done. However, one of the last bugs i need to fix is that enemy sprites can overlap each other and spawn on top of one another off screen. I want to make it so if enemy sprites collide they can only touch but not overlap. here is the code for the enemy class.
import java.awt.Graphics;
import java.awt.Image;
import java.awt.Rectangle;
import java.util.ArrayList;
import javax.swing.ImageIcon;
public class Enemy extends Base {
int x;
int y;
int velx = -2;
int vely = 0;
public Enemy(int x, int y) {
super(x,y);
this.x = x;
this.y = y;
}
public void update() {
movement();
x += velx;
y += vely;
if (x < - 15) {
x = -15;
movement();
}
if (x > 1100) {
x = 1100;
movement();
}
if (y > 660) {
y = 660;
movement();
}
if (y < 10) {
y = 10;
movement();
}
}
public void draw(Graphics g) {
g.drawImage(getEnemyImage(), x, y, null);
}
public static Image getEnemyImage(){
ImageIcon ic = new ImageIcon("enemy.gif");
return ic.getImage();
}
public Rectangle getBounds(){
return new Rectangle(x, y, getEnemyImage().getWidth(null), getEnemyImage().getHeight(null));
}
public void checkColision(){
ArrayList<Base> enemies = GamePanel.getEnemyList();
if (x <= 0) {
velx = 2;
}
if (x >= 1096) {
velx = -2;
}
for (int a = 0; a < enemies.size(); a++) {
Base temp = GamePanel.enemy.get(a);
if (getBounds().intersects(temp.getBounds())) {
// where the collision check should happen.
}
}
}
public void movement(){
if (y > 16) {
if (x > GamePanel.p.getX()) {
velx = - 2;
}
if (x < GamePanel.p.getX()) {
velx = 2;
}
if (y > GamePanel.p.getY()) {
vely = -2;
}
if (y < GamePanel.p.getY()) {
vely = 2;
}
}
}
}
and this is where the enemies are spawned.
for (int a = 0; a < enemy_amount; a++) {
space += 50;
int rand = (int)(Math.random() * 2) + 1;
if (rand == 1) {
int randp = (int)(Math.random() * 2) + 1;
int x = 0;
int y = 0;
if (randp == 1) {
x = 1250 + space;
y = 500;
}
if (randp == 2) {
x = 1250 + space;
y = 100;
}
enemy.add(new Enemy(x,y));
}
if (rand == 2) {
int randp = (int)(Math.random() * 2) + 1;
int x = 0;
int y = 0;
if (randp == 1) {
x = 250;
y = -100 - space;
}
if (randp == 2) {
x = 850;
y = -100 - space;
}
enemy.add(new Enemy2(x,y));
}
}
any help would be great because i am really stuck.

Processing (the Java visualization language): edge checking function fails when used with an array of objects

I'm working my way through Daniel Shiffman's excellent The Nature of Code book and have completed simulating gravitational attraction and repulsion between objects.
The gravitational force works well, but my objects fly right off the screen. I would like to constrain them to the drawing canvas. I am using an edge-checking function written in the previous chapter, and it is failing to keep any objects on the canvas.
I would love to know why.
The offending function is the last block of code in the Mover class.
class Mover {
PVector location;
PVector velocity;
PVector acceleration;
float mass;
float G = 0.4; //universal gravitational constant
Mover(float m, float x, float y) {
location = new PVector(x,y);
velocity = new PVector(1,0);
acceleration = new PVector(0,0);
mass = m;
}
PVector attract(Mover m) {
PVector force = PVector.sub(location,m.location);
float distance = force.mag();
distance = constrain(distance,5,25);
force.normalize();
float strength = ((G * mass * m.mass) / (distance * distance)*-1);
force.mult(strength);
return force;
}
void applyForce(PVector force) {
PVector f = PVector.div(force,mass);
acceleration.add(f);
}
void update() {
velocity.add(acceleration);
location.add(velocity);
acceleration.mult(0);
}
void display() {
stroke(0);
strokeWeight(2);
fill(0,100);
ellipse(location.x,location.y, mass*25 , mass*25);
}
void checkEdges() { // why doesn't this work?
if (location.x > width) {
location.x = width;
velocity.x *= -1;
} else if (location.x < 0) {
velocity.x *= -1;
location.x = 0;
}
if (location.y > height) {
velocity.y *= -1;
location.y = height;
} else if (location.y < 0) {
velocity.y *= -1;
location.y = 0;
}
}
}
//////////////////////////////////////////////////////////////////////
Mover [] movers = new Mover [10];
void setup() {
size(1000,1000);
for (int i = 0; i < movers.length; i++) {
movers[i] = new Mover(random(0.1,2),random(width),random(height)); //mass, loc.x, loc.y //each mover initialized randomly
}
}
void draw() {
background(255);
for(int i = 0; i < movers.length; i++){
for(int j = 0; j < movers.length; j++){
if(i != j){
PVector force = movers[j].attract(movers[i]); //Calculate attraction force
movers[i].applyForce(force); //Apply attraction force
}
}
movers[i].update();
movers[i].display();
}
}

Shouldn't you be calling checkEdges() inside Update()? If not that, then how about in your main code between
movers[i].update();
movers[I].checkEdges(); // check after the update and before the display
movers[i].display();

You're not checking whether location.y is less than zero.
Compare what you're doing for X:
if (location.x > width) {
location.x = width;
velocity.x *= -1;
} else if (location.x < 0) {
velocity.x *= -1;
location.x = 0;
}
to what you're doing for Y:
if (location.y > height) {
velocity.y *= -1;
location.y = height;
}

Can anyone help me understand how to simulate fluids?

I'm trying to make a program that simulates the physics of fluids in Processing. In the IDE there's an included example:
/**
* Fluid
* by Glen Murphy.
*
* Click and drag the mouse to move the simulated fluid.
* Adjust the "res" variable below to change resolution.
* Code has not been optimised, and will run fairly slowly.
*/
int res = 2;
int penSize = 30;
int lwidth;
int lheight;
int pnum = 30000;
vsquare[][] v;
vbuffer[][] vbuf;
particle[] p = new particle[pnum];
int pcount = 0;
int mouseXvel = 0;
int mouseYvel = 0;
void setup()
{
size(200, 200);
noStroke();
frameRate(30);
lwidth = width/res;
lheight = height/res;
v = new vsquare[lwidth+1][lheight+1];
vbuf = new vbuffer[lwidth+1][lheight+1];
for (int i = 0; i < pnum; i++) {
p[i] = new particle(random(res,width-res),random(res,height-res));
}
for (int i = 0; i <= lwidth; i++) {
for (int u = 0; u <= lheight; u++) {
v[i][u] = new vsquare(i*res,u*res);
vbuf[i][u] = new vbuffer(i*res,u*res);
}
}
}
void draw()
{
background(#666666);
int axvel = mouseX-pmouseX;
int ayvel = mouseY-pmouseY;
mouseXvel = (axvel != mouseXvel) ? axvel : 0;
mouseYvel = (ayvel != mouseYvel) ? ayvel : 0;
for (int i = 0; i < lwidth; i++) {
for (int u = 0; u < lheight; u++) {
vbuf[i][u].updatebuf(i,u);
v[i][u].col = 32;
}
}
for (int i = 0; i < pnum-1; i++) {
p[i].updatepos();
}
for (int i = 0; i < lwidth; i++) {
for (int u = 0; u < lheight; u++) {
v[i][u].addbuffer(i, u);
v[i][u].updatevels(mouseXvel, mouseYvel);
v[i][u].display(i, u);
}
}
}
class particle {
float x;
float y;
float xvel;
float yvel;
int pos;
particle(float xIn, float yIn) {
x = xIn;
y = yIn;
}
void updatepos() {
float col1;
if (x > 0 && x < width && y > 0 && y < height) {
int vi = (int)(x/res);
int vu = (int)(y/res);
vsquare o = v[vi][vu];
float ax = (x%res)/res;
float ay = (y%res)/res;
xvel += (1-ax)*v[vi][vu].xvel*0.05;
yvel += (1-ay)*v[vi][vu].yvel*0.05;
xvel += ax*v[vi+1][vu].xvel*0.05;
yvel += ax*v[vi+1][vu].yvel*0.05;
xvel += ay*v[vi][vu+1].xvel*0.05;
yvel += ay*v[vi][vu+1].yvel*0.05;
o.col += 4;
x += xvel;
y += yvel;
}
else {
x = random(0,width);
y = random(0,height);
xvel = 0;
yvel = 0;
}
xvel *= 0.5;
yvel *= 0.5;
}
}
class vbuffer {
int x;
int y;
float xvel;
float yvel;
float pressurex = 0;
float pressurey = 0;
float pressure = 0;
vbuffer(int xIn,int yIn) {
x = xIn;
y = yIn;
pressurex = 0;
pressurey = 0;
}
void updatebuf(int i, int u) {
if (i>0 && i<lwidth && u>0 && u<lheight) {
pressurex = (v[i-1][u-1].xvel*0.5 + v[i-1][u].xvel + v[i-1][u+1].xvel*0.5 - v[i+1][u-1].xvel*0.5 - v[i+1][u].xvel - v[i+1][u+1].xvel*0.5);
pressurey = (v[i-1][u-1].yvel*0.5 + v[i][u-1].yvel + v[i+1][u-1].yvel*0.5 - v[i-1][u+1].yvel*0.5 - v[i][u+1].yvel - v[i+1][u+1].yvel*0.5);
pressure = (pressurex + pressurey)*0.25;
}
}
}
class vsquare {
int x;
int y;
float xvel;
float yvel;
float col;
vsquare(int xIn,int yIn) {
x = xIn;
y = yIn;
}
void addbuffer(int i, int u) {
if (i>0 && i<lwidth && u>0 && u<lheight) {
xvel += (vbuf[i-1][u-1].pressure*0.5
+vbuf[i-1][u].pressure
+vbuf[i-1][u+1].pressure*0.5
-vbuf[i+1][u-1].pressure*0.5
-vbuf[i+1][u].pressure
-vbuf[i+1][u+1].pressure*0.5
)*0.25;
yvel += (vbuf[i-1][u-1].pressure*0.5
+vbuf[i][u-1].pressure
+vbuf[i+1][u-1].pressure*0.5
-vbuf[i-1][u+1].pressure*0.5
-vbuf[i][u+1].pressure
-vbuf[i+1][u+1].pressure*0.5
)*0.25;
}
}
void updatevels(int mvelX, int mvelY) {
if (mousePressed) {
float adj = x - mouseX;
float opp = y - mouseY;
float dist = sqrt(opp*opp + adj*adj);
if (dist < penSize) {
if (dist < 4) dist = penSize;
float mod = penSize/dist;
xvel += mvelX*mod;
yvel += mvelY*mod;
}
}
xvel *= 0.99;
yvel *= 0.99;
}
void display(int i, int u) {
float tcol = 0;
if (col > 255) col = 255;
if (i>0 && i<lwidth-1 && u>0 && u<lheight-1) {
tcol = (+ v[i][u+1].col
+ v[i+1][u].col
+ v[i+1][u+1].col*0.5
)*0.4;
tcol = (int)(tcol+col*0.5);
}
else {
tcol = (int)col;
}
fill(tcol, tcol, tcol);
rect(x,y,res,res);
}
}
It's not really commented and I'm somewhat new to programming, so I have no idea where to start as far as understanding it. Is there any good reading on fluid physics? I'm more interesting in the visual effect than the accuracy of the simulation.

A good starting point could be the paper Stable Fluids, it will show you the math behind the fluid simulation, and in the third chapter it describe the implementation of a fluid solver. There is also an open source implementation available in sourceforge (you will need to checkout the source with cvs).