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Java how to have two variables point to the same primitive object

I am writing a platform game in Java in which there are various game objects such as platforms, the player, and (eventually) enemies. I describe the position of these game objects with rectangles: x position, y position, width, and height. However, I would also like to add in other variables to describe the position: left, top, right, and bottom. For the last two I know I will need to change them whenever x, y, width, or height gets modified, but as left and top are the same as x and y, I was wondering how I could get them to point to the same value as x and y. I think this can be done in C with the #define function, but sadly this is Java, not C.
How can I get two different variable names to point to the same value in Java such that if one changes, the other changes as well?
Edit: Here is the basics of my GameObject class:
public abstract class GameObject {
protected float x;
protected float y;
protected float right;
protected float bottom;
protected float width;
protected float height;
public float getX() {
return x;
}
public float getY() {
return y;
}
public void setX(float x) {
this.x = x;
right=this.x+width;
}
public void setY(float y) {
this.y = y;
bottom=this.y+height;
}
//A bunch of other getters/setters
//Subclasses must have render (for displaying graphics) and tick (mainly for updating position) functions.
public abstract void render(Graphics g);
public abstract void tick();
}
What I would like to do is add in two more variables:
protected float left = x;
protected float top = y;
and have these refer to the same primitive as x and y (not copy the primitives). This, apparently, does not seem possible. Thanks for your answers!

This can't be done in Java in the sense that you mean it. You can have two object references that point to the same object, but if you have a normal primitive variable, those are always distinct and don't refer to each other. You can make an object to hold a primitive, have two different references to that same object, and a change to the contents of that object through one reference will be reflected in the other, but you cannot have an int x and and int y where writing x = 10 will make y == 10.

If you really need something behaving like a reference to a primitive, you need to create an object holding that primitive value and then share the "holder" object.
However, I don't think this is the best way to tackle your problem.
As I understand your game objects, they can be entirely described with x, y, height and width variable. left, top, right and bottom on the other hand, are values that can derived from the four basic ones.
In fact, it would be possible to have a game object containing only x, y, height and width and to compute the other four values outside of the object whenever they are needed. Such an object could look like this :
public class GameObject {
private int x, y, height, width;
public int getX() { return x; }
public int getY() { return y; }
public int getHeight() { return height; }
public int getWidth() { return width; }
public void setX(int x) { this.x = x; }
public void setY(int y) { this.y = y; }
public void setHeight(int height) { this.height = height; }
public void setwidth(int width) { this.width = width; }
}
Notice that the actual variables are private and only readable through getters. This helps to avoid the use of a variable reference later on.
Now, to have an easy and unique way to access to left, top, right and bottom, while you could add four variables and keep them in sync with the four other variable already present, I think it is not the best way to do it.
I suggest that you just compute them on the fly inside slightly smarter-than-average getters. It would look like that :
public class GameObject {
// getters and setters omitted for brevity
private int x, y, height, width;
public int getLeft() {
return x;
}
public int getRight() {
// assuming x increase toward the right
return x + width;
}
public int getTop() {
return y
}
public int getBottom() {
// assuming y increase toward the bottom
return y + height;
}
}
This will also help if you need to create some special update methods, like moveRight(int delta), scale(double factor) since you will only need to apply changes to the basic variables and not the values that are derived from them.

int/integer is passed by value, not by reference in java.
You can define a new class, For example,
public class Position {
int x;
int y;
}
Let two variables point to the same instance of Position.

In java there are two types of variables.
1.Primitive types
2.Reference types
You need to use reference type variable if you want to refer the same object through different variables.
In your case for gaming application those parameters might change frequently.
So if all the time values, left and top are same as x and y, you can use use one variable to represent the both parameters which saves memory.
e.g : for left and x use one variable.
Or else using java OOP concepts,
you can make those x, y, left, top parameters private and assign getters and setters for those.
public int setX(int x){
this.x =x;
}
public setLeft(int left){
this.left = left;
}
public int setY(int y){
this.y =y;
}
public setTop(int top){
this.top= top;
}
public setXnLeft(int xnleft){
setX(xnleft);
setleft(xnleft);
}
public setYnTop(int yntop){
setY(yntop);
setTop(yntop);
}

This can easily be done by creating a new Class. Java is object-oriented, so, basically, everything is about classes and it's instances. Take a look at an example below:
class GameObject
{
//Instance variables
//(all objects of this class will have their own)
int x;
int y;
int width;
int height;
//Constructor
//(you call a constructor when creating a new object)
GameObject(int x, int y, int width, int height)
{
//Here you are assigning the values received to the instance variables (marked with "this", that represents the current object)
this.x = x;
this.y = y;
this.width = width;
this.height = height;
}
//This method will return the ordinate ("y") of the object's top position (assuming the "y axis" points down)
int getTop() {
return (y - height/2);
}
}
After creating the class GameObject, you can create instances (objects) of it. Look below:
GameObject player = new GameObject(0, 0, 50, 50); //creates a new object with coordinates (0,0), width = 50 and height = 50
GameObject enemy1 = new GameObject(30, 50, 100, 100);
//Getting the "x" position of the player and the monster
System.out.println("Player X = " + player.x);
System.out.println("Enemy1 X = " + enemy1.x);
//Get the top position "y"
System.out.println("Player top = " + player.getTop());
//Change player's "y"
player.y = 10;
//Get player's top position again
System.out.println("Player top = " + player.getTop()); //it will have changed, since "y" was changed.
Oracle has a really good lesson about Object-Oriented Programming, you should check it out here. The C language is structured, while Java is Object-Oriented, these are two different paradigms. I hope this was of help. Cheers!

Rectangle Project Error Fixing

I am currently working on a project called Rectangle project in which I am supposed to do the following on Java:
Make the following methods:
setOrigin
area
move
Also make a method that determines if two rectangles intersect and returns a new intersection Rectangle. Test all your methods in the ObjectDemo program for the following rectangles:
A: Origin 0,0: width 10: height 20
B: Origin 5,5: width 15, height 15
C: Origin 20,12: width 10: height 20
What is the area of each? Test if each of them intersect with the other two and what is the intersection area. Move A by 5,5; B by -5,-5: and C by -20, 0. Now give the intersection area of each.
I need to finish this by Monday but I keep getting a ton of errors like unrecognized variables, etc., and I'm not sure how to fix them. Please let me know!
I have three files: Point, RectangleTest, and Rectangle.
Here are their codes:
Point code:
public class Point
{
//Class variables
private int xCoord; //Private (instead of Public) because we are going to use this class in the other file
//We don't want people changing the values unless we let them
private int yCoord; //Variables are not in a function so will maintain their value
//Constructor
Point()
{
xCoord = 0;
yCoord = 0;
}
//Constructor
Point(int startX, int startY)
{
xCoord = startX;
yCoord = startY;
}
public int getX()
{
return xCoord;
}
public int getY()
{
return yCoord;
}
public void setX(int newX)
{
xCoord = newX;
}
public void setY(int newY)
{
yCoord = newY;
}
public void move(int moveX, int moveY)
{
xCoord+=moveX;
yCoord+=moveY;
}
Point(Point p)
{
xCoord = p.getX();
yCoord = p.getY();
}
}
RectangleTest Code:
public class RectangleTest
{
public static void main(String [] args)
{
Rectangle A = new Rectangle(0,0,10,20);
Rectangle B = new Rectangle(5,5,15,15);
Rectangle C = new Rectangle(20,12,10,20);
//Move rectangles
A.moveby(5,10);
B.moveby(-5,-5);
C.moveby(-20,0);
int areaA = A.getarea;
System.out.println("The area of rectangle A is " +areaA);
int areaB = B.getarea;
System.out.println("The area of rectangle B is " +areaB);
int areaC = C.getarea;
System.out.println("The area of rectangle C is " +areaC);
Rectanlge iAB = A.intersect(B);
Rectangle iAC = A.intersect(C);
Rectangle iBC = B.intersect(C);
if(iab != null)
{
System.out.println("The area of intersection rectangle iab = " +iAB.area());
}
if(iac != null)
{
System.out.println("The area of intersection rectangle iac = " +iAC.area());
}
if (ibc != null)
{
System.out.println("The area of intersection area ibc = " +iBC.area());
}
}
}
Rectangle Code:
public class Rectangle
{
Point origin;
int height;
int width;
//Constructor for rectangle object
Public Rectangle(int startX, int startY, int startW, int startH)
{
origin = new Point (startX, startY);
width = startW;
height = startH;
}
//Set origin point for NEW rectangle origins
//FIX
public void setOrigin(int newX, int newY)
{
origin.setX(newX);
origin.setY(newY);
}
public int moveBy(int moveX, int moveY)
{
origin.move(moveX, moveY);
}
public int getArea()
{
int recArea = height*width;
return recArea;
}
public Rectangle intersect(Rectangle testR)
{
int meTRX = origin.getX() + width;
int meTRY = origin.getY() + height;
int testTRX = testR.origin.getX() + width;
int testTRY = testR.origin.getY() + height;
//Boolean to get iTRX
if(meTRX>testTRX)
{
int iTRX = testTRX;
}
else
{
int iTRX = meTRX;
}
//Boolean to get iTRY
if(meTRY>testTRY)
{
int iTRY = testTRY;
}
else
{
int iTRY = meTRY;
}
//Boolean to get iBLX
if(testBLX>meBLX)
{
int iBLX = testBLX;
}
else
{
int iBLX = meBLX;
}
//Boolean to get iBLY
if(testBLY>meBLY)
{
int iBLY = testBLY;
}
else
{
int iBLY = meBLY;
}
//Testing for whether or not there is an intersection rectangle
if(iTRX-iBLX<0 || iTRY-iBLY<0)
{
return null;
}
int iH = iTRY - iBLY;
int iW = iTRX - iBLX;
int intersectArea = iH * iW;
}
}
Please point out any problems! I'm rather new to programming, so I usually make a lot of simple mistakes. Also, I would appreciate if there are no newly introduced commands or anything because my teacher is pretty strict about doing it this way.
Thanks!
P.S. I would appreciate any extra knowledge or info on code improvement (just in general). Thanks!

Couple of Issues:
Java is case sensitive so Public is not same as public in your rectangle class.
When your method doesnt return anything you should use void as return type. So in your method public int moveBy(int moveX, int moveY), you should change it to public void moveBy(int moveX, int moveY)
You need to define variables before using them. So variables like testBLX, meBLX, testBLY, meBLY, iTRX, iTRY, iBLX, iBLY are undefined. I am not sure from where the values will get populated. But you could avoid the compilation error by defining them as int testBLX = 0; and similarly the others.

In your Rectangle class:
In the constructor your wrote Public Rectangle(int startX, int startY, int startW, int startH), but you actually want public Rectangle(int startX, int startY, int startW, int startH). In Java keywords start always with a lower case.
Your method for changing the origin of a rectangle public int moveBy(int moveX, int moveY) has int as a return type, so the compiler wants you to return an integer value. I suppose you did not want to return anything at all so you can change the return type to void.
In your intersect method public Rectangle intersect(Rectangle testR) you declare your variables (iTRX, iTRY, iBLX, iBLY) such as int iTRX = testTRX; only in the scope of your if/else statements which means that after every if/else statement these variables are not available anymore. To learn more about the different scopes of variables: Variable scopes
In your RectangleTest class:
You forgot a part of your task: What is the area of each? Test if each of them intersect with the other two and what is the intersection area.
Some general leads:
The use of more descriptive variable names improves the readability. For example the variable name meTRX does not have any meaning for me as person who did not work on your code or maybe for you if you review your code two months later.
Before you start coding, you could check if Java has built-in classes which you can use. In your case Java provides a Point class in the package java.awt.Point. You do not have to reinvent the wheel.
I would also recommend to read the Java Code Conventions Code Conventions which can bring you and others who read your code on a common denominator in the future.

Method takes another object as a parameter

I am doing homework for java programming. I am asked to write a method that returns a distance between two points. I should use given formula that distance = square root((x2 - x1)*(x2 - x1) +(y2 - y1)*(y2 - y1)).
In the below codes, an object a will be contained current coordinate x1 and y1 and b will be coordinate x2 and y2, passed to move at some where.
How can I write the method in this class without having other classes and other elements such as x2, y2? In the objects there are two values but how can I assign each to x1 and x2, and y1 and y2? I found definition of vector for java but I am not sure it is applicable for this. Does anybody have an idea?
public class MyPoint{
private int x;
private int y;
}
public MyPoint(int x, int y){
this.x = x;
this.y = y;
}
public int distanceTo(MyPoint a, MyPoint b){
MyPoint.x1 = a;
MyPoint.y1 = a;
MyPoint.x2 = b;
MyPoint.y2 = b;
double distance = Math.sqrt((x2-x1)*(x2-x1) + (y2-y1)*(y2-y1));
return distance;
}
}

The distanceTo method should only take one parameter, a MyPoint object, not two parameters:
public double distanceTo(MyPoint other) {
// ...
}
The first object of the comparison will be the current object, the one whose method is being called.
Then in the method body you compare the current object's fields, the this.x and this.y with the x and y values of the object passed in to the method's parameter, other.x and other.y.
Also, the method probably should return a double, not an int as you have it defined.
Regarding,
How can I write the method in this class without having other classes and other elements such as x2, y2?
I'm not sure exactly what you mean by this.

You don't need to declare MyPoint.x1 = a. That isn't really doing anything. Instead you can reference the points such as a.x or b.y.
Additionally you should make sure that your return type is what you want.
EDIT: Hovercraft has essentially said the same as I did but a little nicer.

public class Point
{
int x,y;
public Point(int x, int y)
{
this.x = x;
this.y = y;
}
public static double distance(Point a, Point b)
{
return Math.sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
public static void main(String[] args)
{
Point a = new Point(0,0);
Point b = new Point(3,5);
System.out.println(distance(a,b));
}
}

How can I move a point and print the new position?

I'm new to Java and I received an assignment that asks me to write a class called PointerTester that has two Points as instance variables. I need to initialize one of these at coordinate (0.0,0.0) and one at (10.0,12.0). Then I need to move each point by +2.0 in x and -3.0 in Y, query the coordinates of the points, and print out the values.
So far I have this:
public class PointerTester{
/*instance variables*/
private double pointOneX;
private double pointOneY;
private double pointTwoX;
private double pointTwoY;
private double deltaX;
private double deltaY;
/*constructor to initialize*/
public PointerTester (){
pointOneX = 0.0;
pointOneY = 0.0;
pointTwoX = 10.0;
pointTwoY = 12.0;
deltaX = 2.0;
deltaY = -3.0;
}
/*constructor to initialize to specific value*/
PointerTester(double pointOneX, double pointOneY, double pointTwoX, double pointTwoY){
this.pointOneX = pointOneX;
this.pointOneY = pointOneY;
this.pointTwoX = pointTwoX;
this.pointTwoY = pointTwoY;
}
/*command to change value*/
public void moveBy(double deltaX, double deltaY){
this.pointOneX = this.pointOneX + deltaX;
this.pointOneY = this.pointOneY + deltaY;
this.pointTwoX = this.pointTwoX + deltaX;
this.pointTwoY = this.pointTwoY + deltaY;
}
/*Queries*/
public double getOneX(){
return pointOneX;
}
public double getOneY(){
return pointOneY;
}
public double getTwoX(){
return pointTwoX;
}
public double getTwoY(){
return pointTwoY;
}
/*print values*/
public static void main(String[] args){
PointerTester pointOne = new PointerTester();
PointerTester pointTwo = new PointerTester();
System.out.println("Point One after move (" + pointOne + ")");
System.out.println("Point Two after move (" + pointTwo + ")");
}
}
I cannot figure out how to correctly output the values or if I am completely wrong in working on this problem.
Edit It seems I needed to use this code that I at first thought was supposed to be separate
public class Point{
public double x;
public double y;
public Point(double x, double y){
this.x = x;
this.y = y;
}
public void setX(double x){
this.x = x;
}
public void setY(double y){
this.y = y;
}
public double getX(){
return x;
}
public double getY(){
return y;
}
}
How do I incorporate this into my code?

I think to get started you should have a separate class called "Point" that encapsulates an X and Y value, and includes a "moveBy" method. It could also implement "toString()" such that "System.out.println" will print something nice for it. [Edit: Or just use java.awt.Point.]
Beyond that, I'll leave that for you to do as your homework.

You might want to use some Point objects.
Point p1 = new Point();
Point p2 = new Point(10,12);
you can then use the setLocation or translate methods found in the point class. Maybe the whole thing would look something like this? Hope this helps.
main(){
//make some points
Point p1 = new Point();
Point p2 = new Point(10,12);
//move our points
p1.translate(2,-3);
p2.translate(2,-3);
//print our points
System.out.println(p1);
System.out.println(p2);
}

You can implement the toString method. There are examples here:
http://www.javapractices.com/topic/TopicAction.do?Id=55

You're trying to print the PointerTester object instead of the values contained within. System.out.println(pointOne.getOneX()) should at least stop throwing exceptions and allow you to print correctly.

Trouble getting java methods combined in one main method

class Bug {
// An ant is represented by the coordinates of its location,
// and the direction it is facing.
Integer x;
Integer y;
Dir dir;
enum Dir { E,W,N,S }
}
Bug(Integer x, Integer y, Dir dir) {
this.x = x;
this.y = y;
this.dir = dir;
}}
class BugWorld {
Integer theBreadth, theHeight;
Board board;
Bug bug;
BugWorld(Integer breadth, Integer height) {
board = new board(breadth, height);
bug = new Bug(breadth/2, height/2, Ant.Direction.Y);
theBreadth = breadth;
theHeight = height;
}
I already have the following:
Status status(Integer x, Integer y) {
return board[x][y];
}
void update(Integer x, Integer y) {
board[x][y].next();
}
This part below is where I'm having some trouble:
/* Take the world of the bug to the next step. */
void step() {
// 1) Get the state at the present bug position.
//I've done the following (next line) so far.
Bug status(Integer x, Integer y); ...?
// 2) Change the 'status' at that position.
.............?
}
It's just combining these that I'm having trouble with.

It's hard to understand what you're trying to do, but I think here's what you want
void step(){
//get the location of the bug
int x = bug.x;
int y = bug.y;
//gets the status of the location of the bug
Status s = status(x,y);
//move the bug, or do something based on the status of the place
//update the status of the location the bug was originally on
update(x,y)
}