Java Platforming Game Land on Platform

Question

My Collision code for platforms in my Player class isn't working and sometimes glitches out. I created different methods for applying gravity and whatnot.

When the player is under a platform and they jump, I want them to collide with the bottom of the platform and fall back down using the gravity method I created.

When the player walks into both ends of the platform I want them to just stop beside it.

I want to make sure at all times that the player cannot go through the platform and that seems to be my downfall.

I would really appreciate it if anyone could help me out.

Here is my Player class, the applet screen size is 600x400:

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;


public class Player implements KeyListener
{
int x, y, width, height;

boolean jump, left, right;

PlayerThread playerThread;

int maxHeight = 40;
double heightC = 0;
boolean onPlatform = false;
boolean landed = true;
int prevY;

public Player()
{
    x = 500;
    y = 350;
    width = 40;
    height = 50;
    playerThread = new PlayerThread();
    playerThread.start();
}

public void paint(Graphics g)
{
    String str = String.valueOf(x);
    String str2 = String.valueOf(y);

    g.setColor(Color.RED);
    g.fillRect(x, y, width, height);

    g.drawString("X: " + str + ", Y: " + str2, 100, 100);
}

public void update(Platform p)
{
    CheckForCollision(p);
}

public void CheckForCollision(Platform p)
{
    int pX = p.getX();
    int pY = p.getY();
    int pWidth = p.getWidth();
    int pHeight = p.getHeight();

    //THIS IS COLLISION DETECTION CODE THAT DOES NOT WORK
    if (y + height > pY && y  < pY + pHeight && x + width > pX && x < pX + pWidth)
    {
        y = prevY;
        landed = true;
    }
}

public class PlayerThread extends Thread implements Runnable
{
    public void run()
    {
        while (true)
        {
            if (left)
            {
                x -= 2;
            }

            if (right)
            {
                x += 2;
            }

            if (jump)
            {
                if (heightC >= maxHeight)
                {
                    System.out.println(heightC);
                    jump = false;
                    heightC = 0;
                }
                else
                {
                    heightC += 1.5;
                    prevY = y;
                    y -= 5;
                    landed = false;
                }
            }

                            //GRAVITY CODE
            if (!jump)
            {
                if (y < 400 - height && !landed)
                {
                    prevY = y;
                    y += 5;
                    landed = false;
                }
            }

            if (y >= 400 - height)
            {
                y = 400 - height;
                landed = true;
            }

            try
            {
                sleep(17);
            } 
            catch (InterruptedException e)
            {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}

@Override
public void keyPressed(KeyEvent e)
{
    switch (e.getKeyCode())
    {
    case KeyEvent.VK_LEFT:
        left = true;
        break;
    case KeyEvent.VK_RIGHT:
        right = true;
        break;
    case KeyEvent.VK_SPACE:
        if (landed)
        {
        jump = true;
        }
        break;
    }
}

@Override
public void keyReleased(KeyEvent e)
{
    switch (e.getKeyCode())
    {
    case KeyEvent.VK_LEFT:
        left = false;
        break;
    case KeyEvent.VK_RIGHT:
        right = false;
        break;
    case KeyEvent.VK_SPACE:
        break;
    }
}



public int getX()
{
    return x;
}

public void setX(int x)
{
    this.x = x;
}

public int getY()
{
    return y;
}

public void setY(int y)
{
    this.y = y;
}

public int getWidth()
{
    return width;
}

public void setWidth(int width)
{
    this.width = width;
}

public int getHeight()
{
    return height;
}

public void setHeight(int height)
{
    this.height = height;
}

@Override
public void keyTyped(KeyEvent e)
{
    // TODO Auto-generated method stub

}

}

Answer 1

Your problem is with your detection code. I assume, you are using basic rectangle collisions, based on the if statement, however your clauses seem to be a bit strange.

Lets break them down: y + height > pY - If the bottom of the player is below than the top of the platform. y < pY + pHeight - If the top of the player is above than the bottom of the platform x + width > pX - If the right of the player is to the right of the left of the platform x < pX + pWidth - The the left of the player is to the left of the right of the platform

Now note the logic here: This will only trigger iff the player is completly inside the platform, which I expect is not what you want. What you probably want is more akin to a proof by negation:

if(!(y > pY + pHeight || y + height < pY || x > pX + pWidth || x + width < pX)){
    //There is a collision
}

On a side note, although I would generally agree with Mr. D and his comment of using a game engine, it is often useful from a learning perspective to know how these things work, so as you are not constrained to someone elses codebase.