Jumping

    //you need this variables for a better movement; the first two are the players exact location (double instead of int) and the second two are the movement in x and y direction; 
    private double exactX;
    private double exactY;
    private double velX;
    private double velY;
    
    private static final double GRAVITY = 0.15;
    
    public void setLocation(int x, int y) {
        exactX = x;
        exactY = y;
        super.setLocation(x, y);
    }
    public void setLocation(double x, double y) {
        exactX = x;
        exactY = y;
        super.setLocation((int) x, (int) y);
    }
    
    public double getExactX(){
        return exactX;
    }
    public double getExactY(){
        return exactY;
    }
    
    public void act() {
        if (Greenfoot.isKeyDown("up")) {
            velY = -5;
        }
        if (!onEarth) {//you need to find out when the actor is on the ground and when he is in the air;
            velY += GRAVITY;
        }
        else {
            velY = 0;
        } 
        if(Greenfoot.isKeyDown("a")) {
            left();
        }
        if(Greenfoot.isKeyDown("d")) {
            right();
        }
        move();
    }

    public void move() {
        setLocation(getExactX() + velX, getExactY() + velY);
    }

public void left() { turn(-1); } 
public void right() { turn(1); }