Jumping error

public class player extends Actor
{
    private int vSpeed = 0; //vertical speed, falling
    private int acceleration = 2; //fall faster when higher up
    private boolean jumping; //checks if the player is jumping or not
    private int jumpStrength = 100; //how high player jumps
    
    public void act() 
    {
        checkFall();
        checkKey();
    }
    
    public void checkKey(){
        if(Greenfoot.isKeyDown("z") && jumping == false)
        {
            jump();
        }
    }
    
    public void fall(){
        setLocation(getX(), getY() +vSpeed);
        if(vSpeed <=9)
        {
            vSpeed = 2 + acceleration; //if character is high up then fall faster
        }
        jumping = true; //stops you jumping while falling
    }
    
    public boolean onGround(){
        int spriteHeight = getImage().getHeight();
        int groundFind = (int)(spriteHeight/2) + 5;
        
        Actor ground = getOneObjectAtOffset(0, groundFind, platform.class);
        
        if(ground == null)
        {
            jumping = true; //stops jumping will not on ground
            return false;
        }
        else
        {
            moveToGround(ground); //passing our ground variable to our function
            return true;
        }
    }
    
    public void moveToGround(Actor ground){
        int groundHeight = ground.getImage().getHeight();
        int newY = ground.getY() - (groundHeight + getImage().getHeight())/2; //get ground height and add it to image of character then divide by 2
        
        setLocation(getX(), newY); //sets player location to our new calculated y location
        jumping = false; //ready to jump
    }
    
    public void checkFall(){
        if(onGround())
        {
            vSpeed = 0;
        }
        else
        {
            fall();
        }
    }
    
    public void jump(){
        vSpeed = vSpeed - jumpStrength;
        jumping = true;
        fall();
    }
}