Moving Animation

import greenfoot.*;
   
public class Mario extends Actor
{
    private int speed = 7;
    
    private int acceleration = 2;
    
    public int armour = 3;
    //frame is to keep track of what frame of animation you are on.
	private int frame;
	//direction is supoosed to be either less than 0 for left, or more than 0 for right.
	private int direction;
	//to check if Mario is standing still.
	private boolean standingStill;
	//the power of the jump.
	private int jumpStrength;
	//to keep track of whether Mario is jumping.
	private boolean jumping;
	//the power of gravity.
	private double gravityStrength;
	//the vertical speed of Mario
	private double vSpeed;
    
	public Mario()
	{
	    setImage("Mario_fr0.png");
	    frame = 0;
	    direction = 0;
	    jumpStrength = 30;
	    jumping = false;
	    gravityStrength = 3;
	    vSpeed = 0;
	}
    
	public void act() 
    {
       if (! getWorld().getObjects(GameOver.class).isEmpty()) 
       {
           return;
       }      
       checkFall();
       if(!jumping) {
            standingStill = true;
       }
       if(Greenfoot.isKeyDown("left")){
        	moveLeft();
        	direction = -1;
            if(onGround()) {
            	animateWalking();
                mirror();
            }
        	//if Mario is walking left, she is not StandingStill.
            standingStill = false;
       }
        else if(Greenfoot.isKeyDown("right")){
        	moveRight();
        	direction = 1;
            if(onGround())
            	animateWalking();
        	//if Mario is walking right, she is not StandingStill.
            standingStill = false;
       }
       if(Greenfoot.isKeyDown("z")){
            if(!jumping)
                jump();
            animateJumping();
            if(direction < 0)
                mirror();
       }
       attack();
       checkFall();
       hitGoomba();
       hitCastle_l1();
       hitCastle_l2();
       checkArmour();
    }
    
    public void checkArmour()
    {
        Actor sphere = getOneIntersectingObject(Sphere_left.class);
        if(sphere != null)
        {
           armour = armour - 1;
           ((Health_Bar) getWorld().getObjects(Health_Bar.class).get(0)).updateImage(armour);
           getWorld().removeObject(sphere);
        }
        
        if(armour == 0)
        {
            getImage().setTransparency(0);
            World world = new Game_Over_Lose();
            Greenfoot.setWorld(world);
        }
    }
    
    public void animateWalking()
    {
        if(frame == 0) {setImage("Mario_fr0.png");}
        else if(frame == 1) {setImage("Mario_fr1.png");}
        else if(frame == 2) {setImage("Mario_fr2.png");}
        else if(frame == 3) {setImage("Mario_fr3.png");}
        else if(frame == 4) {setImage("Mario_fr4.png");}
        else if(frame == 5) {setImage("Mario_fr5.png");}
        else if(frame == 6) {setImage("Mario_fr6.png");}
        else if(frame == 6) {    
            setImage("Mario_fr7.png");
            frame = 0;
            return;
        }
        frame++;
    }
    
    public void animateJumping()
    {
        if(frame == 0) {setImage("Mario_fr0.png");}
        else if(frame == 1) {setImage("Mario_fr1.png");}
        else if(frame > 1) {
            setImage("Mario_fr2.png");
        }
        frame++;
    }
    
    private void moveLeft()
    {
        setLocation(getX() - 10, getY());
    }
    
    private void moveRight()
    {
        setLocation(getX() + 10, getY());
    }
    
    private void jump()
    {
        frame = 0;
        vSpeed -= jumpStrength;
        fall();
        jumping = true;
    }
    
    private void checkFall()
    {
        if(!onGround()){
        	fall();
        }
        if(underGround()) {
        	setLocation(getX(), getY() - 2);
            jumping = false;
            vSpeed = 0;
        }
    }
    
    private void fall()
    {
        setLocation(getX(), getY() + (int)vSpeed);
        vSpeed += gravityStrength;
    }
    
    public boolean onGround()
    {
        if(getY() > 400) {
        	return true;
        }
        return false;
    }
    
    public boolean underGround()
    {
        if(getY() > 420) {
        	return true;
        }
        return false;
    }
    
    public void mirror()
    {
        getImage().mirrorHorizontally();
    }
    
    public void attack()
    {
        if("space".equals(Greenfoot.getKey()))
        {
            getWorld().addObject(new Sphere_right(), getX() + 5, getY());
        }
    }

    public void hitGoomba()
    {
        Actor goomba = getOneIntersectingObject(Goomba.class);
        if(goomba != null)
        {
            armour = armour - 1;
            getWorld().removeObject(goomba);
            ((Health_Bar) getWorld().getObjects(Health_Bar.class).get(0)).updateImage(armour);
        }
    }
     
    public void hitCastle_l1()
    {
        Actor castle = getOneIntersectingObject(Castle.class);
        if(castle != null)
        {
            World world = new Level_2();
            Mario mario2 = (Mario)world.getObjects(Mario.class).get(0);
            mario2.armour = this.armour;
            Greenfoot.setWorld(world);
            ((Health_Bar)world.getObjects(Health_Bar.class).get(0)).updateImage(armour);
        }
    }
    
    public void hitCastle_l2()
    {
        Actor castle = getOneIntersectingObject(Castle_l2.class);
        if(castle != null)
        {
            World world = new Final_level();
            Mario mario3 = (Mario)world.getObjects(Mario.class).get(0);
            mario3.armour = this.armour; 
            Greenfoot.setWorld(world);
            ((Health_Bar)world.getObjects(Health_Bar.class).get(0)).updateImage(armour);
        }
    }
}