animation for enemys

public class monster extends Actor
{
    private GreenfootImage image1 = new GreenfootImage ("spr_monster_idle_0.png"); 
    private GreenfootImage image2 = new GreenfootImage ("spr_monster_idle_1.png"); 
    private int time = 0;
    public void act() 
    {
        if(time > 0){
            time --;}
            else{
        animate();
    }
    }  
    public void animate()
    { 
        if(getImage().equals("spr_monster_idle_0.png")){
        setImage("spr_monster_idle_1.png");
        time = 30;
    }
    else if(getImage().equals("spr_monster_idle_1.png")){
        setImage("spr_monster_idle_0.png");
        time = 30;
    }
    }