if (onGround() && Greenfoot.isKeyDown("up"))
{
    System.out.println("Actor should jump");
    System.out.println("Actor at: (" + getX() + ", " + getY() + ")");
    jump();
}

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)

/**
 * Write a description of class actor here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class actor extends Actor
{
    /**
     * Act - do whatever the actor wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    /**
     * An example of a method - replace this comment with your own
     *
     * @param  y   a sample parameter for a method
     * @return     the sum of x and y
     */
     
    private int gravity = 0;
    
        public void act()  
   {  
    //fill this in with what you want,  
    //but add this here:  
   if (onGround() && Greenfoot.isKeyDown("up")) 
   { 
   System.out.println("Actor should jump");  
    System.out.println("Actor at: (" + getX() + ", " + getY() + ")");  
    jump();  
       
    }
  }  
      
    private void checkFall()  
  {  
      if(!onGround())  
        fall();  
  }  
    private boolean onGround()  
  {  
       int myHeight = getImage().getHeight();  
       Actor ground = getOneObjectAtOffset(0, myHeight/2, Ground.class);  
       if(ground != null)//if the ground is really there...  
          return true;  
       //(if not, return false)  
       return false;  
  }  
    //this is quite like SPower's method  
    private void fall()  
    {  
       setLocation(getX(), getY() + gravity);  
       gravity++;//the player will accelerate as he falls  
  }  
    private void jump()  
    {  
         
          gravity -= 20;//or choose 10 or 15 or whatever works  
  }  

}

public void act()
{
    // THIS IS OUTSIDE THE "IF" STATEMENT BODY
    if (...) {
       // THIS IS INSIDE THE "IF" STATEMENT BODY
    }
    // THIS IS OUTSIDE THE "IF" STATEMENT BODY
}