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public class crane extends Actor {
     
    private Container container;
     
    public void act() {
        //some other stuff for moving etc.
        if (container == null && Greenfoot.isKeyDown("p") && getObjectsInRange(25, Container.class)) {
            //"p" for pick up; you can use any other key if you want;
            //25 is the radius in which a container needs to be to pick it up; You can also use any other value;
            container = (Container) getObjectsInRange(25, Container.class).get(0);
        }
        if (container != null && Greenfoot.isKeyDown("d")) {//"d" for drop;
            //drop the container;
            //you should add a method in the container that does something when it's droped;
            container.doSomething();//this line will not compile because there is no doSomething() method in the container class;
            //It's just an example so you can delete this line;
            container = null;
        }
    }
     
    public void setLocation(int x, int y) {
        //by overriding this method you move the container when you have picked up one;
        super.setLocation(x, y);
        if (container != null) {
            container.setLocation(x, y+5);
            //the +5 makes the container stay five fields underneath the crane; You can again use any value you want here;
        }
    }
 
 
}