import greenfoot.*;  
import java.awt.Color; 

public class enemy  extends Actor
{
    private int size=5; // move this up here

    public enemy() // had a semicolon here
    {
        size=Greenfoot.getRandomNumber(274);
    }
    
    public void act()
    {
        // enemy(); - you can't call your constructor
        move();
        lookForRocket();
    }

    public boolean canSee(Class clss)
    {
        Actor actor = getOneObjectAtOffset(0, 0, clss);
        return actor != null;        
    }

    public void eat(Class clss)
    {
        Actor actor = getOneObjectAtOffset(0, 0, clss);
        if(actor != null) {
            getWorld().removeObject(actor);
        }
    } // forgot this bracket   

    public void move()
    {
        setLocation(getX()+2,getY());
    }

    public void lookForRocket()
    {
        if ( canSee(Rocket.class) ) 
        {
            eat(Rocket.class);
        }
    }
}

public void act() {
    //write this at the end of your act method; otherwhile there might be exceptions;
    if (atWorldEdge()) {
        getWorld().removeObject(this);
    }
}


    public boolean atWorldEdge() {
        if(getX() < 20 || getX() > getWorld().getWidth() - 20) {
            return true;
        }
        if(getY() < 20 || getY() > getWorld().getHeight() - 20) {
            return true;
        }
        else {
            return false;
        }
    }

//first you have to import java.util.List;

//in your rocket class;
public void act() {
    if (enemyHit()) {
        //whatever should happen if the game is over;
    }
}

public boolean enemyHit() {
    List<Enemy> enemiesInWorld = getWorld().getObjects(Enemy.class);//Enemy has to be the classname; If this is not the right classname you have to change it;
    for (Enemy enemy : enemiesInWorld) {
        if (intersects(enemy)) {
            return true;
        }
    }
    return false;
}