最后添加的元素将覆盖列表中的所有对象。 爪哇

Question

I've check the contents of res(the next state given action a) while in the loop and they are correct. It does add to the frontier list but the values are all the same, the last iteration of res.

public State BFSearch(){     
         State initial = new State(array);
         LinkedList<State> frontier = new LinkedList<State>();
         frontier.add(initial);


         while(!frontier.isEmpty()){
            State currentState = new State();
            currentState = frontier.removeFirst();

            if(goalTest(currentState)){
                System.out.println("goal");
                return currentState;
            }
            else{
                for (Point a : Actions(currentState)) {
                    State res = new State();
                    res = Result(currentState,a);
                    frontier.add(res);
                }

            }
         }
         return null; 
    }

Answer 1

I guess you're tried to implement Breadth-first search .

From your implementation it looks like you missed this step: if n is not in S: So it could works just if you don't have a loop in your graph. In other case, you'll stuck in infinite loop.

Also, a lot of errors in this code:

public State BFSearch(){     
     State initial = new State(array);
     LinkedList<State> frontier = new LinkedList<State>();
     frontier.add(initial); <-- initially you have 1 element in list


     while(!frontier.isEmpty()){
        State currentState = new State(); <-- You don't need to create new instance because you'll overwrite it on next step
        currentState = frontier.removeFirst(); <-- you remove initial element

        if(goalTest(currentState)){
            System.out.println("goal");
            return currentState;
        }
        else{
            for (Point a : Actions(currentState)) { <-- I guess Actions is method which returns N points
                State res = new State(); <-- You don't need to create instance because you overwrite it on next step
                res = Result(currentState,a); <-- is it method with just return state or you just missed `new` 
                 <-- here should be check for existent `res` in `frontier` -->

                frontier.add(res); <-- there you put N new steps
            }

        }
     }
     return null; 
}

Your implementation could looks like:

public State BFSearch() {
    State initial = new State(arrays);
    List<State> frontier = new LinkedList<>();
    frontier.add(initial);


    while (!frontier.isEmpty()) {
        State currentState = frontier.removeFirst();

        if (goalTest(currentState)) {
            System.out.println("goal");
            return currentState;
        } else {
            for (Point a : actions(currentState)) {
                State res = result(currentState, a);
                if (!frontier.contains(res)) {
                    frontier.add(res);
                }
            }

        }
    }
    return null;
}

Important Be sure that you implemented equals and hashCode methods properly in your State implementation

Answer 2

That's quite evident in the statement currentState = frontier.removeFirst(); As it is in while loop, eventually all the elements get removed. If you do not wish to remove, you may try the method peekFirst() .

Answer 3

Hard to follow exactly what it is your doing or what your question is but from what i gather you're removing the first element (from a list that is presumably a single element list at first), then you're checking to see if it matches a criteria, you're returning it from the function if it is, if not you're populating with elements based off another collection. Then repeating the above until the first element is the one you want? Have you tried printing out "Result(currentState,a)" in the loop and see what it gives you? or printing out the list before you enter the next iteration?