Solution to problem using breadth first :
Exception in thread "main" java.lang.NullPointerException
at SolvingProblem.isGoal(SolvingProblem.java:24)
at AbstractTreeSearch.solve(AbstractTreeSearch.java:31)
at EightP.main(EightP.java:15)

import java.util.ArrayList;
import java.util.Collection;
import java.util.Arrays;

public class SolvingProblem implements Problem {

State initialState = new State(State.arrayA);
State GoalState = new State(State.arrayG);

@Override
public Object getInitialState() {
    return initialState;
}

@Override
public boolean isGoal(Object state) {
    return state.equals(GoalState);
}

import java.util.ArrayList;
import java.util.Collection;

public abstract class AbstractTreeSearch implements Search {

Collection<Node> frontier;

public Node solve(Problem problem) {

    //initialize the search tree using the initial state of problem
    frontier = initFrontier();
    frontier.addAll(expand(new Node(problem.getInitialState()), problem));
    //Starting frontier
    boolean done = false;
    Node solution = null;
    while (!done) {
        if (frontier.isEmpty()) {
            System.out.println("Blank frontier");
            done = true;
        } else {
            Node node = chooseLeafNode(frontier, problem);
            //inspecting node
            if (problem.isGoal(node.getState())) {
                System.out.println("Solution found");
                System.out.println();
                solution = node;
                done = true;
            } else {
                //Expanding node, frontier is..
                frontier.addAll(expand(node, problem));

            }
        }
    }
    return solution;
}

SolvingProblem p2 = new SolvingProblem();
    System.out.println("Solution to problem using breadth first : ");
    System.out.println(new BreadthFirstTreeSearch().solve(p2).pathToString());