Question

我知道调试后儿子列表被更改了我不知道为什么会改变这个修改导致我的算法出现问题

public class IDS extends Algo {
public State found = null;

public IDS(char[][] input, int size) {
    super(input, size);
}

public String solve() {
    State root = new State(0, 0, 0, 0, "", 0, 0);
    for (int i = 0; i < size * size; i++) {
        found = DLS(root, i);
        if (found != null) {
            return found.path + " " + found.dist;
        }
    }
    return "no path";
}

private State DLS(State node, int depth) {
    if (depth == 0 && super.inp[node.row][node.col] == 'G') {
        return node;
    }
    if (depth > 0) {
        // List<State> sons = super.find_neighbors(node.row, node.col, node.prod_time);
        // for (int j = 0; j < sons.size(); j++) {
        //State cur = sons.get(j);
        for(State cur : super.find_neighbors(node.row, node.col, node.prod_time)){
            cur.addCost(node.dist);
            cur.path = cur.addpath(node.path);
            found = DLS(cur, depth - 1);
            if (found != null) {
                return found;
            }
        }
    }
    return null;
}

}

这是抽象类：

abstract class Algo {
public static Map<Integer,String> ways = new TreeMap<>();
protected char[][]inp;
protected int size;
private Map<Character,Integer>costs = new HashMap<>();
public List<State>sons = new ArrayList<>();

public Algo(){}
public Algo(char[][] input,int size){
   inp = input;
    this.size = size;
    ways.put(1,"R");
    ways.put(2,"RD");
    ways.put(3,"D");
    ways.put(4,"LD");
    ways.put(5,"L");
    ways.put(6,"LU");
    ways.put(7,"U");
    ways.put(8,"RU");
    costs.put('R',1);
    costs.put('D',3);
    costs.put('H',10);
    costs.put('G',0);
    costs.put('S',0);
}
private void addDirection(int row,int col,int prod, int loc){
    int cos = costs.get(inp[row][col]);
    State s = new State(row,col,0,prod,"",loc,cos);
    sons.add(s);
}
public abstract String solve();
//gets the parent i,j and the prod_t of the sons
public List<State> find_neighbors(int i, int j,int prod_t){
    List<String>openD = new ArrayList<>();
    sons.clear();

    if(j+1 < size) {//right
        if (inp[i][j + 1] != 'W') {
            addDirection(i, j + 1, prod_t, 1);
            openD.add("R");
        }
    }
    if(i-1 >= 0) {
        if (inp[i - 1][j] != 'W') {//up
            addDirection(i - 1, j, prod_t, 7);
            openD.add("U");
        }
    }
    if(i+1 < size) {
        if (inp[i + 1][j] != 'W') {//down
            addDirection(i + 1, j, prod_t, 3);
            openD.add("D");
        }
    }
    if(j-1 >= 0){
        if(inp[i][j-1]!='W'){//left
            addDirection(i,j-1,prod_t,5);
            openD.add("L");
        }
    }

    if(openD.contains("L")){
        if(openD.contains("U")){
            if(inp[i-1][j-1]!='W'){
                addDirection(i-1,j-1,prod_t,6);
            }
        }
        if(openD.contains("D")){
            if(inp[i+1][j-1]!='W'){
                addDirection(i+1,j-1,prod_t,4);
            }
        }
    }
    if(openD.contains("R")){
        if(openD.contains("U")) {
            if (inp[i - 1][j + 1] != 'W') {
                addDirection(i - 1, j + 1, prod_t, 8);
            }
        }
        if(openD.contains("D")){
            if(inp[i+1][j+1]!='W'){
                addDirection(i+1,j+1,prod_t,2);
            }
        }
    }
    return sons;
}

}

异常只是帮助我知道列表已经修改但是仍然不知道原因：

Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:859)
at java.util.ArrayList$Itr.next(ArrayList.java:831)
at IDS.DLS(IDS.java:33)
at IDS.solve(IDS.java:17)
at ex1.main(ex1.java:34)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

Answer 1

super.find_neighbors 修改并返回 sons 列表。在循环内部，递归调用方法 DLS ，再次调用 super.find_neighbors （修改当前迭代上面一次递归的相同列表）。因为在迭代过程中更改了列表，所以抛出了异常。

Answer 2

你在Algo.find_neighbours（）里面做了一个sons.clear（）这将导致循环中的ConcurrentModification。此外，由于find_neighbours（）总是返回一个空列表，那么试图迭代它的是什么？

递归中的java.util.ConcurrentModificationException

2 个答案: