java - ArrayList#size() 的奇怪返回

Question

我试图在HashMap中获取一个随机ArrayList，当我查看HashMap中的所有ArrayList时，大小为一切都应该是 2。

当我尝试将 Hashmap 中的 ArrayList 添加到另一个中时，就会出现问题。然后它返回大小为 0，即使 HashMap 中所有列表的大小都是 2。有人明白为什么吗？

for(Point p : defensePossibilities.keySet())
    System.out.println(defensePossibilities.get(p).size());

ArrayList<Point> points = new ArrayList<>();
while(points == null) {
    try {
        int random = rnd.nextInt(defensePossibilities.size());
        points.addAll(
                defensePossibilities.get(random));
    } catch(Exception e) {}
}
System.out.println("PointsSize: " + points.size());

int piece2 = rnd.nextInt(points.size());

这是它在堆栈跟踪中返回的内容

2
2
2
2
2
2
2
2
PointsSize: 0
Exception in thread "Thread-0" java.lang.IllegalArgumentException: bound must be positive
    at java.util.Random.nextInt(Unknown Source)
    at me.xthegamerplayz.FirstGame.board.White_AI.move(White_AI.java:95)
    at me.xthegamerplayz.FirstGame.board.ChessBoard.tick(ChessBoard.java:29)
    at me.xthegamerplayz.FirstGame.board.ChessBoard.<init>(ChessBoard.java:21)
    at me.xthegamerplayz.FirstGame.Game.render(Game.java:124)
    at me.xthegamerplayz.FirstGame.Game.run(Game.java:70)
    at java.lang.Thread.run(Unknown Source)

第 95 行是

int piece2 = rnd.nextInt(

如果HashMap中所有ArrayList的大小都是2，为什么points的大小却是0？

Answer 1

将points == null替换为points.isEmpty()

points 变量不为 null，因为您在 while 循环之前为 points 变量分配了一个数组列表。我假设您想要做的是在点列表为空时进行检查。因此，进行建议的更改将解决您的问题。

for(Point p : defensePossibilities.keySet())
    System.out.println(defensePossibilities.get(p).size());

ArrayList<Point> points = new ArrayList<>();
while(points == null) { // points is not null, it is just empty
// replace points == null with points.isEmpty()
    try {
        int random = rnd.nextInt(defensePossibilities.size());
        points.addAll(
                defensePossibilities.get(random));
    } catch(Exception e) {}
}
System.out.println("PointsSize: " + points.size());

int piece2 = rnd.nextInt(points.size());