为什么我的第二个 while 循环没有执行？

Question

public void compare(ArrayList list_old, ArrayList list_new) {
        try {
            Iterator<User> iterator_old = list_old.iterator();
            Iterator<User> iterator_new = list_new.iterator();

            //Check New User Is Added
            while (iterator_new.hasNext()) {
                Log.i("Test", "inside!");
                User user_new = iterator_new.next();
                boolean NEW = true;

                while (iterator_old.hasNext() && NEW) {
                    User user_old = iterator_old.next();
                    if (user_new.getUsername().equals(user_old.getUsername())) {
                        NEW = false;
                    }
                }
                if (NEW) {
                    generateNotification(getApplicationContext(), user_new.getUsername() + " has been added.");
                }
            }
            //Check User Is Removed
            while (iterator_old.hasNext()) {
                Log.i("Test", "inside");
                User user_old = iterator_old.next();
                boolean NEW = true;

                while (iterator_new.hasNext() && NEW) {
                    User user_new = iterator_new.next();
                    if (user_old.getUsername().equals(user_new.getUsername())) {
                        NEW = false;
                    }
                }
                if (NEW) {
                    generateNotification(getApplicationContext(), user_old.getUsername() + " has been removed.");
                }
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

在这个函数中，我将旧列表与新列表进行比较。
第一个 while 循环检查是否添加了任何新用户，第二个 while 循环检查是否删除了任何用户。
当我运行这个应用程序只执行我的第一个 while 循环，它不执行第二个 while 循环。
我检查了 Log.i("Test", "inside!");我发现它不在我的第二个 while 循环中。

请帮忙

Answer 1

您需要重新初始化迭代器，

 public void compare(ArrayList list_old, ArrayList list_new) {
        try {
            Iterator<User> iterator_old;
            Iterator<User> iterator_new = list_new.iterator();

            //Check New User Is Added
            while (iterator_new.hasNext()) {
                Log.i("Test", "inside!");
                User user_new = iterator_new.next();
                boolean NEW = true;
                iterator_old = list_old.iterator();
                while (iterator_old.hasNext() && NEW) {
                    User user_old = iterator_old.next();
                    if (user_new.getUsername().equals(user_old.getUsername())) {
                        NEW = false;
                    }
                }
                if (NEW) {
                    generateNotification(getApplicationContext(), user_new.getUsername() + " has been added.");
                }
            }
            //initialize again
            iterator_old = list_old.iterator();
            //Check User Is Removed
            while (iterator_old.hasNext()) {
                Log.i("Test", "inside");
                User user_old = iterator_old.next();
                boolean NEW = true;
                 iterator_new = list_new.iterator();    
                while (iterator_new.hasNext() && NEW) {
                    User user_new = iterator_new.next();
                    if (user_old.getUsername().equals(user_new.getUsername())) {
                        NEW = false;
                    }
                }
                if (NEW) {
                    generateNotification(getApplicationContext(), user_old.getUsername() + " has been removed.");
                }
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

Answer 2

您没有在任何地方重置您的迭代器，所以我认为任何代码都没有按您希望的那样工作。如果您使用所有 Iterable 类支持的 for-each 循环，您的代码可能会工作并且更具可读性。这将避免完全创建和重置迭代器的需要。

for (User newUser : list_new) {
    for (User oldUser : list_old) 
        // Compare
}

for (User oldUser : list_old) {
    for (User newUser : list_new) 
        // Compare
}

为什么我的第二个 while 循环没有执行？

Why my 2nd while loop in not executing?

java

while-loop