为什么我的树状图没有按 java 排序？

Question

我有一个树状图：

private Map<String, Integer> dataMap = new TreeMap<String, Integer>();

稍后在代码中我有以下部分：

for (String item : data) {
    JSONObject itemJson = new JSONObject(item);
    System.out.println("-- before " + itemJson.getString("dataLabel") + " " + itemJson.getInt("dataValue"));
    dataMap.put(itemJson.getString("dataLabel"), itemJson.getInt("dataValue"));
}

for(String data : dataMap.keySet()) {
    System.out.println("-- after " + data);
}

第一个循环显示排序后的元素，如：

-- before January 1
-- before February 2
-- before March 3
-- before April 4
.
.
.

但是第二个循环混合了顺序：

-- after April
-- after February
-- after January
-- after March
.
.
.

为什么会这样？以及如何在第二个循环中获得排序结果？排序的意思是与我在第一个循环中添加它们的顺序相同吗？谢谢

Answer 1

TreeMap 按键而不是按值对条目进行排序。来自文档（强调我的）：

A Red-Black tree based NavigableMap implementation. The map is sorted according to the natural ordering of its keys (this is your case), or by a Comparator provided at map creation time, depending on which constructor is used.

输出是正确的，因为字符串是按字母顺序排序的。

how can I get sorted results on the second loop? And by sorted I mean the same order as I was adding them during the first loop?

改用 LinkedHashMap，因为根据插入的顺序存储条目。来自文档（强调我的）：

Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order). Note that insertion order is not affected if a key is re-inserted into the map.

Answer 2

keySet() 方法用于获取此映射中包含的键的 Set 视图。

for (String data : dataMap.keySet())

这给了你钥匙。

现在在该循环中，使用键 (data) 从 dataMap 获取每个值，并打印它。

检查以下示例

for (Map.Entry<K, V> data: dataMap.entrySet()) {
     System.out.println("Key: " + data.getKey() + ". Value: " + data.getValue());
}

或

HashMap 未定义其元素的任何特定顺序。因此，"reverse" 顺序也未定义。

对于TreeMap，您可以使用descendingMap()。

为什么我的树状图没有按 java 排序？

why does my treemap is not sorted in java?

java

hashmap

treemap