将值添加到 Map 中已存在的键的列表中 [英] Adding a value to a list to an already existing key in Map

问题描述

晚上！

我有以下地图:

HashMap<String, ArrayList> myMap = new HashMap<String, ArrayList>();

然后我向其中添加了以下数据:

I then added the following data to it:

ArrayList myList = new ArrayList();
myList.add("Test 1");
myList.add("Test 2");
myList.add("Test 3");
myMap.put("Tests", myList);

这给我留下了以下数据:

This left me with the following data:

关键:测试

值:测试 1、测试 2、测试 3

Values: Test 1, Test 2, Test 3

我的问题是，如何在我已经存在的键上添加新值?例如，如何将值Test 4"添加到我的键Tests"中.

My question is, how do I then add new values on to my already existing key? So for example, how could I add the value "Test 4" onto my key "Tests".

谢谢.

推荐答案

只需从地图中获取列表，然后将元素添加到列表中:

Simply get the list from the map and then add the element to the list:

ArrayList list = myMap.get("Tests");
list.add("Test4");

对于您的代码，还有其他一些需要注意的地方.首先，不要使用原始的输入 ArrayList.使用泛型:

There are some other things that can be remarked about your code. First of all, don't use the raw type ArrayList. Use generics:

HashMap<String, ArrayList<String>> myMap = new HashMap<String, ArrayList<String>>();

ArrayList<String> myList = new ArrayList<String>();
myList.add("Test 1");
myList.add("Test 2");
myList.add("Test 3");
myMap.put("Tests", myList);

第二，程序到接口，而不是实现.换句话说，程序使用接口Map 和List 而不是实现HashMap 和ArrayList.这是众所周知的 OO 编程原则，例如，如果需要，可以更轻松地切换到不同的实现.

Second, program to interfaces, not implementations. In other words, program using interfaces Map and List rather than the implementations HashMap and ArrayList. This is a well-known OO programming principle, which makes it for example easier to switch to a different implementation, if necessary.

Map<String, List<String>> myMap = new HashMap<String, List<String>>();

List<String> myList = new ArrayList<String>();
myList.add("Test 1");
myList.add("Test 2");
myList.add("Test 3");
myMap.put("Tests", myList);

最后，语法提示:如果您使用的是 Java 7 或更新版本，则可以使用 <> 并且不必重复类型参数:

Finally, a syntax tip: if you're using Java 7 or newer you can use <> and you don't have to repeat the type arguments:

Map<String, List<String>> myMap = new HashMap<>();

List<String> myList = new ArrayList<>();
myList.add("Test 1");
myList.add("Test 2");
myList.add("Test 3");
myMap.put("Tests", myList);

myMap.get("Tests").add("Test 4");

这篇关于将值添加到 Map 中已存在的键的列表中的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！