实例化map< String，List< String>>的方式为:在Java中 [英] The way to instantiate map<String, List<String>> in Java

问题描述

我想实例化Java中的 Map< String，List< String>>

I would like to instantiate Map<String, List<String>> in Java,

我尝试过

Map<String, List<String>> foo = new <String, List<String>>();

和

Map<String, List<String>> foo = new <String, ArrayList<String>>();

它们都不起作用.有人知道如何用Java实例化此映射吗?

None of them work. Does any one know how to instantiate this map in Java?

推荐答案

new HashMap<String, List<String>>();

或gparyani评论:

or as gparyani commented:

new HashMap<>(); // type inference

注意:每个条目都需要被实例化为值的列表.您不能get("myKey").add("some_string_for_this_key");第一次从列表中获取列表.

Note: each entry needs to be given an instantiated List as a value. You cannot get("myKey").add("some_string_for_this_key"); the very first time you get() a List from it.

因此，获取一个列表，检查它是否为空.

So, fetch a List, check if it's null.

如果为空，则创建一个新列表，向其中添加字符串，然后将列表放回去.如果它不是null，则添加它，或执行您想要的操作.

If it's null, make a new list, add the string to it, put the List back. If it's anything but null, add to it, or do what you want.

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