java 在 HashMap 中查找值的数量？
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Finding number of values in a HashMap?
提问by java123999
What is the best / most efficient way to find the total number of Valuesin a HashMap.
什么是发现的总数最好/最有效的方式值的HashMap。
I do not mean the .size()method as it counts the number of keys. I want the total number of valuesin all the keys.
我的意思不是.size()方法，因为它计算键的数量。我想要所有键中值的总数。
I want to do so as my key is a String, but my value is a List.
我想这样做，因为我的键是 a String，但我的值是 a List。
采纳答案by Suresh Atta
Easiest would be, iterate and add over list sizes.
最简单的方法是迭代并添加列表大小。
int total = 0;
for (List<Foo> l : map.values()) {
total += l.size();
}
// here is the total values size
回答by Clashsoft
In Java 8, you can also utilize the StreamAPI:
在 Java 8 中，您还可以使用StreamAPI：
int total = map.values()
.stream()
.mapToInt(List::size) // or (l -> l.size())
.sum()
This has the advantage that you don't have to repeat the List<Foo>type for a forvariable, as in the pre-Java 8 solution:
这样做的好处是您不必像在 Java 8 之前的解决方案中那样重复变量的List<Foo>类型for：
int total = 0;
for (List<Foo> list : map.values())
{
total += list.size();
}
System.out.println(total);
In addition to that, although not advised, you could also use that value inline without needing a temp variable:
除此之外，虽然不建议这样做，但您也可以在不需要临时变量的情况下内联使用该值：
System.out.println(map.values().stream().mapToInt(List::size).sum());
回答by Mureinik
If I understand the question correctly, you have a Map<String, List<Something>>, and you want to count the total number of items in all the Lists in the Map's values. Java 8 offers a pretty easy way of doing this by streaming the values, mapping them to their size()and then just summing them:
如果我正确理解了这个问题，那么您有一个Map<String, List<Something>>，并且您想计算's 值中所有Lists 中的项目总数Map。Java 8 通过流式传输值，将它们映射到它们size()，然后将它们相加，提供了一种非常简单的方法来做到这一点：
Map<String, List<Something>> map = ...;
int totalSize = map.values().stream().mapToInt(List::size).sum());
回答by dinomario10
Say you have a map
假设你有一张地图
Map<String, List<Object>> map = new HashMap<>();
You can do this by calling the values()method and calling size()method for all the lists:
您可以通过为所有列表调用values()方法和调用size()方法来执行此操作：
int total = 0;
Collection<List<Object>> listOfValues = map.values();
for (List<Object> oneList : listOfValues) {
total += oneList.size();
}
回答by Tunaki
Starting with Java 8, given a Map<K, List<V>> mapyou could use the Stream API and have:
从 Java 8 开始，假设Map<K, List<V>> map您可以使用 Stream API 并具有：
int size = map.values().stream().mapToInt(List::size).sum();
This creates a Streamof the values with stream(), maps each of them to their size with mapToInt, where the mapper is the method reference List::sizerefering to List#size(), and sum the results with sum().
这就产生了一个Stream与所述值stream()，其中每一个分别映射到它们的大小与mapToInt，其中所述映射器是该方法的参考List::size指的List#size()，总结的结果sum()。
回答by itohro
With Eclipse Collections, the following will work using MutableMap.
使用Eclipse 集合，以下内容将使用MutableMap工作。
MutableMap<String, List<String>> map =
Maps.mutable.of("key1", Lists.mutable.of("a", "b", "c"),
"key2", Lists.mutable.of("d", "e", "f", "g"));
long total = map.sumOfInt(List::size);
Note: I am a committer for Eclipse Collections.
注意：我是 Eclipse Collections 的提交者。
回答by Mostafa Hussien
import java.util.HashMap;
public class Solution {
public static void main(String args[]) {
int total = 0;
HashMap<String,String> a = new HashMap<String,String>();
a.put("1.","1");
a.put("2.","11");
a.put("3.","21");
a.put("4.","1211");
a.put("5.","111221");
for (String l : a.values()) {
total ++;
}
System.out.println(total);
}
}
