Java Hashtable computeIfPresent()方法

java.util.Hashtable.computeIfPresent() 如果指定键的值存在且非空，则用于给定键及其当前映射值的情况下计算新映射。

1 语法

public Object computeIfPresent(Object key,BiFunction remappingFunction)

2 参数

key：与值关联的键。

remappingFunction：对值进行运算的函数。

3 返回值

此方法返回与指定键关联的新的重新映射的值；如果映射返回null，则返回null。

4 示例 

package com.yiidian;

/**
 * 一点教程网： http://www.yiidian.com
 */
/**
 * java.util.Hashtable.computeIfPresent()方法的例子
 */
import java.util.*;

public class Demo {
    public static void main(String args[]) {

        // Create a Hashtable and add some values
        Hashtable<String, Integer> wordCount = new Hashtable<>();
        wordCount.put("Yiidian", 1);
        wordCount.put("for", 2);
        wordCount.put("Java", 3);

        // print Hashtable details
        System.out.println("Hashtable before operation :\n "
                + wordCount);

        // provide new value for keys which is present
        // using computeIfPresent method
        wordCount.computeIfPresent("Yiidian",
                (key, val) -> val + 100);

        // print new mapping
        System.out.println("Hashtable after operation :\n " + wordCount);
    }
}

输出结果为：

Hashtable before operation :
 {Java=3, for=2, Yiidian=1}
Hashtable after operation :
 {Java=3, for=2, Yiidian=101}