Java HashMap computeIfPresent(key, BiFunction)用法及代码示例

HashMap类的computeIfPresent(Key，BiFunction)方法，如果key已经与某个值关联(或映射为null)，则可以使用该方法为指定的key计算映射值。

如果此方法的映射函数返回null，则将删除该映射。
如果重新映射函数引发异常，则重新引发该异常，并且映射保持不变。
在计算过程中，不允许使用此方法修改此Map。

用法:

public Object computeIfPresent(Object key,
                  BiFunction remappingFunction)

参数：此方法接受两个参数：

key与值关联的：key。
remappingFunction：function对值进行运算。

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。

以下示例程序旨在说明computeIfPresent(Key，BiFunction)方法：

范例1：本示例说明了 key 不在哈希图中的情况。

// Java program to demonstrate 
// computeIfPresent(Key, BiFunction) method. 
  
import java.util.concurrent.*; 
import java.util.*; 
  
public class GFG { 
    public static void main(String[] args) 
    { 
  
        // Create a HashMap and add some values 
        HashMap<String, Integer> wordCount = new HashMap<>(); 
        wordCount.put("Geeks", 1); 
        wordCount.put("for", 2); 
        wordCount.put("geeks", 3); 
  
        // print HashMap details 
        System.out.println("Hashmap before operation:\n "
                           + wordCount); 
  
        // provide new value for keys which is present 
        // using computeIfPresent method 
        wordCount.computeIfPresent("Geek", 
                                   (key, val) -> val + 100); 
  
        // print new mapping 
        System.out.println("HashMap after operation:\n "
                           + wordCount); 
    } 
}

输出：

Hashmap before operation:
 {geeks=3, Geeks=1, for=2}
HashMap after operation:
 {geeks=3, Geeks=1, for=2}

范例2：此示例说明了哈希映射中存在键的情况。

// Java program to demonstrate 
// computeIfPresent(Key, BiFunction) method. 
  
import java.util.concurrent.*; 
import java.util.*; 
  
public class GFG { 
    public static void main(String[] args) 
    { 
  
        // Create a HashMap and add some values 
        HashMap<String, Integer> wordCount = new HashMap<>(); 
        wordCount.put("Geeks", 1); 
        wordCount.put("for", 2); 
        wordCount.put("geeks", 3); 
  
        // print HashMap details 
        System.out.println("Hashmap before operation:\n "
                           + wordCount); 
  
        // provide new value for keys which is present 
        // using computeIfPresent method 
        wordCount.computeIfPresent("for", 
                                   (key, val) -> val + 1); 
  
        // print new mapping 
        System.out.println("HashMap after operation:\n "
                           + wordCount); 
    } 
}

输出：

Hashmap before operation:
 {geeks=3, Geeks=1, for=2}
HashMap after operation:
 {geeks=3, Geeks=1, for=3}

参考： https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html#computeIfPresent-K-java.util.function.BiFunction-

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注：本文由纯净天空筛选整理自piyush25pv大神的英文原创作品 HashMap computeIfPresent(key, BiFunction) method in Java with Examples。非经特殊声明，原始代码版权归原作者所有，本译文未经允许或授权，请勿转载或复制。