Java HashMap computeIfPresent(key, BiFunction)方法及示例

HashMap类的 computeIfPresent(Key, BiFunction) 方法允许你计算指定键的映射值，如果键已经与一个值相关联（或被映射为null）。

• 如果此方法的映射函数返回null，则映射被删除。
• 如果重映射函数抛出一个异常，该异常会被重新抛出，而映射则保持不变。
• 在计算过程中，不允许使用此方法修改此映射。

语法

public Object computeIfPresent(Object key,
                  BiFunction remappingFunction)

参数： 该方法接受两个参数。

• key ：将与该值关联的键。
• remappingFunction ：对值进行操作的函数。

返回： 该方法返回 与指定的键相关联的新的重映射值，如果映射返回空，则返回空

下面的程序说明了 computeIfPresent(Key, BiFunction) 方法。

例子1：这个例子演示了当键不在hashhmap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}

例子2：这个例子展示了密钥存在于哈希姆表中的情况。

// 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("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}

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