Java HashMap computeIfPresent(key, BiFunction)用法及代碼示例

HashMap類的computeIfPresent(Key，BiFunction)方法，如果key已經與某個值關聯(或映射為null)，則可以使用該方法為指定的key計算映射值。

如果此方法的映射函數返回null，則將刪除該映射。
如果重新映射函數引發異常，則重新引發該異常，並且映射保持不變。
在計算過程中，不允許使用此方法修改此Map。

用法:

public Object computeIfPresent(Object key,
                  BiFunction remappingFunction)

參數：此方法接受兩個參數：

key與值關聯的：key。
remappingFunction：function對值進行運算。

返回值：此方法返回與指定鍵關聯的新的重新映射的值；如果映射返回null，則返回null。

以下示例程序旨在說明computeIfPresent(Key，BiFunction)方法：

範例1：本示例說明了 key 不在哈希圖中的情況。

// 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); 
    } 
}

輸出：

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

範例2：此示例說明了哈希映射中存在鍵的情況。

// 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); 
    } 
}

輸出：

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

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

相關用法

注：本文由純淨天空篩選整理自piyush25pv大神的英文原創作品 HashMap computeIfPresent(key, BiFunction) method in Java with Examples。非經特殊聲明，原始代碼版權歸原作者所有，本譯文未經允許或授權，請勿轉載或複製。