Java Properties computeIfAbsent(Key, Function)用法及代碼示例

Properties類的computeIfAbsent(Key，Function)方法，如果key尚未與某個值關聯(或映射為null)，則該方法允許您為指定的key計算映射值。

如果此方法的映射函數返回null，則不記錄任何映射。
如果重新映射函數引發異常，則重新引發該異常，並且不記錄任何映射。
在計算過程中，不允許使用此方法修改此Map。
如果重新映射函數在計算過程中修改了此映射，則此方法將引發ConcurrentModificationException。

用法:

public Object computeIfAbsent?(Object key,
                              Function mappingFunction)

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當前(現有的或計算的)值；如果映射返回null，則返回null。

異常：如果檢測到重新映射函數修改了此映射，則此方法將引發ConcurrentModificationException。

以下示例程序旨在說明computeIfAbsent(Key，Function)方法：

程序1:

// Java program to demonstrate 
// computeIfAbsent(Key, Function) method. 
  
import java.util.*; 
  
public class GFG { 
  
    // Main method 
    public static void main(String[] args) 
    { 
  
        // Create a properties and add some values 
        Properties properties = new Properties(); 
        properties.put("Pen", 10); 
        properties.put("Book", 500); 
        properties.put("Clothes", 400); 
        properties.put("Mobile", 5000); 
  
        // print Properties details 
        System.out.println("Current Properties:"
                           + properties.toString()); 
  
        // provide value for new key which is absent 
        // using computeIfAbsent method 
        properties.computeIfAbsent("newPen", k -> 600); 
        properties.computeIfAbsent("newBook", k -> 800); 
  
        // print new mapping 
        System.out.println("New Properties:"
                           + properties.toString()); 
    } 
}

輸出：

Current Properties:{Book=500, Mobile=5000, Pen=10, Clothes=400}
New Properties:{newPen=600, Book=500, newBook=800, Mobile=5000, Pen=10, Clothes=400}

程序2:

// Java program to demonstrate 
// computeIfAbsent(Key, Function) method. 
  
import java.util.*; 
  
public class GFG { 
  
    // Main method 
    public static void main(String[] args) 
    { 
  
        // Create a properties and add some values 
        Properties properties = new Properties(); 
  
        properties.put(1, "100RS"); 
        properties.put(2, "500RS"); 
        properties.put(3, "1000RS"); 
  
        // print Properties details 
        System.out.println("Current Properties:"
                           + properties.toString()); 
  
        // provide value for new key which is absent 
        // using computeIfAbsent method 
        properties.computeIfAbsent(4, k -> "600RS"); 
  
        // this will not effect anything 
        // because key 1 is present 
        properties.computeIfAbsent(1, k -> "800RS"); 
  
        // print new mapping 
        System.out.println("New Properties:"
                           + properties.toString()); 
    } 
}

輸出：

Current Properties:{3=1000RS, 2=500RS, 1=100RS}
New Properties:{4=600RS, 3=1000RS, 2=500RS, 1=100RS}

參考：https：//docs.oracle.com/javase/9/docs/api/java/util/Properties.html#computeIfAbsent-java.lang.Object-java.util.function.Function-

相關用法

注：本文由純淨天空篩選整理自Kirti_Mangal大神的英文原創作品 Properties computeIfAbsent(Key, Function) 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。