属性计算 Java 中缺少的(键、函数)方法，示例
原文:https://www . geeksforgeeks . org/properties-computeifexkey-function-method-in-Java-with-examples/
属性类的计算缺少(键，函数)方法，如果键还没有与值相关联(或被映射为空)，该方法允许您计算指定键的映射值。
- If the mapping function of this method returns null, the mapping is not recorded.
- If the remapping function throws an exception, it throws an exception again and records the no mapping.
- This method is not allowed to modify the map during the calculation.
- If the remapping function modifies this mapping during calculation, this method will throw a Concurrentmodification Exception.
语法:
public Object computeIfAbsent?(Object key,
Function mappingFunction)
参数:此方法接受两个参数:
- key : the key with which the value is to be associated.
- [T0】 remapingfunction 【T1]: the function of computing numerical values.
返回:此方法返回与指定键关联的当前(现有或计算的)值，如果映射返回空值，则返回空值。
异常:如果检测到重映射函数修改了该地图，则该方法抛出ConcurrentModificationException。
以下程序说明了计算无(键，函数)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());
}
}
输出:
Current Properties: {Book=500, Mobile=5000, Pen=10, Clothes=400}
New Properties: {newPen=600, Book=500, newBook=800, Mobile=5000, Pen=10, Clothes=400}
程序二:
// 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());
}
}
输出:
Current Properties: {3=1000RS, 2=500RS, 1=100RS}
New Properties: {4=600RS, 3=1000RS, 2=500RS, 1=100RS}
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