Properties类的computeIfAbsent(Key,Function)方法,如果key尚未与某个值关联(或映射为null),则该方法允许您为指定的key计算映射值。
- 如果此方法的映射函数返回null,则不记录任何映射。
- 如果重新映射函数引发异常,则重新引发该异常,并且不记录任何映射。
- 在计算过程中,不允许使用此方法修改此Map。
- 如果重新映射函数在计算过程中修改了此映射,则此方法将引发ConcurrentModificationException。
用法:
public Object computeIfAbsent?(Object key, Function mappingFunction)
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当前(现有的或计算的)值;如果映射返回null,则返回null。
异常:如果检测到重新映射函数修改了此映射,则此方法将引发ConcurrentModificationException。
以下示例程序旨在说明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());
}
}
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());
}
}
输出:
Current Properties:{3=1000RS, 2=500RS, 1=100RS} New Properties:{4=600RS, 3=1000RS, 2=500RS, 1=100RS}
参考:https://docs.oracle.com/javase/9/docs/api/java/util/Properties.html#computeIfAbsent-java.lang.Object-java.util.function.Function-
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注:本文由纯净天空筛选整理自Kirti_Mangal大神的英文原创作品 Properties computeIfAbsent(Key, Function) 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。