Java 中的 ConcurrentHashMap compute()方法，示例
原文:https://www . geeksforgeeks . org/concurrenthashmap-compute-method-in-Java-with-examples/
ConcurrentHashMap 类的计算(键，双功能)方法用于计算指定键及其当前映射值的映射(如果没有找到当前映射，则为空)。
- This method is used to automatically update the value of a given key in ConcurrentHashMap.
- If the remapping function throws an exception, it throws an exception again, and the current mapping remains unchanged.
- During the calculation, the updating process of this mapping by other threads has been blocked, so the calculation must not try to update any other mappings of this mapping.
-
For example, this mapping appends the mapped string value:
```java ConcurrentHashMap.compute(key, (key, value) -> (value == null) ? msg : value.concat(msg))
```
语法:
public V
compute(K key,
BiFunction<? super K, ? super V,
? extends V> remappingFunction)
参数:该方法接受两个参数:
- 键:与数值相关联的键。
- 重编程功能:对数值进行运算的功能。
返回:该方法返回与指定键关联的新值，如果没有则返回空值。
异常:如果指定的键或 remappingFunction 为空，此方法将引发以下异常:
- [null pointer exception: .
- [T0】 llegalstatexception: 【T1] If the calculation attempts to recursively update the map, otherwise the update will never be completed.
- Runtime Exception: If remappingFunction does this, the mapping will not change in this case.
以下程序说明了计算(键，双功能)方法:
程序 1:
// Java program to demonstrate
// compute(Key, BiFunction) method.
import java.util.concurrent.*;
public class GFG {
// Main method
public static void main(String[] args)
{
// create a ConcurrentHashMap and add some values
ConcurrentHashMap<String, Integer>
map = new ConcurrentHashMap<>();
map.put("Book1", 10);
map.put("Book2", 500);
map.put("Book3", 400);
// print map details
System.out.println("ConcurrentHashMap: "
+ map.toString());
// remap the values of ConcurrentHashMap
// using compute method
map.compute("Book2", (key, val)
-> val + 100);
map.compute("Book1", (key, val)
-> val + 512);
// print new mapping
System.out.println("New ConcurrentHashMap: "
+ map);
}
}
输出:
ConcurrentHashMap: {Book3=400, Book1=10, Book2=500}
New ConcurrentHashMap: {Book3=400, Book1=522, Book2=600}
程序二:
// Java program to demonstrate
// compute(Key, BiFunction) method.
import java.util.concurrent.*;
public class GFG {
// Main method
public static void main(String[] args)
{
// create a ConcurrentHashMap and add some values
ConcurrentHashMap<Integer, String>
map = new ConcurrentHashMap<>();
map.put(1, "Kolkata");
map.put(2, "Nadia");
map.put(3, "Howrah");
// print map details
System.out.println("ConcurrentHashMap: "
+ map.toString());
// remap the values of ConcurrentHashMap
// using compute method
map.compute(2, (key, val)
-> val.concat(" (West-Bengal)"));
map.compute(3, (key, val)
-> val.concat(" (West-Bengal)"));
// print new mapping
System.out.println("New ConcurrentHashMap: "
+ map);
}
}
输出:
ConcurrentHashMap: {1=Kolkata, 2=Nadia, 3=Howrah}
New ConcurrentHashMap: {1=Kolkata, 2=Nadia (West-Bengal), 3=Howrah (West-Bengal)}
程序 3: 显示空指针异常
// Java program to demonstrate NullPointerException
// for compute(Key, BiFunction) method.
import java.util.concurrent.*;
public class GFG {
// Main method
public static void main(String[] args)
{
// create a ConcurrentHashMap and add some values
ConcurrentHashMap<Integer, String>
map = new ConcurrentHashMap<>();
map.put(1, "Kolkata");
map.put(2, "Nadia");
map.put(3, "Howrah");
try {
// remap the values of ConcurrentHashMap
// using compute method
map.compute(null, (key, val)
-> val.concat(" (West-Bengal)"));
}
catch (NullPointerException e) {
// print Exception
System.out.println("Exception: " + e);
}
}
}
输出:
Exception: java.lang.NullPointerException
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