HashMap 的 clone()、putIfAbsent()、computeIfAbsent()、computeIfPresent() Java 中的方法（示例）

已发表: 2021-05-11

Java中HashMap的clone()、putIfAbsent()、computeIfAbsent()、computeIfPresent()方法（附示例）

在本 Java 教程中，我们将详细介绍如何使用 HashMap 的以下方法：

克隆（）
putIfAbsent(K 键，V 值)
computeIfAbsent(K key, java.util.function.Function mappingFunction)
computeIfPresent(K key, java.util.function.BiFunction BiFunction )

hashMap.clone():

返回此 HashMap 实例的浅表副本：键和值本身没有被克隆。

hashMap.putIfAbsent():

如果指定的键尚未与值关联（或映射到 null），则将其与给定值关联并返回 null，否则返回当前值。

hashMap.computeIfAbsent():

如果指定的键尚未与值关联（或映射为 null），则尝试使用给定的映射函数计算其值并将其输入到此映射中，除非为 null。

hashMap.computeIfPresent():

如果指定键的值存在且非空，则尝试在给定键及其当前映射值的情况下计算新映射。

让我们开始吧：

创建 Java 类CrunchifyCloneHashmap .java
将以下代码放入其中

package crunchify . com . java . tutorials ;

import java . util . HashMap ;

/**

* @author Crunchify.com

* HashMap's clone(), putIfAbsent(), computeIfAbsent(), computeIfPresent() Methods in Java (Example attached)

public class CrunchifyCloneHashmap {

public static void main ( String [ ] args ) {

// Create our first Hashmap crunchifyHashMap

HashMap < String , Integer > crunchifyHashMap = new HashMap < > ( ) ;

// Add elements to HashMap crunchifyHashMap

crunchifyHashMap . put ( "Crunchify" , 11 ) ;

crunchifyHashMap . put ( "Apple" , 12 ) ;

crunchifyHashMap . put ( "Twitter" , 13 ) ;

crunchifyHashMap . put ( "Facebook" , 14 ) ;

crunchifyHashMap . put ( "Chase" , 15 ) ;

crunchifyPrint ( "crunchifyHashMap Details: " + crunchifyHashMap ) ;

HashMap < String , Integer > crunchifyHashMapNew ;

// Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.

crunchifyHashMapNew = ( HashMap < String , Integer > ) crunchifyHashMap . clone ( ) ;

// If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value.

crunchifyHashMapNew . putIfAbsent ( "Crunchify" , 16 ) ;

crunchifyHashMapNew . putIfAbsent ( "Google" , 17 ) ;

crunchifyHashMapNew . put ( "WordPress" , 18 ) ;

// If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.

//If the mapping function returns null, no mapping is recorded. If the mapping function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in:

// map.computeIfAbsent(key, k -> new Value(f(k)));

//Or to implement a multi-value map, Map<K,Collection<V>>, supporting multiple values per key:

// map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);

crunchifyHashMapNew . computeIfAbsent ( "Paypal" , k - > 100 + 200 ) ;

// If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.

//If the remapping function returns null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

//The remapping function should not modify this map during computation.

//This method will, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation.

crunchifyHashMapNew . computeIfPresent ( "Crunchify" , ( key , val ) - > 300 ) ;

crunchifyPrint ( "crunchifyHashMapNew Details: " + crunchifyHashMapNew ) ;

// Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation),

// the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via

// the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

crunchifyPrint ( "crunchifyHashMap keyset(): " + crunchifyHashMap . keySet ( ) ) ;

crunchifyPrint ( "crunchifyHashMapNew keyset(): " + crunchifyHashMapNew . keySet ( ) ) ;

// Returns the number of key-value mappings in this map.

crunchifyPrint ( "crunchifyHashMap size(): " + crunchifyHashMap . size ( ) ) ;

crunchifyPrint ( "crunchifyHashMapNew size(): " + crunchifyHashMapNew . size ( ) ) ;

}

private static void crunchifyPrint ( String crunchifyString ) {

System . out . println ( "===> " + crunchifyString + "\n" ) ;

}

在上面的 Java 代码中，你不会看到 crunchifyHashMapNew.putIfAbsent(“Crunchify”, 16); 添加到新的 HashMap。

只需在 Eclipse IDE 或 IntelliJ IDEA 中将上述程序作为 Java 应用程序运行。

您应该会看到与此类似的控制台结果：

/ Library / Java / JavaVirtualMachines / jdk - 15.jdk / Contents / Home / bin / java - javaagent : / Applications / IntelliJ 5.1.3.RELEASE.jar crunchify . com . java . tutorials . CrunchifyCloneHashmap

=== > crunchifyHashMap Details : { Apple = 12 , Twitter = 13 , Facebook = 14 , Crunchify = 11 , Chase = 15 }

=== > crunchifyHashMapNew Details : { Paypal = 300 , Google = 17 , Apple = 12 , WordPress = 18 , Twitter = 13 , Facebook = 14 , Crunchify = 300 , Chase = 15 }

=== > crunchifyHashMap keyset ( ) : [ Apple , Twitter , Facebook , Crunchify , Chase ]

=== > crunchifyHashMapNew keyset ( ) : [ Paypal , Google , Apple , WordPress , Twitter , Facebook , Crunchify , Chase ]

=== > crunchifyHashMap size ( ) : 5

=== > crunchifyHashMapNew size ( ) : 8

Process finished with exit code 0

关于 computeIfAbsent()、computeIfPresent() 方法的 Hashmap 详细信息

如果您在运行代码时遇到任何问题，请告诉我。