Java中的ConcurrentMap接口

ConcurrentMap是一个接口，它是Java Collections Framework 的成员，它在 JDK 1.5 中引入，表示能够处理对它的并发访问而不影响映射中条目的一致性的 Map。 Java.util.concurrent包中存在 ConcurrentMap 接口。除了从 SuperInterface 继承的方法（即Java.util.Map）之外，它还提供了一些额外的方法。它继承了嵌套接口 Map.Entry。

HashMap 操作不同步，而 Hashtable 提供同步。虽然 Hashtable 是线程安全的，但它的效率不是很高。为了解决这个问题， Java Collections Framework 在Java 1.5 中引入了ConcurrentMap 。

ConcurrentMap 的层次结构

宣言：

public interface ConcurrentMap extends Map

这里， K是 key Object 的类型， V是 value Object 的类型。

• 它扩展了Java中的 Map 接口。
• ConcurrentNavigableMap 是子接口。
• ConcurrentMap 由 ConcurrentHashMap、 ConcurrentSkipListMap类实现。
• ConcurrentMap 被称为同步 Map。

实现类

由于它属于Java.util.concurrent包，我们必须 import is using

import java.util.concurrent.ConcurrentMap
                or
import java.util.concurrent.*

ConcurrentMap 有两个实现类，它们是ConcurrentSkipListMap和ConcurrentHashMap 。 ConcurrentSkipListMap 是 ConcurrentNavigableMap 接口的可扩展实现，它扩展了 ConcurrentMap 接口。 ConcurrentSkipListMap 中的键按自然顺序或在构造对象时使用 Comparator 排序。 ConcurrentSkipListMap 的插入、删除和搜索操作的预期时间成本为log(n) 。它是一个线程安全的类，因此，所有的基本操作都可以同时完成。

句法：

// ConcurrentMap implementation by ConcurrentHashMap
CocurrentMap numbers = new ConcurrentHashMap();

// ConcurrentMap implementation by ConcurrentSkipListMap
ConcurrentMap< ? , ? > objectName = new ConcurrentSkipListMap< ? , ? >();

例子：

Java

// Java Program to illustrate methods
// of ConcurrentMap interface
import java.util.concurrent.*;
  
class ConcurrentMapDemo {
  
    public static void main(String[] args)
    {
        // Since ConcurrentMap is an interface,
        // we create instance using ConcurrentHashMap
        ConcurrentMap m = new ConcurrentHashMap();
        m.put(100, "Geeks");
        m.put(101, "For");
        m.put(102, "Geeks");
  
        // Here we cant add Hello because 101 key
        // is already present
        m.putIfAbsent(101, "Hello");
  
        // We can remove entry because 101 key
        // is associated with For value
        m.remove(101, "For");
  
        // Now we can add Hello
        m.putIfAbsent(101, "Hello");
  
        // We can replace Hello with For
        m.replace(101, "Hello", "For");
        System.out.println("Map contents : " + m);
    }
}

输出

Map contents : {100=Geeks, 101=For, 102=Geeks}

基本方法

1.添加元素

ConcurrentSkipListMap 的 put() 方法是Java中的一个内置函数，它将指定的值与此映射中的指定键相关联。如果映射先前包含键的映射，则替换旧值。

Java

// Java Program to demonstrate adding
// elements
  
import java.util.concurrent.*;
  
class AddingElementsExample {
    public static void main(String[] args)
    {
        // Instantiate an object
        // Since ConcurrentMap
        // is an interface so We use
        // ConcurrentSkipListMap
        ConcurrentMap mpp = new ConcurrentSkipListMap();
  
        // Adding elements to this map
          // using put() method
        for (int i = 1; i <= 5; i++)
            mpp.put(i, i);
  
        // Print map to the console
        System.out.println("After put(): " + mpp);
    }
}

输出

After put(): {1=1, 2=2, 3=3, 4=4, 5=5}

2.删除元素

ConcurrentSkipListMap 的 remove() 方法是Java中的一个内置函数，用于从该映射中删除指定键的映射。如果该特定键没有映射，则该方法返回 null。执行此方法后，地图的大小会减小。

Java

// Java Program to demonstrate removing
// elements
  
import java.util.concurrent.*;
  
class RemovingElementsExample {
    public static void main(String[] args)
    {
        // Instantiate an object
        // Since ConcurrentMap
        // is an interface so We use
        // ConcurrentSkipListMap
        ConcurrentMap mpp = new ConcurrentSkipListMap();
  
        // Adding elements to this map
        // using put method
        for (int i = 1; i <= 5; i++)
            mpp.put(i, i);
  
        // remove() mapping associated
        // with key 1
        mpp.remove(1);
  
        System.out.println("After remove(): " + mpp);
    }
}

输出

After remove(): {2=2, 3=3, 4=4, 5=5}

3. 访问元素

我们可以使用 get() 方法访问 ConcurrentSkipListMap 的元素，下面给出了这个示例。

Java

// Java Program to demonstrate accessing
// elements
  
import java.util.concurrent.*;
  
class AccessingElementsExample {
  
    public static void main(String[] args)
    {
  
        // Instantiate an object
        // Since ConcurrentMap
        // is an interface so We use
        // ConcurrentSkipListMap
        ConcurrentMap chm = new ConcurrentSkipListMap();
  
        // insert mappings using put method
        chm.put(100, "Geeks");
        chm.put(101, "for");
        chm.put(102, "Geeks");
        chm.put(103, "Contribute");
  
        // Displaying the HashMap
        System.out.println("The Mappings are: ");
        System.out.println(chm);
  
        // Display the value of 100
        System.out.println("The Value associated to "
                           + "100 is : " + chm.get(100));
  
        // Getting the value of 103
        System.out.println("The Value associated to "
                           + "103 is : " + chm.get(103));
    }
}

输出

The Mappings are: 
{100=Geeks, 101=for, 102=Geeks, 103=Contribute}
The Value associated to 100 is : Geeks
The Value associated to 103 is : Contribute

4. 遍历

我们可以使用 Iterator 接口来遍历 Collection Framework 的任何结构。由于迭代器使用一种类型的数据，我们使用 Entry< ? , ? > 将两种不同的类型解析为兼容的格式。然后使用 next() 方法我们打印 ConcurrentSkipListMap 的元素。

Java

import java.util.concurrent.*;
import java.util.*;
  
public class TraversingExample {
  
    public static void main(String[] args)
    {
  
        // Instantiate an object
        // Since ConcurrentMap
        // is an interface so We use
        // ConcurrentSkipListMap
        ConcurrentMap chmap = new ConcurrentSkipListMap();
  
        // Add elements using put()
        chmap.put(8, "Third");
        chmap.put(6, "Second");
        chmap.put(3, "First");
        chmap.put(11, "Fourth");
  
        // Create an Iterator over the
        // ConcurrentSkipListMap
        Iterator > itr
            = chmap.entrySet().iterator();
  
        // The hasNext() method is used to check if there is
        // a next element The next() method is used to
        // retrieve the next element
        while (itr.hasNext()) {
            ConcurrentSkipListMap
                .Entry entry
                = itr.next();
            System.out.println("Key = " + entry.getKey()
                               + ", Value = "
                               + entry.getValue());
        }
    }
}

输出

Key = 3, Value = First
Key = 6, Value = Second
Key = 8, Value = Third
Key = 11, Value = Fourth

ConcurrentMap 的方法

• K - 映射中键的类型。
• V – 映射中映射的值的类型。

METHOD	DESCRIPTION
compute​(K key, BiFunction  V,? extends V> remappingFunction)	Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
computeIfAbsent​(K key, Function mappingFunction)	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.
computeIfPresent​(K key, BiFunction remappingFunction)	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.
forEach​(BiConsumer action)	Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
getOrDefault​(Object key, V defaultValue)	Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.
merge​(K key, V value, BiFunction remappingFunction)	If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
putIfAbsent​(K key, V value)	If the specified key is not already associated with a value, associates it with the given value.
remove​(Object key, Object value)	Removes the entry for a key only if currently mapped to a given value.
replace​(K key, V value)	Replaces the entry for a key only if currently mapped to some value.
replace​(K key, V oldValue, V newValue)	Replaces the entry for a key only if currently mapped to a given value.
replaceAll​(BiFunction function)	Replaces each entry’s value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.

从接口Java .util.Map 继承的方法

METHOD	DESCRIPTION
clear​()	Removes all of the mappings from this map (optional operation).
containsKey​(Object key)	Returns true if this map contains a mapping for the specified key.
containsValue​(Object value)	Returns true if this map maps one or more keys to the specified value.
entry​(K k, V v)	Returns an immutable Map.Entry containing the given key and value.
entrySet​()	Returns a Set view of the mappings contained in this map.
equals​(Object o)	Compares the specified object with this map for equality.
get​(Object key)	Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
hashCode​()	Returns the hash code value for this map.
isEmpty​()	Returns true if this map contains no key-value mappings.
keySet​()	Returns a Set view of the keys contained in this map.
of​()	Returns an immutable map containing zero mappings.
of​(K k1, V v1)	Returns an immutable map containing a single mapping.
of​(K k1, V v1, K k2, V v2)	Returns an immutable map containing two mappings.
of​(K k1, V v1, K k2, V v2, K k3, V v3)	Returns an immutable map containing three mappings.
of​(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4)	Returns an immutable map containing four mappings.
of​(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5)	Returns an immutable map containing five mappings.
of​(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6)	Returns an immutable map containing six mappings.
of​(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7)	Returns an immutable map containing seven mappings.
of​(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8)	Returns an immutable map containing eight mappings.
of​(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9)	Returns an immutable map containing nine mappings.
of​(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10)	Returns an immutable map containing ten mappings.
ofEntries​(Map.Entry… entries)	Returns an immutable map containing keys and values extracted from the given entries.
put​(K key, V value)	Associates the specified value with the specified key in this map (optional operation).
putAll​(Map m)	Copies all of the mappings from the specified map to this map (optional operation).
remove​(Object key)	Removes the mapping for a key from this map if it is present (optional operation).
size​()	Returns the number of key-value mappings in this map.
values​()	Returns a Collection view of the values contained in this map.

参考： Java : Java