Java中的ConcurrentNavigableMap接口

ConcurrentNavigableMap接口是Java集合框架的成员。它从 NavigableMap 接口和 ConcurrentMap 接口扩展而来。 ConcurrentNavigableMap 提供对地图元素的线程安全访问以及方便的导航方法。它属于Java.util.concurrent包。

宣言：

public interface ConcurrentNavigableMap extends ConcurrentMap, NavigableMap

这里， K是键对象类型， V是值对象类型。

ConcurrentNavigableMap 的层次结构

它实现了 ConcurrentMap、Map、NavigableMap、SortedMap 接口。 ConcurrentSkipListMap实现 ConcurrentNavigableMap。

例子：

Java

// Java Program to demonstrate the
// ConcurrentNavigableMap Interface
import java.util.concurrent.ConcurrentNavigableMap;
import java.util.concurrent.ConcurrentSkipListMap;
 
public class GFG {
 
    public static void main(String[] args)
    {
 
        // Instantiate an object
        // Since ConcurrentNavigableMap
        // is an interface so We use
        // ConcurrentSkipListMap
        ConcurrentNavigableMap cnmap
            = new ConcurrentSkipListMap();
 
        // Add elements using put() method
        cnmap.put(1, "First");
        cnmap.put(2, "Second");
        cnmap.put(3, "Third");
        cnmap.put(4, "Fourth");
 
        // Print the contents on the console
        System.out.println(
            "Mappings of ConcurrentNavigableMap : "
            + cnmap);
 
        System.out.println("HeadMap(3): "
                           + cnmap.headMap(3));
        System.out.println("TailMap(3): "
                           + cnmap.tailMap(3));
        System.out.println("SubMap(1, 3): "
                           + cnmap.subMap(1, 3));
    }
}

输出：

Mappings of ConcurrentNavigableMap : {1=First, 2=Second, 3=Third, 4=Fourth}
HeadMap(3): {1=First, 2=Second}
TailMap(3): {3=Third, 4=Fourth}
SubMap(1, 3): {1=First, 2=Second}

实现类

ConcurrentNavigableMap 有一个实现类，即ConcurrentSkipListMap 类。 ConcurrentSkipListMap 是 ConcurrentNavigableMap 接口的可扩展实现。 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：

ConcurrentSkipListMap< ? , ? > objectName = new ConcurrentSkipListMap< ? , ? >();

示例：在下面给出的代码中，我们简单地实例化一个名为 cslmap 的 ConcurrentSkipListMap 类的对象。 put() 方法用于添加元素， remove() 用于删除元素。对于 remove() 方法，语法是objectname.remove(Object key) 。 keySet() 显示映射中的所有键（上面给出的方法表中的描述）。

Java

// Java Program to demonstrate the ConcurrentSkipListMap
import java.util.concurrent.*;
 
public class ConcurrentSkipListMapExample {
 
    public static void main(String[] args)
    {
 
        // Instantiate an object of
        // ConcurrentSkipListMap named cslmap
        ConcurrentSkipListMap cslmap
            = new ConcurrentSkipListMap();
 
        // Add elements using put()
        cslmap.put(1, "Geeks");
        cslmap.put(2, "For");
        cslmap.put(3, "Geeks");
 
        // Print the contents on the console
        System.out.println(
            "The ConcurrentSkipListMap contains: "
            + cslmap);
 
        // Print the key set using keySet()
        System.out.println(
            "\nThe ConcurrentSkipListMap key set: "
            + cslmap.keySet());
 
        // Remove elements using remove()
        cslmap.remove(3);
 
        // Print the contents on the console
        System.out.println(
            "\nThe ConcurrentSkipListMap contains: "
            + cslmap);
    }
}

输出：

The ConcurrentSkipListMap contains: {1=Geeks, 2=For, 3=Geeks}

The ConcurrentSkipListMap key set: [1, 2, 3]

The ConcurrentSkipListMap contains: {1=Geeks, 2=For}

ConcurrentNavigableMap 的基本操作

1.添加元素

要将元素添加到 ConcurrentNavigableMap 我们可以使用 Map 接口的任何方法。下面的代码展示了如何使用它们。在代码中可以观察到，在构造时没有提供 Comparator 时，遵循自然顺序。

Java

// Java Program for adding elements to a
// ConcurrentNavigableMap
import java.util.concurrent.*;
 
public class AddingElementsExample {
 
    public static void main(String[] args)
    {
 
        // Instantiate an object
        // Since ConcurrentNavigableMap is an interface
        // We use ConcurrentSkipListMap
        ConcurrentNavigableMap cnmap
            = new ConcurrentSkipListMap();
 
        // Add elements using put()
        cnmap.put(8, "Third");
        cnmap.put(6, "Second");
        cnmap.put(3, "First");
 
        // Print the contents on the console
        System.out.println(
            "Mappings of ConcurrentNavigableMap : "
            + cnmap);
    }
}

输出：

Mappings of ConcurrentNavigableMap : {3=First, 6=Second, 8=Third}

2. 移除元素

为了删除元素，我们也使用 Map 接口的方法，因为 ConcurrentNavigableMap 是 Map 的后代。

Java

// Java Program for deleting
// elements from ConcurrentNavigableMap
 
import java.util.concurrent.*;
 
public class RemovingElementsExample {
 
    public static void main(String[] args)
    {
 
        // Instantiate an object
        // Since ConcurrentNavigableMap
        // is an interface
        // We use ConcurrentSkipListMap
        ConcurrentNavigableMap cnmap
            = new ConcurrentSkipListMap();
 
        // Add elements using put()
        cnmap.put(8, "Third");
        cnmap.put(6, "Second");
        cnmap.put(3, "First");
        cnmap.put(11, "Fourth");
 
        // Print the contents on the console
        System.out.println(
            "Mappings of ConcurrentNavigableMap : "
            + cnmap);
 
        // Remove elements using remove()
        cnmap.remove(6);
        cnmap.remove(8);
 
        // Print the contents on the console
        System.out.println(
            "\nConcurrentNavigableMap, after remove operation : "
            + cnmap);
 
        // Clear the entire map using clear()
        cnmap.clear();
        System.out.println(
            "\nConcurrentNavigableMap, after clear operation : "
            + cnmap);
    }
}

输出：

Mappings of ConcurrentNavigableMap : {3=First, 6=Second, 8=Third, 11=Fourth}

ConcurrentNavigableMap, after remove operation : {3=First, 11=Fourth}

ConcurrentNavigableMap, after clear operation : {}

3. 访问元素

我们可以使用 get() 方法访问 ConcurrentNavigableMap 的元素，下面给出了这个例子。

Java

// Java Program for accessing
// elements in a ConcurrentNavigableMap
 
import java.util.concurrent.*;
 
public class AccessingElementsExample {
 
    public static void main(String[] args)
    {
        // Instantiate an object
        // Since ConcurrentNavigableMap is an interface
        // We use ConcurrentSkipListMap
        ConcurrentNavigableMap cnmap
            = new ConcurrentSkipListMap();
 
        // Add elements using put()
        cnmap.put(8, "Third");
        cnmap.put(6, "Second");
        cnmap.put(3, "First");
        cnmap.put(11, "Fourth");
 
        // Accessing the elements using get()
        // with key as a parameter
        System.out.println(cnmap.get(3));
        System.out.println(cnmap.get(6));
        System.out.println(cnmap.get(8));
        System.out.println(cnmap.get(11));
 
        // Display the set of keys using keySet()
        System.out.println(
            "\nThe ConcurrentNavigableMap key set: "
            + cnmap.keySet());
    }
}

输出：

First
Second
Third
Fourth

The ConcurrentNavigableMap key set: [3, 6, 8, 11]

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() 方法打印 ConcurrentNavigableMap 的元素。

Java

// Java Program for traversing a ConcurrentNavigableMap
 
import java.util.concurrent.*;
import java.util.*;
 
public class TraversalExample {
 
    public static void main(String[] args)
    {
 
        // Instantiate an object
        // Since ConcurrentNavigableMap is an interface
        // We use ConcurrentSkipListMap
        ConcurrentNavigableMap cnmap
            = new ConcurrentSkipListMap();
 
        // Add elements using put()
        cnmap.put(8, "Third");
        cnmap.put(6, "Second");
        cnmap.put(3, "First");
        cnmap.put(11, "Fourth");
 
        // Create an Iterator over the
        // ConcurrentNavigableMap
        Iterator > itr
            = cnmap.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()) {
            ConcurrentNavigableMap
                .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

注意：每次我们说“ConcurrentNavigableMap 的元素”时，必须注意元素实际上存储在 ConcurrentNavigableMap 的实现类的对象中，在这种情况下是 ConcurrentSkipListMap。

ConcurrentNavigableMap 的方法

ConcurrentNavigableMap 继承了 Map 接口、SortedMap 接口、ConcurrentMap 接口、NavigableMap 接口的方法。添加元素、移除元素和遍历的基本方法由父接口给出。 ConcurrentNavigableMap 的方法如下表所示。这里，

• K - 映射中键的类型。
• V – 映射中映射的值的类型。

Method	Description
descendingKeySet()	Returns a reverse order NavigableSet view of the keys contained in the map.
descendingMap()	Returns a reverse order view of the mappings in the map.
headMap​(K toKey)	Returns the view of the portion of the map wherein the keys are less than toKey.
headMap​(K toKey, boolean inclusive)	Returns the view of the portion of the map wherein the keys are less than toKey, and equal to toKey if inclusive is true.
keySet()	Returns a NavigableSet view of the keys contained in this map.
navigableKeySet()	Returns a NavigableSet view of the keys contained in this map.
subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive)	Returns a view of the portion of the map, keys ranging from fromKey to toKey.
subMap(K fromKey, K toKey)	Returns a view of the portion of the map, keys ranging from fromKey, inclusive, to toKey, exclusive.
tailMap(K fromKey)	Returns a view of the map wherein the keys are greater than fromKey.
tailMap(K fromKey, boolean inclusive)	Returns a view of the map wherein the keys are greater than fromKey, and equal to if inclusive is true.

在接口Java.util.concurrent.ConcurrentMap 中声明的方法

METHOD	DESCRIPTION
compute​(K key, BiFunction ​? super 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 ? extends V> 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 super V,​? extends V> 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 ,​? super V,​? extends V> 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 ,​? extends V> 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.

在接口Java.util.Map 中声明的方法

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.
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.
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.

在接口Java.util.NavigableMap 中声明的方法

METHOD	DESCRIPTION
ceilingEntry​(K key)	Returns a key-value mapping associated with the least key greater than or equal to the given key, or null if there is no such key.
ceilingKey​(K key)	Returns the least key greater than or equal to the given key, or null if there is no such key.
firstEntry()	Returns a key-value mapping associated with the least key in this map, or null if the map is empty.
floorEntry​(K key)	Returns a key-value mapping associated with the greatest key less than or equal to the given key, or null if there is no such key.
floorKey​(K key)	Returns the greatest key less than or equal to the given key, or null if there is no such key.
higherEntry​(K key)	Returns a key-value mapping associated with the least key strictly greater than the given key, or null if there is no such key.
higherKey​(K key)	Returns the least key strictly greater than the given key, or null if there is no such key.
lastEntry()	Returns a key-value mapping associated with the greatest key in this map, or null if the map is empty.
lowerEntry​(K key)	Returns a key-value mapping associated with the greatest key strictly less than the given key, or null if there is no such key.
lowerKey​(K key)	Returns the greatest key strictly less than the given key, or null if there is no such key.
pollFirstEntry()	Removes and returns a key-value mapping associated with the least key in this map, or null if the map is empty.
pollLastEntry()	Removes and returns a key-value mapping associated with the greatest key in this map, or null if the map is empty.

在接口Java.util.SortedMap 中声明的方法

METHOD	DESCRIPTION
comparator()	Returns the comparator used to order the keys in this map, or null if this map uses the natural ordering of its keys.
entrySet()	Returns a Set view of the mappings contained in this map.
firstKey()	Returns the first (lowest) key currently in this map.
lastKey()	Returns the last (highest) key currently in this map.
values()	Returns a Collection view of the values contained in this map.