JavaIdentityHashMap、WeakHashMap、EnumMap的区别

IdentityHashMap、WeakHashMap 和 EnumMap 都是Java集合中实现 Map 接口的类。但它们之间几乎没有区别。

1. IdentityHashMap ： IdentityHashMap 实现了 Map 接口。在比较键(和值)时，它遵循引用相等而不是对象相等。当用户需要通过引用来比较对象时，将使用此类。它不是同步的，必须在外部同步。此类中的迭代器是快速失败的，在迭代时抛出ConcurrentModificationException以尝试修改。

IdentityHashMap 的工作：

Java

// Java program to illustrate the
// working of IdentityHashmap
  
import java.util.*;
  
public class IteratingIdentityHashMap {
  
    public static void main(String[] args)
    {
  
        // Creating an empty IdentityHashMap
        IdentityHashMap ihmap
            = new IdentityHashMap();
  
        // Mapping string values to int keys
        ihmap.put(10, "Geeks");
        ihmap.put(20, "4");
        ihmap.put(30, "Geeks");
        ihmap.put(40, "Welcomes");
        ihmap.put(50, "You");
  
        // Displaying the size of IdentityHashMap
        System.out.println("IdentityHashMap size : "
                           + ihmap.size());
  
        // Displaying the IdentityHashMap
        System.out.println("Initial identity hash map: "
                           + ihmap);
  
        // Create an Iterator over the
        // IdentityHashMap
        Iterator >
            itr = ihmap.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())
        {
            IdentityHashMap.Entry entry = itr.next();
            
            System.out.println("Key = " + entry.getKey()
                               + ", Value = "
                               + entry.getValue());
        }
    }
}

输出

IdentityHashMap size : 5
Initial identity hash map: {10=Geeks, 40=Welcomes, 50=You, 30=Geeks, 20=4}
Key = 10, Value = Geeks
Key = 40, Value = Welcomes
Key = 50, Value = You
Key = 30, Value = Geeks
Key = 20, Value = 4

2.弱HashMap： WeakHashMap 是 Map 接口的实现，它只存储弱键。在 WeakHashMap 中，我们只能存储其键的弱引用，这使得键值对在其键不再正常使用时被垃圾回收。 WeakHashMap 是基于 HashTable 的实现，但它不是同步的。它允许您存储空键和空值。

WeakHashMap 的工作：

Java

// Java program to illustrate
// the WeakHashMap
  
import java.util.Collection; 
import java.util.Map; 
import java.util.Set; 
import java.util.WeakHashMap; 
  
class WeakHashMapdemo {
    public static void main(String[] arg)
    {
        Map whmap = new WeakHashMap();
        
        whmap.put(1, "geeks");
        whmap.put(2, "4");
        whmap.put(3, "geeks");
        whmap.put(4, "welcomes");
        whmap.put(5, "you");
  
        // Displaying weak hash map
        System.out.println("WeakHashMap is : " + whmap);
  
        // Checking if "welcomes" exist
        if (whmap.containsValue("welcomes"))
            System.out.println("Yes welcomes exist");
  
        // Checking if 3 exist as a key in map
        if (whmap.containsKey(3))
            System.out.println("Yes 3 exist");
        
        // Creating set for key
        Set keyset = whmap.keySet();
        
        // Displaying key set
        System.out.println("key Set : " + keyset);
        
        Collection values = whmap.values();
        
        // Displaying values of map
        System.out.println("Values : " + values);
  
        // Removing all data
        whmap.clear();
  
        // Checking whether map is empty or not
        if (whmap.isEmpty())
            System.out.println("Empty WeakHashMap: " + whmap);
    }
}

输出

WeakHashMap is : {5=you, 4=welcomes, 3=geeks, 2=4, 1=geeks}
Yes welcomes exist
Yes 3 exist
key Set : [5, 4, 3, 2, 1]
Values : [you, welcomes, geeks, 4, geeks]
Empty WeakHashMap: {}

3. EnumMap : EnumMap 是枚举类型的 Map 接口的特殊实现。它扩展了 AbstractMap 并在Java实现了 Map 接口。 EnumMap 的几个重要特性如下：

• EnumMap 类是Java Collections Framework 的成员，它不是同步的。
• EnumMap 是一个有序的集合，它们是按照键的自然顺序维护的(键的自然顺序是指在枚举类型中声明枚举常量的顺序)
• EnumMap 比 HashMap 快得多。
• 每个 EnumMap 实例的所有键必须是相同枚举类型的键。
• 如果我们尝试插入空键，EnumMap 不允许插入空键，它会抛出NullPointerException 。
• EnumMap 在内部表示为数组，因此它提供了更好的性能。

EnumMap 的工作：

Java

// Java program to illustrate working
// of EnumMap
  
import java.util.*;
  
class EnumMapExample {
  
    public enum Months {
        January,
        February,
        March,
        April;
    }
  
    public static void main(String[] args)
    {
        // Creating an EnumMap of the Days enum
        EnumMap enumMap = new EnumMap<>(Months.class);
  
        // Insert using put() method
        enumMap.put(Months.January, 31);
        enumMap.put(Months.February, 28);
        enumMap.put(Months.March, 31);
        enumMap.put(Months.April, 30);
  
        // Printing size of EnumMap 
        System.out.println("Size of EnumMap: "
                           + enumMap.size());
        // Printing the EnumMap
        for (Map.Entry m : enumMap.entrySet())
        {
            System.out.println(m.getKey() + " "
                               + m.getValue());
        }
    }
}

输出

Size of EnumMap: 4
January 31
February 28
March 31
April 30

IdentityHashMap、WeakHashMap 和 EnumMap 的区别：

PROPERTIES	IdentityHashMap	WeakHashMap	EnumMap
References	IdentityHashMap stores strong key reference.	WeakHashMap stores the weak key reference.	EnumMap stores the strong key reference.
Search and get the values	It uses equality operator (==) to search and get the values.	It uses equals() method for that purpose.	It also uses equals() method for that purpose.
Keys	It allows to store any type of keys.	It also allows to store any type of keys.	It allows to store only enum type keys.
Underlined data structure	It uses the array as an underlined data structure.	It uses the HashTable as an underlined data structure.	It uses the array as an underlined data structure.
Iterator	Iterator used in IdentityHashMap is Fail-fast.	Iterator used in WeakHashMap is Fail-fast.	Iterator used in EnumMap is weakly consistent.
Null Values	It allows to store null values.	It allows to store null values.	It doesn’t allow to store null values