JavaHashMap和IdentityHashMap的区别

Java中的HashMap是一个类，它是Java集合的一部分。它实现了Java的 Map 接口。它以键值对的形式存储数据。键应该是唯一的，但值可能是重复的。如果您尝试插入重复键，它将替换相应键的元素。 HashMap 类似于哈希表，但它是不同步的。它允许存储空键和空值，但应该只有一个空键并且可以有任意数量的空值。

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，抛出 ConcurrentModificationException 试图在迭代时进行修改。

HashMap 和 IdentityHashMap 都是实现 Map 接口的类。但是它们之间几乎没有区别。

示例 1： HashMap

Java

// Java program to illustrate
// the working of Java HashMap
// to demonstrate
// internal working difference between them
  
// Importing HashMap class from
// java.util package
import java.util.HashMap;
  
// Class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
        // Creating an empty HashMap object
        HashMap map = new HashMap<>();
  
        // Add elements to the map
        // Custom inputs
        map.put(10, "Geeks");
        map.put(20, "for");
        map.put(30, "geeks");
        map.put(40, "welcome");
        map.put(50, "you");
  
        // Printing the size of map
        System.out.println("Size of map is:- "
                           + map.size());
  
        // Printing the HashMap content
        System.out.println("HashMap content: " + map);
  
        // Removing a key 50
        map.remove(50);
  
        // Printing the HashMap after the removal
        System.out.println("HashMap after removal : "
                           + map);
    }
}

输出

Size of map is:- 5
HashMap content: {50=you, 20=for, 40=welcome, 10=Geeks, 30=geeks}
HashMap after removal : {20=for, 40=welcome, 10=Geeks, 30=geeks}

示例 2： IdentityHashMap

Java

// Java program to illustrate
// working of IdentityHashmap
// to demonstrate
// internal working difference between them
  
// Importing all classes of
// java.util package
import java.util.*;
  
// Class for iterating IdentityHashMap
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Creating an empty IdentityHashMap object
        IdentityHashMap ihmap
            = new IdentityHashMap();
  
        // Mapping string values to int keys
        // Custom inputs --> Custom mappings
        ihmap.put(10, "Geeks");
        ihmap.put(20, "for");
        ihmap.put(30, "Geeks");
        ihmap.put(40, "Welcomes");
        ihmap.put(50, "You");
  
        // Display the size of IdentityHashMap
        System.out.println("IdentityHashMap size : "
                           + ihmap.size());
  
        // Display the IdentityHashMap
        System.out.println("Initial identity hash map: "
                           + ihmap);
  
        // Create an Iterator over the IdentityHashMap
        Iterator >
            itr = ihmap.entrySet().iterator();
  
        // Condition check using hasNext() method()
  
        // Condition check using hasNext() method holding
        // true if there is any next element remaining
        while (itr.hasNext()) {
  
            // next() method which is used to
            // retrieve the next element
            IdentityHashMap.Entry entry
                = itr.next();
  
            // Print and display key and value pairs
            // using getKey() method
            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=for}
Key = 10, Value = Geeks
Key = 40, Value = Welcomes
Key = 50, Value = You
Key = 30, Value = Geeks
Key = 20, Value = for

JavaHashMap和IdentityHashMap的区别

S.NO.	HashMap	IdentityHashMap
1.	HashMap implements the Map interface but it doesn’t violate the map general contract.	IdentityHashMap also implements the Map interface but it intentionally violates the map general contract.
2.	HashMap uses object equality to compare the key and values.	IdentityHashMap uses reference equality to compare the key and values.
3.	HashMap uses the hashCode() method of HashMap class to find the bucket location.	IdentityHashMap doesn’t use the hashCode() method instead it uses the System.IdentityHashCode() method to find the bucket location.
4.	HashMap uses chaining.	IdentityHashMap uses a simple liner probe hash table.
5.	To safely store the objects in HashMap the keys need to be immutable.	IdentityHashMap doesn’t require the key to be immutable.
6.	HashMap performs slightly less than the IdentityHashMap.	IdentityHashMap performs better than HashMap.