Java IdentityHashMap Usage Guide

IdentityHashMap in Java is a unique implementation of HashMap that uses object references as keys, instead of using the equals() and hashCode() methods to determine if objects are equal.

The primary purpose of using IdentityHashMap is when there is a need to compare if two object references are equal, such as when there is a need to retain objects with different references, or when using objects as keys.

The usage of IdentityHashMap is similar to HashMap, where you can add key-value pairs to the mapping using the put() method, retrieve values by keys using the get() method, and check for the existence of specific keys using the containsKey() method.

Here is a simple example:

IdentityHashMap<String, Integer> map = new IdentityHashMap<>();

// 添加键值对
map.put("one", 1);
map.put("two", 2);
map.put(new String("one"), 3); // 不同的引用，仍然可以添加成功

// 获取值
System.out.println(map.get("one")); // 输出3，因为使用了不同的引用

// 检查键是否存在
System.out.println(map.containsKey("one")); // 输出true

// 遍历映射
for (Map.Entry<String, Integer> entry : map.entrySet()) {
    System.out.println(entry.getKey() + " : " + entry.getValue());
}

The output results in:

3
true
one : 3
two : 2

It is important to note that IdentityHashMap uses object references for comparing keys, rather than using the equals() and hashCode() methods, so it may not be suitable in some cases. In general, using HashMap should suffice for most needs.