Java Container-Reference classification and partial map usage

First, directory

1. Reference classification

2, understand Weakhashmap, Identityhashmap, Enummap

3. Synchronous control and read-only setting

Second, the code implementation

1. Reference Classification (interview)

Strong references (strongreference): References to Objects, GC runtime, are not recycled.

Weak reference (SoftReference): GC Runtime may be recycled when the JVM is out of memory.

Soft Reference (WeakReference): objects are recycled when the GC is running.

Virtual Reference (Phantomreference): Similar to non-silver, the main tracking object is recycled objects, cannot be used alone, to be used in conjunction with Referencequeue

Objective: To avoid the long-term presence of objects in memory and to solve the problem of garbage collection mechanism timing.

2, Weakhashmap (key if referenced, after recycling automatically delete Key-value objects)

 public  static  void   main (String [] args) { // 
   
     result: Map with size 2,new String () for soft objects is recycled  weakhashmap weakhashmap=
    new  
     Weakhashmap ();  
    // 
     "Key1", "Key2" exists in constant pool as strong objects are not recycled         weakhashmap.put ("Key1", "Hello" 
     "Key2", "Hello2" ); Weakhashmap.put ( new  String ("Key3"), "Hello3"        );        System.GC ();        System.runfinalization ();    System.out.println (Weakhashmap.size ()); }
   

3. Synchronous control and read-only operation

(1) Synchronization control

Because some of the containers are thread-safe and some are thread insecure, how can you adjust thread-insecure containers to thread-safe?

such as: Hashtale (SAFE)-hashmap (unsafe) arrylist-linkedlist (unsafe)-vector (security)

　　

(2) Read-only operation

1) An immutable set of empty

　　

2) An immutable set of elements

　　

3) Non-variable container

　　

Java Container-Reference classification and partial map usage