1. Strong References (Objects)

Features:
Strong references provide direct access to the target object.
Will not be recycled.

Two, soft reference (SoftReference class)

Features:
When GC reclaims based on JVM memory, JVM reclaims when it finds it is out of memory
Conditions for freeing up space: JVM found insufficient memory.
While Android's Andler and Bitmap are in use, they are not released during the life cycle of the Activity, being pointer-to-pointer is empty, and memory is not actually released

Third, WeakReference

Features:
As long as the system is GC-enabled, it will be reclaimed regardless of whether there is enough memory.

Fourth, PhantomReference

get() is an empty null value
Use of storage when null is empty

package com.songli.ref;

import java.lang.ref.Reference;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;



/**
    > Author: songli
    > QQ: 2734030745
    > Mail: 15850774503@163.com
    > CSDN: http://my.csdn.net/Poisx
    > github: https://github.com/chensongpoixs
 */


public class RefSongli {
    public static class Bean {

        String name;
        Integer va;
        public Bean(String name, Integer value) {
            this.name = name;
            this.va = value;
        }

        @Override
        public String toString() {
            // TODO Auto-generated method stub
            return name +": " + va;
        }

    }

    public static void main(String[] args) {
        //Strong Reference

        Bean bean = new Bean("songli", 67);
         Bean[] referent = new Bean[20000]; 
         for (int i=0;i<referent.length;i++){ 
             referent[i] = new Bean("mybean:" + i,100);// Throw Exception 
         }
         System.out.println(referent[1000].toString());

        //Soft reference
         Reference<Bean>[] referent = new SoftReference[20000]; 
         for (int i=0;i<referent.length;i++){ 
             referent[i] = new SoftReference<Bean>(new Bean("mybean:" + i,100)); 
         } 
         System.out.println("finish");
         System.out.println(referent[499].get() + " " + 
                 referent[10000].get() + " " + 
                 referent[19999].get());// "null"
////         
        Object object = new Object();
        WeakReference<Object> weakReference = new WeakReference<Object>(object);
//      SoftReference<Object> weakReference = new SoftReference<Object>(object);
        object = null;
        System.gc();
        System.out.println(weakReference.get()); //
    }
}