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: [email protected] > 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()); // } }