thinking of = = and equals comparison of Java Integer ( -128~127) value __java

Recently encountered a problem in the project, two values of the same integer value for = = comparison, found some of the mysteries of the integer, by the way also review the difference between = = and equals, first through the Damo code explained as follows:

System.out.println ("integer x = value) in the value of <-128~127 within." > ");
Integer i = 127;
Integer j = 127;
System.out.println ("i=" + i + ", j =" + j);
System.out.println ("i = = J:" + (i = = j) + "<--comparison-->i.equals (j):" + i.equals (j));
System.out.println ("An integer value other than <-128~127, integer x = value," is assigned the way.) > ");
Integer m = 128;
Integer n = 128;
System.out.println ("m=" + M + ", n =" + N);
System.out.println ("M = = N:" + (M = = N) + "<--comparison-->m.equals (n):" + m.equals (n));
System.out.println ();
		
System.out.println ("< arbitrary integer value, integer x = new Integer (value), the way to assign a value." > ");
Integer x = new Integer (299);
Integer y = new integer (299);
System.out.println ("x=" + x + ", y =" + y);
System.out.println ("x = = y:" + (x = = y) + "<--comparison-->x.equals (y):" + x.equals (y));

The output results are:

Integer x = value in the <-128~127 within the value. >
i=127,j =127
i = = j:true<--comparison-->i.equals (j):
integer value other than true <-128~127, integer x = value; The way you assign the value. >
m=128,n =128
m = = n:false<--comparison-->m.equals (n): True


< arbitrary integer value, integer x = new Integer (value ), the way to assign a value. >
x=299,y =299
x = = y:false<--comparison-->x.equals (y): True

Through the above code and output results, we must have seen the mystery. The first is summed up as follows:
1. The first and second paragraphs of the above code are intended to indicate that the integer value that is assigned in the -128~127 integer value and in integer x = value will return True when the = = and equals are compared. Because Java inside the -128~127 between the integer value, with the primary data type int, will be used in memory for reuse, that is, the integer value for = = comparison is only for the INT native data type numeric comparison, and beyond the -128~127 range , the address and numerical comparison are performed when the = = comparison is made.


2, the third paragraph to explain: = = and equals difference, = = is the address and value comparison, can not be overloaded with = = operator, and for the Equals method, Integer inside the Equals method rewrite object Equals method, View the integer source can see that the Equals method is a numerical comparison.

continue to explain:

  First look at a piece of code (using JDK 5) as follows: [HTML]   View plain copy public class hello     {      public static void main (String[] args)        {        int a = 1000, b =  1000;        system.out.println (a == b);            Integer c = 1000, d = 1000;        system.out.println (c == d);            Integer e = 100, f = 100;         System.out.println (e == f);      }   }   

Output result: [HTML] view plain copy true false

The Java Language specification, 3rd Edition wrote: [HTML] view plain Copy to save memory, for two instances of the following wrapper objects, when their basic values are the same , they are always = =: Boolean Byte Character, \u0000-\u007f (7f is a decimal 127) Integer,-128-127

View the JDK source code as follows:[Java]  View plain copy/**       * Cache to support the  object identity semantics of autoboxing for values between        * -128 and 127  (inclusive)  as required by  JLS.       *       * The cache  is initialized on first usage. during vm initialization the       * getAndRemoveCacheProperties method may be used  to get and remove any system       * properites  that configure the cache size. At this time, the size  of the       * cache may be controlled by the  vm option -xx:autoboxcachemax=<size>.       */           // value of java.lang.Integer.IntegerCache.high property  (obtained  During vm init)        private static String  integercachehighpropvalue;          static void  Getandremovecacheproperties ()  {           if  (! Sun.misc.VM.isBooted ())  {                properties props = system.getproperties ();                integerCacheHighPropValue =                     (String) Props.remove (" Java.lang.Integer.IntegerCache.high ");   &NBsp;           if  (integerCacheHighPropValue ! = null)                     system.setproperties (props);  // remove from system props