Why Equal Operator Works for Integer Value Until 128 Number

Why equal operator works for Integer value until 128 number?

Check out the source code of Integer . You can see the caching of values there.

The caching happens only if you use Integer.valueOf(int), not if you use new Integer(int). The autoboxing used by you uses Integer.valueOf.

According to the JLS, you can always count on the fact that for values between -128 and 127, you get the identical Integer objects after autoboxing, and on some implementations you might get identical objects even for higher values.

Actually in Java 7 (and I think in newer versions of Java 6), the implementation of the IntegerCache class has changed, and the upper bound is no longer hardcoded, but it is configurable via the property "java.lang.Integer.IntegerCache.high", so if you run your program with the VM parameter -Djava.lang.Integer.IntegerCache.high=1000, you get "Same values" for all values.

But the JLS still guarantees it only until 127:

Ideally, boxing a given primitive value p, would always yield an identical reference. In practice, this may not be feasible using existing implementation techniques. The rules above are a pragmatic compromise. The final clause above requires that certain common values always be boxed into indistinguishable objects. The implementation may cache these, lazily or eagerly.
For other values, this formulation disallows any assumptions about the identity of the boxed values on the programmer's part. This would allow (but not require) sharing of some or all of these references.
This ensures that in most common cases, the behavior will be the desired one, without imposing an undue performance penalty, especially on small devices. Less memory-limited implementations might, for example, cache all characters and shorts, as well as integers and longs in the range of -32K - +32K.

Why is 128==128 false but 127==127 is true when comparing Integer wrappers in Java?

When you compile a number literal in Java and assign it to a Integer (capital I) the compiler emits:

Integer b2 =Integer.valueOf(127)

This line of code is also generated when you use autoboxing.

valueOf is implemented such that certain numbers are "pooled", and it returns the same instance for values smaller than 128.

From the java 1.6 source code, line 621:

public static Integer valueOf(int i) {
    if(i >= -128 && i <= IntegerCache.high)
        return IntegerCache.cache[i + 128];
    else
        return new Integer(i);
}

The value of high can be configured to another value, with the system property.

-Djava.lang.Integer.IntegerCache.high=999

If you run your program with that system property, it will output true!

The obvious conclusion: never rely on two references being identical, always compare them with .equals() method.

So b2.equals(b3) will print true for all logically equal values of b2,b3.

Note that Integer cache is not there for performance reasons, but rather to conform to the JLS, section 5.1.7; object identity must be given for values -128 to 127 inclusive.

Integer#valueOf(int) also documents this behavior:

this method is likely to yield significantly better space and time performance by caching frequently requested values. This method will always cache values in the range -128 to 127, inclusive, and may cache other values outside of this range.

Why Comparison Of two Integer using == sometimes works and sometimes not?

The Java Language Specification says that the wrapper objects for at least -128 to 127 are cached and reused by Integer.valueOf(), which is implicitly used by the autoboxing.

why is (Integer) 1 == (Integer) 1 returns true, but (Integer) 200 == (Integer) 200 returns false?

I guess we're talking about java? (:

Java has a very weird way of storing integers. I can give you the long answer, but it can be easily found around the forums.

Long story short, when integers pass 127, their length becomes too long to store normally, and thus the default comparing operator between them does not work properly. If you need to make this comparison, I'd advise using some other numeric type, but usually just storing them and using the .equals() function would do - It compares objects by value. Example:

//in some package , inside some function
// if((Integer)128==(Integer)128) 
// {
//    // this will never happen
// }
// if((Integer)128).equals((Integer)128))
   {
      // this will happen.
   }

Java: Integer equals vs. ==

The JVM is caching Integer values. Hence the comparison with == only works for numbers between -128 and 127.

Refer: #Immutable_Objects_.2F_Wrapper_Class_Caching