class D {
public static void main(String args[]) {
Integer b2=128;
Integer b3=128;
System.out.println(b2==b3);
}
}
Output:
false
class D {
public static void main(String args[]) {
Integer b2=127;
Integer b3=127;
System.out.println(b2==b3);
}
}
Output:
true
Note: Numbers between -128 and 127 are true.
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.