I know that '==' doesn't work with values outside the range of [-128,127] as there is a cache maintained of Integer objects within this range and the same reference is returned if value is within the range. But why does '>', '<', '>=', '<=' give correct answers even outside the range ?
Integer a=150;
Integer b=150;
System.out.println(a==b); //returns false
Integer a=150;
Integer b=150;
System.out.println(a>=b); // returns true
Why is this happening ?
The <, >, <= and >= operators are only defined for primitive types. Therefore using them on wrapper types causes the compiler to unbox the objects into primitives.
This means
System.out.println(a>=b);
is equivalent to
System.out.println(a.intValue()>=b.intValue());
However, the == and != operators exist for both primitive types and reference types, so using them to compare two objects of primitive wrapper types compares the references instead of the primitive values they wrap.
As Holger commented, comparison of object references with == and != existed in the Java language before auto-boxing and auto-unboxing were introduced, but comparison with <, >, <= and >= was not supported for any reference types before auto-unboxing was introduced.
This means that in the early days of Java, the a>=b of your code snippet would not pass compilation (since a and b are not primitive numeric types). On the other hand your a==b snippet would still pass compilation and return false.
Changing the behavior of == and != for reference types that happen to be wrappers of numeric primitives would change the behavior of existing code, thus breaking backwards compatibility, which is probably the reason it wasn't done.