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

Question

I know that i am comparing reference while i'm using == which is not a good idea but i did not understand why is this happening.

Integer a=100;
Integer b=100;
Integer c=500;
Integer d=500;
System.out.println(a == b); //true
System.out.println(a.equals(b)); //true
System.out.println(c == d); //false
System.out.println(c.equals(d)); //true

Answer 1

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.