checking object reference equality using == (in Java) [duplicate]

Question

Why does...

    String a = new String("a");
    StringBuilder b = new StringBuilder("a");
    System.out.println(a==b);

... result in an incompatible types error when compiling given that...

    String a = new String("b");
    Object b = new StringBuilder("b");
    System.out.println(a==b);

... does not?

Why can I compare the object references of a String and an Object but not a StringBuilder and a String? Aren't they all just addresses to memory locations?

Thanks

Answer 1

Acording to the Java Language Specification (15.21.3):

It is a compile-time error if it is impossible to convert the type of either operand to the type of the other by a casting conversion (§5.5). The run-time values of the two operands would necessarily be unequal (ignoring the case where both values are null).

Which means, in order to "compare" two reference types, one should be able to be cast to the other. String "is" an object, but there is no "cast" between String and StringBuilder.