The equality operators are syntactically left-associative (they group left-to-right),
but this fact is essentially never useful; for example, a==b==c parses as
(a==b)==c. The result type of a==b is always boolean, and c must therefore be
of type boolean or a compile-time error occurs. Thus, a==b==c does not test to
see whether a, b, and c are all equal.
EqualityExpression:
RelationalExpression
EqualityExpression==RelationalExpression
EqualityExpression!=RelationalExpression
The == (equal to) and the != (not equal to) operators are analogous to the relational operators except for their lower precedence. Thus, a<b==c<d is true whenever a<b and c<d have the same truth value.
The equality operators may be used to compare two operands of numeric type, or two operands of type boolean, or two operands that are each of either reference type or the null type. All other cases result in a compile-time error. The type of an equality expression is always boolean.
In all cases, a!=b produces the same result as !(a==b). The equality operators are commutative if the operand expressions have no side effects.