The Java Language Specification, Third Edition

EXPRESSIONS Reference Equality Operators == and != 15.21.3
• The value produced by the == operator is true if the value of the left-hand
operand is equal to the value of the right-hand operand; otherwise, the result is
false.
• The value produced by the != operator is true if the value of the left-hand
operand is not equal to the value of the right-hand operand; otherwise, the
result is false.
15.21.2 Boolean Equality Operators == and !=
If the operands of an equality operator are both of type boolean, or if one operand
is of type boolean and the other is of type Boolean, then the operation is boolean
equality. The boolean equality operators are associative.
If one of the operands is of type Boolean it is subjected to unboxing conversion
(§5.1.8).
The result of == is true if the operands (after any required unboxing conversion)
are both true or both false; otherwise, the result is false.
The result of != is false if the operands are both true or both false; otherwise,
the result is true. Thus != behaves the same as ^ (§15.22.2) when applied
to boolean operands.
15.21.3 Reference Equality Operators == and !=
Things are more like they are now than they ever were before.
—Dwight D. Eisenhower
If the operands of an equality operator are both of either reference type or the null
type, then the operation is object equality.
A compile-time error occurs 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.
At run time, the result of == is true if the operand values are both null or
both refer to the same object or array; otherwise, the result is false.
The result of != is false if the operand values are both null or both refer to
the same object or array; otherwise, the result is true.
While == may be used to compare references of type String, such an equality
test determines whether or not the two operands refer to the same String
object. The result is false if the operands are distinct String objects, even if
they contain the same sequence of characters. The contents of two strings s and t
can be tested for equality by the method invocation s.equals(t). See also
§3.10.5.
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