Is it guaranteed that new Integer(i) == i in Java?

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Yes. JLS §5.6.2 specifies the rules for binary numeric promotion. In part:

When an operator applies binary
numeric promotion to a pair of
operands, each of which must denote a
value that is convertible to a numeric
type, the following rules apply, in
order, using widening conversion
(§5.1.2) to convert operands as
necessary:

If any of the operands is of a
reference type, unboxing conversion
(§5.1.8) is performed.

Binary numeric promotion applies for several numeric operators, including “the numerical equality operators == and !=.”

JLS §15.21.1 (Numerical Equality Operators == and !=) specifies:

If the operands of an equality
operator are both of numeric type, or
one is of numeric type and the other
is convertible (§5.1.8) to numeric
type, binary numeric promotion is
performed on the operands (§5.6.2).

In contrast, JLS §15.21.3 (Reference Equality Operators == and !=) provides:

If the operands of an equality
operator are both of either reference
type or the null type, then the
operation is object equality

This fits the common understanding of boxing and unboxing, that’s it only done when there’s a mismatch.

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