Question

In Java, I can do the following to succinctly guard against a NullPointerException:

if ("myString".equals(someOtherString))

But I cannot do the same with Integers, e.g.

if (5.equals(someOtherInteger))

I get a compile-time error. Any ideas on why this design decision was made? Or any resources that might explain it? Thanks in advance.

EDIT: someOtherInteger is an Integer, not an int.

Answer 1

The JLS specifies that boxing conversions can only occur during assignment conversions, method invocation conversions, or casting conversions. Since you are neither assigning 5 to a variable, passing it as an argument to a method, nor explicitly casting it to Integer, it will not be autoboxed for you.

Assignment conversion (§5.2, §15.26) converts the type of an
expression to the type of a specified variable.

Assignment conversion may cause an OutOfMemoryError (as a result of
boxing conversion (§5.1.7)), a NullPointerException (as a result of
unboxing conversion (§5.1.8)), or a ClassCastException (as a result of
an unchecked conversion (§5.1.9)) to be thrown at run-time.

Method invocation conversion (§5.3, §15.9, §15.12) is applied to each
argument in a method or constructor invocation and, except in one
case, performs the same conversions that assignment conversion does.

Method invocation conversion may cause an OutOfMemoryError (as a
result of boxing conversion (§5.1.7)), a NullPointerException (as a
result of unboxing conversion (§5.1.8)), or a ClassCastException (as a
result of an unchecked conversion (§5.1.9)) to be thrown at run-time.

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Casting contexts allow the use of one of:

…

a boxing conversion (§5.1.7) optionally followed by a widening
reference conversion (§5.1.5)