Why Doesn't Java Allow Casting Boolean -> Int?

It doesn't allow this because the Java designers (correctly) recognized that the boolean / integer overloading in C and C++ was a significant source of errors.

(I recall seeing that in writing in some design rationale, but I can't find it.)

For instance:

if (i = 0) {
    ...
}

is legal but probably a bug in an application that that is written C or C++.

Java avoids this and other problems by making boolean and the integer datatypes distinct types that cannot be converted from one to the other. Thus, the above is a compilation error in Java.

This doesn't work in all cases. For example (in Java):

if (flag = true) {
    ...
}

compiles. However, it does work in enough cases to be worthwhile. Furthermore, the idiomatic way to write the above in Java is:

if (flag) {
    ...
}

which avoids the pitfall entirely. Also, a checker like findbugs or pmd should flag the incorrect version as suspicious.

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Now this doesn't explain why you cannot explicitly type cast a boolean to an int. But I think that can be understood by observing the following:

• You rarely need to do that in real Java programs.

• Number <-> boolean casting wouldn't jell with the way that other type casts work. In particular, for other types there are up-casts and down-casts, and up-casts¹ in Java don't require an explicit type cast.

• You can't typecast between numbers and string either, or between strings an other objects. These are conversions, not type casts. And int <-> boolean is too.

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^{1 - I am being sloppy with my terminology here (deliberately). The correct Java terminology is "widening" and "narrowing". Narrowing conversions require an explicit type cast with a limited exception for literals. Widening conversions don't require a type cast.}

Java supports widening conversions on primitive numeric types. However, boolean is not considered a numeric type.

The supported widening conversions are listed under "Widening Primitive Conversion" in the Java Language Specification.

Because Java is strongly typed, a boolean and an int are two completely different data typesand one can not be casted to the other - it can not be casted, in fact.