Logical operators

Logical operators

The logical operators AND (&&), OR (||) and NOT (!) produce a boolean value of true or false based on the logical relationship of its arguments. This example uses the relational and logical operators:

You can apply AND, OR, or NOT to boolean values only. You can't use a non-boolean as if it were a boolean in a logical expression as you can in C and C++. You can see the failed attempts at doing this commented out with a //! comment marker. The subsequent expressions, however, produce boolean values using relational comparisons, then use logical operations on the results.

One output listing looked like this:

Note that a boolean value is automatically converted to an appropriate text form if it's used where a String is expected.

You can replace the definition for int in the above program with any other primitive data type except boolean. Be aware, however, that the comparison of floating-point numbers is very strict. A number that is the tiniest fraction different from another number is still "not equal." A number that is the tiniest bit above zero is still nonzero.

Short-circuiting

When dealing with logical operators you run into a phenomenon called "short circuiting." This means that the expression will be evaluated only until the truth or falsehood of the entire expression can be unambiguously determined. As a result, all the parts of a logical expression might not be evaluated. Here's an example that demonstrates short-circuiting:

Each test performs a comparison against the argument and returns true or false. It also prints information to show you that it's being called. The tests are used in the expression:

You might naturally think that all three tests would be executed, but the output shows otherwise:

The first test produced a true result, so the expression evaluation continues. However, the second test produced a false result. Since this means that the whole expression must be false, why continue evaluating the rest of the expression? It could be expensive. The reason for short-circuiting, in fact, is precisely that; you can get a potential performance increase if all the parts of a logical expression do not need to be evaluated.