What's the difference between & and &&?

&& (and | vs || is the same) is the shortcut operator. As soon as you can determine the results, it stops. This is useful for things like

if (myObject != null && myObject.doIt())

if the object is null, we know the expression is false, so we don't bother calling myObject.doIt(), which is a good thing, since that would throw a null pointer exception.

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For more details, see JLS - 15.23 Conditional-And Operator &&.

Another thing is that && and || have lower precedences than & ^ (exclusive-or) and |. The single-stroke operators & ^ and | are overloaded; they are applied to integer numbers and secondarily also applied to Boolean values. Boolean values include un-boxed java.lang.Boolean objects and boolean primitives.

Note the Java™ Tutorials refer to && as conditional and on one page, which is correct, and logical and, which may be incorrect, on another page.

Campbell Ritchie wrote:... Note the Java™ Tutorials refer to && as conditional and on one page, which is correct, and logical and, which may be incorrect, on another page.

You're right. The JLS defines a single '&' as "bitwise" or "logical" (JLS - 15.22). The double '&&' is "conditional" (JLS - 15.23).