A static field or member is maintained once in memory and used by all instances of that class. One way to imagine using this is to think of an instance counter. You could increment it every time an object gets made, and each object would then "know" how many total instances had been created.
A static method can be invoked without ever making an instance of the class that contains it. Best straightforward example: all the methods in the Math class. You don't really need a Math object for anything, so why create one just to make use of its functions ? (You can't make Math objects, anyway, but that's because it's final class).
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Michael Ernest, co-author of:
The Complete Java 2 Certification Study Guide