Specifies that the method is impure and that its return value must be used.

For pure methods (annotated with @Contract(pure = true)), it's implied that the resulting value is important, so this annotation would be redundant.

Some impure methods have side effects and still require the return value to be used. For example, java.io.InputStream#read(byte[]) returns the number of bytes actually stored in the byte array. Without checking the return value, it's impossible to say how many bytes were actually read.

This annotation should not be used if the return value of the method provides only additional information. For example, the main purpose of java.util.Collection#add(Object) is to modify the collection, and the return value is only interesting when adding an element to a set, to see if the set already contained that element before.

When used on a type, the annotation applies to all methods that do not return void.

When used on a package, the annotation applies to all types of that package.