Using annotation to ensure that value returned by method is not discarded

You could also check out jsr305. It defines a @CheckReturnValue annotation:

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import javax.annotation.meta.When;

@Documented
@Target( { ElementType.METHOD, ElementType.CONSTRUCTOR, ElementType.TYPE,
        ElementType.PACKAGE })
@Retention(RetentionPolicy.RUNTIME)
public @interface CheckReturnValue {
    When when() default When.ALWAYS;
}

It’s compatible with findbugs and generates a warning when someone forgets to handle the return value.

Guavas Splitter uses it:
http://code.google.com/p/guava-libraries/source/browse/guava/src/com/google/common/base/Splitter.java

I must say that I love annotations that can guide static code analysis.