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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.checker.signedness.qual;

import org.checkerframework.framework.qual.SubtypeOf;

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;

/**
 * Client code may interpret the value either as {@link Signed} or as {@link Unsigned}. This
 * primarily applies to values whose most significant bit is not set (i.e., {@link SignedPositive}),
 * and thus the value has the same interpretation as signed or unsigned.
 *
 * The programmer should not write this annotation. Instead, the programmer should write
 * {@link Signed} or {@link Unsigned} to indicate how the programmer intends the value to be
 * interpreted. For a value whose most significant bit is not set and different clients may treat it
 * differently (say, the return value of certain library routines, or certain constant fields), the
 * programmer should write {@code @}{@link SignedPositive} instead of {@code @SignednessGlb}.
 *
 * 
The Signedness Checker applies this annotation to manifest literals. This permits a value like
 * {@code -1} or {@code 255} or {@code 0xFF} to be used in both signed and unsigned contexts. The
 * Signedness Checker has no way of knowing how a programmer intended a literal to be used, so it
 * does not issue warnings for any uses of a literal. (An alternate design would require the
 * programmer to explicitly annotate every manifest literal whose most significant bit is set. That
 * might detect more errors, at the cost of much greater programmer annotation effort.)
 *
 * The "Glb" in the name stands for "greatest lower bound", because this type is the greatest
 * lower bound of the types {@link Signed} and {@link Unsigned}; that is, this type is a subtype of
 * both of those types.
 *
 * @see SignedPositive
 * @checker_framework.manual #signedness-checker Signedness Checker
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
@SubtypeOf({Unsigned.class, Signed.class})
public @interface SignednessGlb {}