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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.framework.qual;

import java.lang.annotation.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;

/**
 * A meta-annotation that indicates that an annotation is a polymorphic type
 * qualifier.
 *
 * 
 * Any method written using a polymorphic type qualifier conceptually has
 * two or more versions — one version for each qualifier in the
 * qualifier hierarchy.  In each version of the method, all instances of
 * the polymorphic type qualifier are replaced by one of the other type
 * qualifiers.
 * 
 *
 * @checker_framework.manual #qualifier-polymorphism Qualifier polymorphism
 */
@Documented
@Target({ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface PolymorphicQualifier {
    /**
     * Usually, you can write {@code @PolymorphicQualifier} without an
     * argument.  When multiple type hierarchies are supported by a single
     * type system, then each polymorphic qualifier needs to indicate which
     * sub-hierarchy it belongs to.  Do so by passing a qualifier from the
     * given hierarchy, by convention the top qualifier.
     */
    // We use the meaningless PolymorphicQualifier.class as default value and
    // then ensure there is a single top qualifier to use.
    Class value() default PolymorphicQualifier.class;
}