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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
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.ANNOTATION_TYPE})
@AnnotatedFor("nullness")
public @interface PolymorphicQualifier {
  /**
   * Indicates which type system this annotation refers to (optional, and usually unnecessary). 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 the top
   * qualifier from the given hierarchy.
   *
   * @return the top qualifier in the hierarchy of this qualifier
   */
  // We use the meaningless Annotation.class as default value and
  // then ensure there is a single top qualifier to use.
  Class value() default Annotation.class;
}