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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.nullness.qual;

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 org.checkerframework.framework.qual.PolymorphicQualifier;

/**
 * A polymorphic qualifier for the Map Key (@KeyFor) type system.
 *
 * Any method written using {@code @PolyKeyFor} conceptually has an arbitrary number of versions:
 * one in which every instance of {@code @PolyKeyFor} has been replaced by {@code @}{@link
 * UnknownKeyFor}, one in which every instance of {@code @}{@link PolyKeyFor} has been replaced by
 * {@code @}{@link KeyForBottom}, and ones in which every instance of {@code @PolyKeyFor} has been
 * replaced by {@code @}{@code KeyFor}, for every possible combination of map arguments.
 *
 * @checker_framework.manual #nullness-checker Nullness Checker
 * @checker_framework.manual #qualifier-polymorphism Qualifier polymorphism
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
@PolymorphicQualifier(UnknownKeyFor.class)
public @interface PolyKeyFor {}