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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.Repeatable;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.checkerframework.framework.qual.InheritedAnnotation;
import org.checkerframework.framework.qual.JavaExpression;
import org.checkerframework.framework.qual.PostconditionAnnotation;
import org.checkerframework.framework.qual.QualifierArgument;

/**
 * Indicates that the value expressions evaluate to a value that is a key in all the given maps, if
 * the method terminates successfully.
 *
 * Consider the following method from {@code java.util.Map}:
 *
 * 
 * @EnsuresKeyFor(value="key", map="this")
 * public @Nullable V put(K key, V value) { ... }
 * 
 *
 * This method guarantees that {@code key} has type {@code @KeyFor("this")} after the method
 * returns.
 *
 * @see KeyFor
 * @see EnsuresKeyForIf
 * @checker_framework.manual #map-key-checker Map Key Checker
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
@PostconditionAnnotation(qualifier = KeyFor.class)
@InheritedAnnotation
@Repeatable(EnsuresKeyFor.List.class)
public @interface EnsuresKeyFor {
  /**
   * Java expressions that are keys in the given maps on successful method termination.
   *
   * @checker_framework.manual #java-expressions-as-arguments Syntax of Java expressions
   */
  String[] value();

  /**
   * Returns Java expressions whose values are maps, each of which contains each expression value as
   * a key (after successful method termination).
   *
   * @return Java expressions whose values are maps, each of which contains each expression value as
   *     a key (after successful method termination)
   * @checker_framework.manual #java-expressions-as-arguments Syntax of Java expressions
   */
  @JavaExpression
  @QualifierArgument("value")
  String[] map();

  /**
   * A wrapper annotation that makes the {@link EnsuresKeyFor} annotation repeatable.
   *
   * Programmers generally do not need to write this. It is created by Java when a programmer
   * writes more than one {@link EnsuresKeyFor} annotation at the same location.
   */
  @Documented
  @Retention(RetentionPolicy.RUNTIME)
  @Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
  @PostconditionAnnotation(qualifier = KeyFor.class)
  @InheritedAnnotation
  @interface List {
    /**
     * Returns the repeatable annotations.
     *
     * @return the repeatable annotations
     */
    EnsuresKeyFor[] value();
  }
}