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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.Repeatable;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * A postcondition annotation to indicate that a method ensures that certain expressions have a
 * certain type qualifier once the method has successfully terminated. The expressions for which the
 * qualifier holds after the method's execution are indicated by {@code expression} and are
 * specified using a string. The qualifier is specified by the {@code qualifier} annotation element.
 *
 * Here is an example use:
 *
 * 

 *  {@literal @}EnsuresQualifier(expression = "p.f1", qualifier = Odd.class)
 *   void oddF1_1() {
 *       p.f1 = null;
 *   }
 * 
 *
 * Some type systems have specialized versions of this annotation, such as {@code
 * org.checkerframework.checker.nullness.qual.EnsuresNonNull} and {@code
 * org.checkerframework.checker.lock.qual.EnsuresLockHeld}.
 *
 * @see EnsuresQualifierIf
 * @checker_framework.manual #java-expressions-as-arguments Syntax of Java expressions
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
@InheritedAnnotation
@Repeatable(EnsuresQualifier.List.class)
public @interface EnsuresQualifier {
    /**
     * Returns the Java expressions for which the qualifier holds after successful method
     * termination.
     *
     * @return the Java expressions for which the qualifier holds after successful method
     *     termination
     * @checker_framework.manual #java-expressions-as-arguments Syntax of Java expressions
     */
    String[] expression();

    /**
     * Returns the qualifier that is guaranteed to hold on successful termination of the method.
     *
     * @return the qualifier that is guaranteed to hold on successful termination of the method
     */
    Class qualifier();

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