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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 precondition annotation to indicate that a method requires certain expressions to have a
 * certain qualifier at the time of the call to the method. The expressions for which the annotation
 * must hold after the method's execution are indicated by {@code expression} and are specified
 * using a string. The qualifier is specified by {@code qualifier}.
 *
 * @checker_framework.manual #java-expressions-as-arguments Syntax of Java expressions
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
@Repeatable(RequiresQualifier.List.class)
public @interface RequiresQualifier {
    /**
     * Returns the Java expressions for which the annotation need to be present.
     *
     * @return the Java expressions for which the annotation need to be present
     * @checker_framework.manual #java-expressions-as-arguments Syntax of Java expressions
     */
    String[] expression();

    /**
     * Returns the qualifier that is required.
     *
     * @return the qualifier that is required
     */
    Class qualifier();

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