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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.common.value.qual;

import org.checkerframework.framework.qual.ConditionalPostconditionAnnotation;
import org.checkerframework.framework.qual.InheritedAnnotation;
import org.checkerframework.framework.qual.QualifierArgument;

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;

/**
 * Indicates that the value of the given expression is a sequence containing at least the given
 * number of elements, if the method returns the given result (either true or false).
 *
 * When the annotated method returns {@code result}, then all the expressions in {@code
 * expression} are considered to be {@code MinLen(targetValue)}.
 *
 * @see MinLen
 * @checker_framework.manual #constant-value-checker Constant Value Checker
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
@ConditionalPostconditionAnnotation(qualifier = MinLen.class)
@InheritedAnnotation
@Repeatable(EnsuresMinLenIf.List.class)
public @interface EnsuresMinLenIf {
    /**
     * Returns the return value of the method under which the postcondition holds.
     *
     * @return the return value of the method under which the postcondition holds
     */
    boolean result();

    /**
     * Returns Java expression(s) that are a sequence with the given minimum length after the method
     * returns {@link #result}.
     *
     * @return an array of Java expression(s), each of which is a sequence with the given minimum
     *     length after the method returns {@link #result}
     * @checker_framework.manual #java-expressions-as-arguments Syntax of Java expressions
     */
    String[] expression();

    /**
     * Returns the minimum number of elements in the sequence.
     *
     * @return the minimum number of elements in the sequence
     */
    @QualifierArgument("value")
    int targetValue() default 0;

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