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checker-qual contains annotations (type qualifiers) that a programmer writes to specify Java code for type-checking by the Checker Framework.

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package org.checkerframework.checker.index.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;

/**
 * An integer that is either -1 or is a valid index for each of the given sequences.
 *
 * The 
 * {@code String.indexOf(String)} method is declared as
 *
 * 

 *   class String {
 *    {@literal @}IndexOrLow("this") int indexOf(String str) { ... }
 *   }
 * 
 *
 * Writing {@code @IndexOrLow("arr")} is equivalent to writing {@link
 * GTENegativeOne @GTENegativeOne} {@link LTLengthOf @LTLengthOf("arr")}, and that is how it is
 * treated internally by the checker. Thus, if you write an {@code @IndexOrLow("arr")} annotation,
 * you might see warnings about {@code @GTENegativeOne} or {@code @LTLengthOf}.
 *
 * @see GTENegativeOne
 * @see LTLengthOf
 * @checker_framework.manual #index-checker Index Checker
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
public @interface IndexOrLow {
    /** Sequences that the annotated expression is a valid index for (or it's -1). */
    String[] value();
}