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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.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();
}