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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 can be used to index any of the given sequences.
 *
 * For example, an expression with type {@code @IndexFor({"a", "b"})} is non-negative and is less
 * than both {@code a.length} and {@code b.length}. The sequences {@code a} and {@code b} might have
 * different lengths.
 *
 * 
The 
 * {@code String.charAt(int)} method is declared as
 *
 * 
{@code
 * class String {
 *   char charAt(@IndexFor("this") index) { ... }
 * }
 * }
 *
 * Writing {@code @IndexFor("arr")} is equivalent to writing {@link NonNegative @NonNegative}
 * {@link LTLengthOf @LTLengthOf("arr")}, and that is how it is treated internally by the checker.
 * Thus, if you write an {@code @IndexFor("arr")} annotation, you might see warnings about
 * {@code @NonNegative} or {@code @LTLengthOf}.
 *
 * @see NonNegative
 * @see LTLengthOf
 * @checker_framework.manual #index-checker Index Checker
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
public @interface IndexFor {
    /** Sequences that the annotated expression is a valid index for. */
    String[] value();
}