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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 java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.checkerframework.framework.qual.SubtypeOf;

/**
 * An annotation indicating the length of an array or a string. If an expression's type has this
 * annotation, then at run time, the expression evaluates to an array or a string whose length is
 * one of the annotation's arguments.
 *
 * For example, {@code String @ArrayLen(2) []} is the type for an array of two strings, and
 * {@code String @ArrayLen({1, 2, 4, 8}) []} is the type for an array that contains 1, 2, 4, or 8
 * strings.
 *
 * @checker_framework.manual #constant-value-checker Constant Value Checker
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE_PARAMETER, ElementType.TYPE_USE})
@SubtypeOf({UnknownVal.class})
public @interface ArrayLen {
  /** The possible lengths of the array. */
  int[] value();
}