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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.framework.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;

/**
 * A marker annotation, written on a class declaration, that signifies that one or more of the
 * class's type parameters can be treated covariantly. For example, if {@code MyClass} has a single
 * type parameter that is treated covariantly, and if {@code B} is a subtype of {@code A}, then
 * {@code SomeClass} is a subtype of {@code SomeClass}.
 *
 * Ordinarily, Java treats type parameters invariantly: {@code SomeClass} is unrelated to
 * (neither a subtype nor a supertype of) {@code SomeClass}.
 *
 * 
It is only safe to mark a type parameter as covariant if the type parameter is used in a
 * read-only way: values of that type are read from but never modified. This property is not
 * checked; the {@code @Covariant} is simply trusted.
 *
 * 
Here is an example use:
 *
 * 
{@code @Covariant(0)
 * public interface Iterator { ... }
 * }

 *
 * @checker_framework.manual #covariant-type-parameters Covariant type parameters
 */
@Documented
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface Covariant {
    /** The zero-based indices of the type parameters that should be treated covariantly. */
    int[] value();
}