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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.aliasing.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.DefaultFor;
import org.checkerframework.framework.qual.TypeUseLocation;
import org.checkerframework.framework.qual.SubtypeOf;

/**
 * An expression with this type has no aliases.
 * In other words, no other expression, evaluated at the same program
 * point, would evaluate to the exact same object value.
 * 
 *
 * A constructor's return type should be annotated with @Unique if the
 * constructor does not leak references to the constructed object.
 * For example, the String() constructor return type
 * is annotated as @Unique.
 *
 * @see MaybeAliased
 * @checker_framework.manual #aliasing-checker Aliasing Checker
 */

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE_USE, ElementType.TYPE_PARAMETER })
@SubtypeOf({MaybeAliased.class})
@DefaultFor(TypeUseLocation.LOWER_BOUND)
public @interface Unique {}