src/java.base/share/classes/jdk/internal/util/ReferenceKey.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * accompanied this code).
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package jdk.internal.util;

import java.lang.ref.Reference;

/**
 * View/wrapper of keys used by the backing {@link ReferencedKeyMap}.
 * There are two style of keys; one for entries in the backing map and
 * one for queries to the backing map. This second style avoids the
 * overhead of a {@link Reference} object.
 *
 * @param <T> key type
 *
 * @since 21
 */
public sealed interface ReferenceKey<T> permits StrongReferenceKey, WeakReferenceKey, SoftReferenceKey {
    /**
     * {@return the value of the unwrapped key}
     */
    T get();

    /**
     * Cleanup unused key.
     */
    void unused();

}