001/* -*- mode: Java; c-basic-offset: 2; indent-tabs-mode: nil; coding: utf-8-unix -*-
002 *
003 * Copyright © 2023–2025 microBean™.
004 *
005 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
006 * the License. You may obtain a copy of the License at
007 *
008 *     http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
011 * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
012 * specific language governing permissions and limitations under the License.
013 */
014package org.microbean.proxy;
015
016/**
017 * An interface whose implementations pretend to be another type and alter the behavior of instances of that type in
018 * some way.
019 *
020 * <p>An implementation of this interface must, somehow, be able to be {@linkplain #$cast() cast} to the type of the
021 * instance it logically proxies.</p>
022 *
023 * @param <T> the proxied type
024 *
025 * @author <a href="https://about.me/lairdnelson" target="_top">Laird Nelson</a>
026 *
027 * @see #$proxied()
028 *
029 * @see #$cast()
030 */
031// TODO: this interface may move to another module or package.
032public interface Proxy<T> {
033
034  /**
035   * Returns the instance being proxied, not this {@link Proxy}.
036   *
037   * @return the instance being proxied; never {@code null}; never {@code this}
038   *
039   * @microbean.nullability Implementations of this method must never return {@code null}.
040   *
041   * @microbean.idempotency Implementations of this method must be idempotent and deterministic. Specifically, they must
042   * always return the instance being proxied, not {@code this} or anything else.
043   *
044   * @microbean.threadsafety Implementations of this method must be safe for concurrent use by multiple threads.
045   */
046  public T $proxied();
047
048  /**
049   * Returns this {@link Proxy}, or a completely substitutable replacement for it, cast to the type of the
050   * instance it proxies.
051   *
052   * <p>The default implementation of this method returns, effectively, {@code (T)this}.</p>
053   *
054   * @return this {@link Proxy}; never {@code null}
055   *
056   * @exception ClassCastException if the cast could not take place for any reason; this indicates a violation of this
057   * interface's structural contracts by the implementation class
058   *
059   * @microbean.nullability Implementations of this method must never return {@code null}.
060   *
061   * @microbean.idempotency Implementations of this method must be idempotent and deterministic. Specifically, they must
062   * always return this {@link Proxy} or a completely substitutable replacement for it, not anything else.
063   *
064   * @microbean.threadsafety Implementations of this method must be safe for concurrent use by multiple threads.
065   */
066  @SuppressWarnings("unchecked")
067  public default T $cast() {
068    return (T)this;
069  }
070
071}