/*
 * Copyright (C) 2010 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.util;

import android.compat.annotation.UnsupportedAppUsage;

/**
 * Singleton helper class for lazily initialization.
 *
 * Modeled after frameworks/base/include/utils/Singleton.h
 *
 * @hide
 */
@android.ravenwood.annotation.RavenwoodKeepWholeClass
public abstract class Singleton<T> {

    @UnsupportedAppUsage
    public Singleton() {
    }

    @UnsupportedAppUsage
    private volatile T mInstance;

    protected abstract T create();

    @UnsupportedAppUsage
    public final T get() {
        T instance = mInstance;
        if (instance == null) {
            synchronized (this) {
                if (mInstance == null) {
                    mInstance = create();
                }
                instance = mInstance;
            }
        }
        return instance;
    }

    public void clear() {
        synchronized (this) {
            mInstance = null;
        }
    }
}