Singleton.java

// 饿汉式
class Singleton {
    private static final Singleton ourInstance = new Singleton();

    static Singleton getInstance() {
        return ourInstance;
    }

    private Singleton() {
    }
}

// 静态内部类
class Singleton2 {
    private Singleton2() {
    }

    public static Singleton2 getInstance() {
        return Singleton2Holder.sInstance;
    }

    private static class Singleton2Holder {
        private static final Singleton2 sInstance = new Singleton2();
    }
}

// 懒汉式 线程安全
class Singleton3 {

    private static Singleton3 sInstance;

    private static final Object sLock = new Object();

    @NonNull
    final Context mContext;

    Singleton3(@NonNull Context context) {
        mContext = context.getApplicationContext();
    }

    @NonNull
    public static Singleton3 getInstance(@NonNull Context context) {
        synchronized (sLock) {
            if (sInstance == null) {
                sInstance = new Singleton3(context);
            }
            return sInstance;
        }
    }
}