Design Patterns

[lveay2]

• Classification of Design Patterns
• Iterator Pattern
• Builder Pattern
• Observer Pattern and Webhook mechanism
• Command Pattern
• Façade Pattern
• Singleton Pattern
• Difference between Factory Pattern and Abstract Factory Pattern
• Decorator Pattern
• Adapter Pattern
• Proxy Pattern

[lveay2]

Singleton

Double checked locking pattern

public class Singleton {

    private volatile static Singleton instance; //声明成 volatile

    private Singleton (){}

    public static Singleton getSingleton() {
        if (instance == null) {                         
            synchronized (Singleton.class) {
                if (instance == null) {       
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

Static final field

public class Singleton{

    private static final Singleton instance = new Singleton(); //类加载时就初始化

    private Singleton(){}

    public static Singleton getInstance(){
        return instance;
    }
}

Static nested class

public class Singleton {

    private static class SingletonHolder {  
        private static final Singleton INSTANCE = new Singleton();  
    }

    private Singleton (){}

    public static final Singleton getInstance() {  
        return SingletonHolder.INSTANCE; 
    }  
}

Enum

public enum EasySingleton{
    INSTANCE;
}

Reference: http://wuchong.me/blog/2014/08/28/how-to-correctly-write-singleton-pattern/

[lveay2]

Enum Singleton

public enum Singleton {
    INSTANCE;
    // A private constructor is hidden here
    private Singleton () {}
}

// Define what the singleton must do.
public interface MySingleton {

    public void doSomething();
}

private enum Singleton implements MySingleton {

    /**
     * The one and only instance of the singleton.
     *
     * By definition as an enum there MUST be only one of these and it is inherently thread-safe.
     */
    INSTANCE {

                @Override
                public void doSomething() {
                    // What it does.
                }

            };
}

public static MySingleton getInstance() {
    return Singleton.INSTANCE;
}

Reference: https://www.jianshu.com/p/4e8ca4e2af6c https://stackoverflow.com/questions/26285520/implementing-singleton-with-an-enum-in-java https://stackoverflow.com/questions/23721115/singleton-pattern-using-enum-version