Home Home » Design Pattern » Java » [Java Design Pattern] [Creational Pattern] - Singleton

[Java Design Pattern] [Creational Pattern] - Singleton

Wednesday, June 12, 2019
Creational Pattern - Singleton 
Purpose: The singleton ensures that a class only one instance and provides a global point of access to it.

This article show how to use Singletone design pattern.  

CarHelper.java
public class CarHelper {
   
    private static CarHelper _instance = null;

    private int distance;
   
    public static synchronized CarHelper getInstance() {
        if (_instance == null) {
            _instance = new CarHelper();
        }
       
        return _instance;
    }

    public int getDistance() {
        return distance;
    }

    public void setDistance(int distance) {
        this.distance += distance;
    }
}

SingletoneMain.java
public class SingletonMain {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        CarHelper.getInstance().setDistance(10);
        System.out.println(CarHelper.getInstance().getDistance());
       
        CarHelper.getInstance().setDistance(20);
        System.out.println(CarHelper.getInstance().getDistance());
       
        CarHelper.getInstance().setDistance(30);
        System.out.println(CarHelper.getInstance().getDistance());

    }

}

Result:
10
30
60

We can see that each time get instance from CarHelper, it same an instance.

Example: we not allow init Statistics object by constructor.
public class Statistics implements Serializable {

    // Definition for the class instance
    private static volatile Statistics instance = new Statistics();

    private List teams = new ArrayList();

    /**
     * Constructor has been made private so that outside classes do not have
     * access to instantiate more instances of Statistics.
     */
    private Statistics() {
    }

    /**
     * Accessor for the statistics class. Only allows for one instance of the
     * class to be created.
     *
     * @return
     */
    public static Statistics getInstance() {
        synchronized (Statistics.class) {
            if (instance == null) {
                instance = new Statistics();
            }
        }
        return instance;
    }

    /**
     * @return the teams
     */
    public List getTeams() {
        return teams;
    }

    /**
     * @param teams
     *            the teams to set
     */
    public void setTeams(List teams) {
        this.teams = teams;
    }

    protected Object readResolve() {
        return instance;
    }
}

public static void main(String[] args) {
        Statistics a = Statistics.getInstance();
        List<String> team = new ArrayList<String>();
        team.add("Deposit");
        team.add("Channel");
       
        a.setTeams(team);
       
        Statistics b = Statistics.getInstance();
        System.out.println(b.getTeams());
       
       
    }
 
Chia sẻ bài viết
Bài liên quan

Comments[ 0 ]


Post a Comment