Singleton Pattern

CREATIONAL PATTERN

As its name suggests, singleton pattern is used to maintain single instance of certain class. There can be situations, when we want to create object of class only one time and keep using same object all the time. We will never want to create second object of same class. For example, There is only one sun in our solar system. Hence we cannot have several objects of class named Sun, but only one. In Banking system, each customer account has unique account number. Hence we would wish to design class AccountNumberGenerator. But we would use singleton only one instance of that class, who would take responsibility to generate unique account number for request generated multiple bankers of bank. If we create instances of this class, it may not guarantee uniqueness of account number. Please follow example below how to achieve it technically.

AccountNumberGenerator.java

public class AccountNumberGenerator {

	private static AccountNumberGenerator AccNoInstance = new AccountNumberGenerator();
	private int lastAccountNo;
	
	private AccountNumberGenerator() {
		// TODO Auto-generated constructor stub
		lastAccountNo=0;
	}
	
	public static AccountNumberGenerator getInstance(){
		if(AccNoInstance == null)
			AccNoInstance = new AccountNumberGenerator();
		return AccNoInstance;
	}

	public int getAccountNumber(){
		return (++this.lastAccountNo);
	}
}

Banker.java

public class Banker {

	private String name; 
	public Banker(String name) {
		// TODO Auto-generated constructor stub
		this.name = name;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	
	int providePassbook(){
		int accountNo;
		AccountNumberGenerator instance = AccountNumberGenerator.getInstance();
		accountNo = instance.getAccountNumber();
		return accountNo;
	}
}

SinglePatternDemo.java

public class SingletonPatternDemo {

	public SingletonPatternDemo() {
		// TODO Auto-generated constructor stub
	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub

		Banker sam = new Banker("Samuel");
		System.out.println("Hey Samuel,I want to open account with your branch");
		System.out.println(sam.getName()+" : Sure sir, we provide you passbook with account No :"+sam.providePassbook());
		
		Banker rajib = new Banker("Rajib");
		System.out.println("Hey rajib,I want to open account with your branch");
		System.out.println(rajib.getName()+" : Sure sir, we provide you passbook with account No :"+sam.providePassbook());
		
		
	}

}

Output