HackerRank Java Singleton Pattern problem solution

In this HackerRank Java Singleton Pattern problem in the java programming language you need to Complete the Singleton class in your editor which contains the following components:

1. A private Singleton non-parameterized constructor.
2. A public String instance variable named str.
3. Write a static method named getSingleInstance that returns the single instance of the Singleton class.

HackerRank Java Singleton Pattern problem solution.

import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;
import java.lang.reflect.*;


class Singleton{
	
	static Singleton instance = new Singleton();
	
	private Singleton() {
		
	}
	
	public String str;
	
	static Singleton getSingleInstance() {
		return instance;
	}

}

Second solution

import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;
import java.lang.reflect.*;


class Singleton{
    public String str;
    private static Singleton instance;
    private Singleton() {
        
    }
    
    public static Singleton getSingleInstance () {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

The third solution in java 8 programming.

import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;
import java.lang.reflect.*;


class Singleton{
    private volatile static Singleton instance;
    public static String str;
    private Singleton() {}
    
    static Singleton getSingleInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }

}