Just saying that from a completeness point of view, something like this would be required:

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;
    }

}

This utilizes the "double-checked locking" design pattern, and guarentees the following for the singleton:

• Lazy initialization (great for resource allocation, and means we don't waste resources with eager initialization)
• No race condition for lazy initialization (two+ more threads each trying to create a singleton will fail, guarenteed by syncronized)
• Acquiring a lock (expensive operation) is done once and only once, thus there is no performance penalty
• Only allows the singleton to be used when it is fully instantiated (thanks to the volatile keyword there)

An even more clever way is this:

class Singleton{
	private Singleton() {}
    
    private static class SingletonHolder {
    	private static final Singleton INSTANCE = new Singleton();
    }
    
    public static Singleton getInstance() {
    	return SingletonHolder.INSTANCE;
    }
}

The second way still retains all the requirements from the first, and is just more concise. (technically speaking it's a form of lazy initialization, since it eagerly initializes on demand).

More reading available here: cs.nyu.edu/courses/fall16/CSCI-UA.0470-001/slides/lecture25.pdf#page=52.

Java 8 (make sure to NOT remove the import statements foolishly like me or you will get errors):

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

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

Mine

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

Guys, why did my solution with static holder class had some weird assertion? It's definetely the best singleton implementation for Java:

class Singleton {
    private Singleton() {}

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

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

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In previous articles, we discussed about singleton design pattern and singleton class implementation in detail. In this article, we will see how we can create singleton classes. After reading this article you will be able to create your singleton class according to your use, simplicity and removed bottlenecks. There are many ways this can be done in Java. All these ways differs in their implementation of the pattern, but in the end, they all achieve the same end result of a single instance.

Eager initialization: This is the simplest method of creating a singleton class. In this, object of class is created when it is loaded to the memory by JVM. It is done by assigning the reference an instance directly. It can be used when program will always use instance of this class, or the cost of creating the instance is not too large in terms of resources and time. // Java code to create singleton class by
// Eager Initialization public class GFG
{ // public instance initialized when loading the class private static final GFG instance = new GFG();

private GFG() { // private constructor } public static GFG getInstance(){ return instance; } } Pros:

Very simple to implement. Cons:

May lead to resource wastage. Because instance of class is created always, whether it is required or not. CPU time is also wasted in creation of instance if it is not required. Exception handling is not possible. Using static block: This is also a sub part of Eager initialization. The only difference is object is created in a static block so that we can have access on its creation, like exception handling. In this way also, object is created at the time of class loading. It can be used when there is a chance of exceptions in creating object with eager initialization.

// Java code to create singleton class // Using Static block public class GFG
{ // public instance public static GFG instance;

private GFG()
{ // private constructor }

{ // static block to initialize instance instance = new GFG(); } } Pros:

Very simple to implement. No need to implement getInstance() method. Instance can be accessed directly. Exceptions can be handled in static block.

This may help you try. https://www.cetpainfotech.com/technology/java-training

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 singleton;
    
    private Singleton() {}
    public static Singleton getSingleInstance() {
        
        if(singleton == null) {
            singleton = new Singleton();
        }
        
        return singleton;
    }
}

I am writing the correct code for the java program .The code is correct and passes all the test cases...

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 singleton;
    
    private Singleton() {}
    public static Singleton getSingleInstance() {
        
        if(singleton == null) {
            singleton = new Singleton();
        }
        
        return singleton;
    }
}