Singleton design pattern in Java
In object-oriented programming , Singleton pattern restricts the instantiation of a class and ensures that only one instance of the class exists in the JVM (Java Virtual Machine). In other words, a class should ensure that only a single instance must be created and single object can be used by all other classes.Usage of Singleton Pattern
When you need to ensure that there's one instance of an object available to a number of other classes, you may want to use the Singleton pattern.
Advantage
- Singleton Pattern can saves memory because object is not created at each request.
- Provide a global point of access to the object.
- Allow multiple instances in the future without affecting a singleton class's clients.
How to create Singleton design pattern?
In order to create a Singleton Pattern, you need the following:
- Static member: It gets memory only once because of static, it contains the instance of the Singleton class.
- Private constructor: It will prevent to instantiate the Singleton class from outside the class.
- Static factory method: It provides the global point of access to the Singleton object and returns the instance to the caller.
class SingletonClass
{
private static SingletonClass sInstance = null;
public String msg;
private SingletonClass()
{
msg = "Singleton Test";
}
public static SingletonClass getInstance()
{
if (sInstance == null)
sInstance = new SingletonClass();
return sInstance;
}
}
public class TestClass {
public static void main(String[] args) {
// instantiating Singleton class with three variable obj1,obj2,obj3
SingletonClass obj1 = SingletonClass.getInstance();
SingletonClass obj2 = SingletonClass.getInstance();
SingletonClass obj3 = SingletonClass.getInstance();
// changing variable of instance obj1
obj1.msg = (obj1.msg).toUpperCase();
System.out.println("msg from obj1 is " + obj1.msg);
System.out.println("msg from obj2 is " + obj2.msg);
System.out.println("msg from obj3 is " + obj3.msg);
System.out.println("\n");
// changing variable of instance obj3
obj3.msg = (obj3.msg).toLowerCase();
System.out.println("msg from obj1 is " + obj1.msg);
System.out.println("msg from obj2 is " + obj2.msg);
System.out.println("msg from obj3 is " + obj3.msg);
}
}
output
msg from obj1 is SINGLETON TEST
msg from obj2 is SINGLETON TEST
msg from obj3 is SINGLETON TEST
msg from obj1 is singleton test
msg from obj2 is singleton test
msg from obj3 is singleton test
SingletonClass Explanation
In the SingletonClass, first time when we request first time call getInstance() method, it creates an object of the class with name sInstance and return it to the variable.
if (sInstance == null)
sInstance = new SingletonClass();
Since sInstance is static, it is changed from null to some object. Second time, when we try to call getInstance() method, since sInstance is not null, then it is returned to the variable, instead of creating new object of SingletonClass.
In the main method, we instantiate the singleton class with 3 objects obj1, obj2, obj3 by calling static method getInstance() . But actually after creation of object obj1, variables obj2 and obj3 are pointed to object obj1. So, if we change the variables of object obj1, that is reflected when we access the variables of objects obj2 and obj3. Also if we change the variables of object obj3, that is reflected when we access the variables of objects obj1 and obj2.
Difference between Normal class and Singleton class
The difference in normal and singleton class in terms of instantiation, That is, for a normal class we use constructor to instantiate, whereas for singleton class we use getInstance() method.
Difference between static class and singleton pattern
- Singleton objects are stored in Heap, but static objects are stored in stack.
- We can clone the singleton object, but we cannot clone the static class object .
- Singleton classes follow the OOP concepts), static classes do not.
- You can create one instance of the object and reuse it whereas you cannot create the instance of static class.
Related Topics
- Java Interview Questions-Core Faq - 1
- Java Interview Questions-Core Faq - 2
- Java Interview Questions-Core Faq - 3
- Important features of Java
- Difference between Java and JavaScript?
- What is the difference between JDK and JRE?
- What gives Java its 'write once and run anywhere' nature?
- What is JVM and is it platform independent?
- What is Just-In-Time (JIT) compiler?
- What is the garbage collector in Java?
- What is NullPointerException in Java
- Difference between Stack and Heap memory in Java
- How to set the maximum memory usage for JVM?
- What is numeric promotion?
- Why do we need Generic Types in Java?
- What does it mean to be static in Java?
- What are final variables in Java?
- How Do Annotations Work in Java?
- How do I use the ternary operator in Java?
- What is instanceof keyword in Java?
- How ClassLoader Works in Java?
- What are fail-safe and fail-fast Iterators in Java
- What are method references?
- "Cannot Find Symbol" compile error
- Difference between system.gc() and runtime.gc()
- How to convert TimeStamp to Date in Java?
- Does garbage collection guarantee that a program will not run out of memory?
- How setting an Object to null help Garbage Collection?
- How do objects become eligible for garbage collection?
- How to calculate date difference in Java
- Difference between Path and Classpath
- Is Java "pass-by-reference" or "pass-by-value"?
- Difference between static and nonstatic methods java
- Why Java does not support pointers?
- What is package in Java?
- What are wrapper classes?
- Difference between Java Local Variable, Instance Variable and a Class Variable?
- Can a top level class be private or protected in java
- Are Polymorphism , Overloading and Overriding similar concepts?
- How to Use Locks in Java
- Why Multiple Inheritance is Not Supported in Java
- Why Java is not a pure Object Oriented language?
- Why can't a Java class be declared as static?
- Difference between Abstract class and Interface in Java
- Why do I need to override the equals and hashCode methods in Java?
- Why does Java not support operator overloading?
- What’s meant by anonymous class in Java?
- Static Vs Dynamic class loading in Java
- Why am I getting a NoClassDefFoundError in Java?
- How to generate random integers within a specific range in Java
- What's the meaning of System.out.println in Java?
- What is the purpose of Runtime and System class?
- What is finally block in Java?
- What is difference between final, finally and finalize?
- What is try-with-resources in java?
- What is a stacktrace?
- What is the meaning of immutable in terms of String?
- What are different ways to create a string object in Java?
- Difference between String and StringBuffer/StringBuilder in Java
- What is the difference between creating String as new() and literal?
- How do I convert String to Date object in Java?
- How do I create a Java string from the contents of a file?
- What actually causes a StackOverflow error in Java?
- Why is char[] preferred over String for storage of password in Java
- What is I/O Filter and how do I use it in Java?
- Serialization and Deserialization in Java
- Understanding transient variables in Java
- What is Externalizable in Java?
- What is the purpose of serialization/deserialization in Java?
- What is the Difference between byte stream and Character streams
- How to append text to an existing file in Java
- Read/convert an InputStream to a String in Java
- What is the difference between Reader and InputStream in Java
- Introduction to Java threads
- What is synchronization Java?
- Static synchronization Vs non static synchronization in Java
- Java Thread Deadlock Tutorial
- What is Daemon thread in Java
- Difference between implements Runnable and extends Thread in Java
- What is the volatile keyword in Java
- What are the basic interfaces of Java Collections Framework
- What are the differences between ArrayList and Vector in Java
- What is the difference between ArrayList and LinkedList?
- What is the difference between List and Set in Java
- Difference between HashSet and HashMap in Java
- Difference between HashMap and Hashtable in Java?
- How does the hashCode() method of java works?
- Difference between capacity() and size() of Vector in Java
- What is a Java ClassNotFoundException?
- How to fix java.lang.UnsupportedClassVersionError