I have an assignment for school where I have to spot patterns in a certain given project. All is well (well, relatively..) but it seems that most patterns I find are some sort of variation. The singleton I had to find is no different.
The code is given below. The strange thing about this is that this class does not seem to instantiate with the following constructor, as one would expect:
public Singleton() {
if (singleton == null) {
singleton = new Singleton();
}
}
However, it is instatiated with this (as you can see in the original code below)? In my understanding this creates some sort of static singleton? I debugged and saw indeed that the first time the constructor is called by
Singleton x = new Singleton();
x.Print();
, it is indeed null. And this is a Singleton instance. However, shouldn't there be
private static Singleton singleton = new Singleton();
on top of the class instead of singelton = this;?
public class Singleton {
private static Singleton singleton;
public static Singleton getDefault() {
return singleton;
}
/**
* The constructor.
*/
public Singleton() {
if (singleton == null) {
singleton = this;
}
}
public void Print()
{
System.out.println("I'm a singleton!");
}
}
I'm pretty sure it is indeed a singleton, it's just of the type I have never seen and I don't get reasoning behind it, that's what I'm basicly asking.
There is generally two approaches to creating a Singleton class, both are based on keeping the constructor private.
The first approach:
public class Singleton
{
public static final Singleton INSTANCE = new Singleton();
private Singleton{...}
}
You create an instance like this: Singleton single = Singleton.INSTANCE;
The second approach:
public class Singleton
{
private static final Singleton INSTANCE = new Singleton();
private Singleton(){...}
public static Singleton getInstance(){ return INSTANCE; }
}
In this case, you create an instance like this: Singleton single = Singleton.getInstance();
The getInstance() method is then regarded as a public factory method.
As you can see, in both cases the new keyword is definitely used to create a single instance of the class, but because of the pattern used, there can only be one instance and no more. You don't need to use the "this" keyword.
Hope this helps.
Your code has not followed the Singleton pattern. I have shown below a class following the Singleton pattern. this refers to the current object instance. When you initialize the singleton reference with this inside constructor, it calls the default (no-args) constructor and assigns it back to the this reference and then it is assigned to singleton class member variable
public class Singleton {
private static Singleton singleton;
public static Singleton getDefault() {
if (singleton == null)
singleton = new Singleton();
return singleton;
}
/**
* The constructor.
*/
private Singleton() {
System.out.println(singleton);
if (singleton == null) {
/*this will call a default constructor
*and instantiate Singleton object
*/
singleton = this;
}
}
public void Print() {
System.out.println("I'm a singleton!");
}
public static void main(String[] args) {
Singleton singleton = Singleton.getDefault();
System.out.println(singleton.Print());
}
}
Hope you can understand!
I think you are confusing the meaning or this. this is not instantiating the class, this references the newly created instance. Then whenever someone calls your constructor a new instance is created (and returned) no matter you store in private static Singleton singleton; or not.
As well said #dic19 you must make your constructor private for preventing more than one instance to exist.
If the point is not to modify the code as you said, then the point is to report that this class is NOT a singleton.
You can check debugging:
Singleton x = new Singleton();
Singleton x1 = new Singleton();
Singleton x2 = new Singleton();
You will see all three are different instances, being Singleton.getDefault() simply the first you created.
If we use singletone we do not need to create instance for use. Because when we are creating singletone then we are creating new instance if not created.
Take a look of below code :
public class Singleton {
private static Singleton singleton;
private Singleton() {
}
public static Singleton getSingleton() {
if (singleton == null) {
singleton = new Singleton();
}
return singleton;
}
public void testSingleTone(){
System.out.println("test");
}
public static void main(String[] args){
Singleton.getSingleton().testSingleTone();
}
}
Related
I went through the wiki https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom to understand Initialization on demand holder Idiom for Singleton pattern, though the wiki mentions how it's thread safe, it doesn't say how this actually ensures only one instance is created ever, i.e if in two different classes Something.getInstance() is called how do they have the same object ?
I am new to Java, so any help would be appreciated.
Basic pattern for a singleton:
//final so it can't be extended, not needed, but I like it for security
public final class MySingleton {
//Private so no other class can access it directly
//static so it stays resident in memory
//final so it can't be nulled out or reassigned;
//The constructor is called when the class is loaded.
private static final MySingleton instance = new MySingleton();
//The constructor is private so no more instances can be created
private MySingleton() {
}
public static final MySingleton getInstance() {
return instance;
}
//Alternatively, you could have the instance initialized to null
//and have creation when getInstance() is called
public static final MySingleton getIntance() {
synchronized(instance) {
if (instance == null) {
instance = new MySingleton();
}
return instance
}
}
Case 1:
public class Singleton {
public static final Singleton INSTANCE = new Singleton();
private Singleton() {
...
}
}
Case 2:
public class Singleton {
private static final Singleton INSTANCE = new Singleton();
private Singleton() {
...
}
public static Singleton getInstance() {
return INSTANCE;
}
}
Is the second method the recommended way to implement the Singleton design pattern, because I have never seen the first one in any example for Singleton pattern.
Without going into all the stuff about singletons being an antipattern (but you should read up on it!), the currently best way to make a singleton in Java is to use an enum.
public enum Singleton {
INSTANCE;
}
The reason this is better is because the JVM guarantees that only one instance of the enum (per classloader) will exist at any time. It is thread safe, and you cannot use reflection to create another instance of the singleton.
Enum-values are also lazily instantiated, so you won't create the singleton before you access Singleton.INSTANCE the first time.
The best way to make a singleton is to use Enum.
public enum Foo {
INSTANCE;
}
What is an efficient way to implement a singleton pattern in Java?
In your example, case 1 is more simple, so it's better.
But just wonder what happen if your constructor function is more complicated with some parameters. At this time, you really need some thing like this:
public class Singleton {
private static final Singleton INSTANCE;
private Singleton(string a, string b, string c) {
...
}
public static Singleton getInstance(string a, string b, string c) {
if (INSTANCE != null) { //note in multiple thread env, need add synchronize here.
......
}
return INSTANCE;
}
}
The first process of creating Singleton object created the instance of Singleton class even before it is being used and that is not the best practice to use.
Also the same problem exists in second implementation.
My preferred way of singleton instance creation:
private static final Singleton INSTANCE = null;
public static Singleton getInstance(){
if(INSTANCE == null)
INSTANCE = new Singleton();
return INSTANCE;
}
Above implementation of Singleton instance creation is okay in single threaded environment but in case of multi-threaded environment two threads may access if block at same time so they will have different instances. This could be solved as below:
private static final Singleton INSTANCE = null;
public static Singleton getInstance(){
if(INSTANCE == null){
synchronized (Singleton.class) {
if(INSTANCE == null){
INSTANCE = new Singleton();
}
}
}
return INSTANCE;
}
Hope this helps.
This how I implement singleton with enum
import static collections.concurrancy.ConcurrentHashMapInstanceEnum.STAFF;
public enum ConcurrentHashMapInstanceEnum {
STAFF;
private ConcurrentHashMap<Integer,Person> concurrentHashMap = null;
private ConcurrentHashMapInstanceEnum() {
concurrentHashMap = new ConcurrentHashMap(10, 5f, 4);
}
public ConcurrentHashMap getConcurrentHashMap() {
return concurrentHashMap;
}
}
and this is how to reach in a thread...
staffList = STAFF.getConcurrentHashMap()
Yes Obviously the second method is preferred way to implement Sigleton pattern , Inside getInstance() you have to check if instance is already created then return the same and Only create new instance if there is No instance of the class.
Also Make getInstance method as static method.
Example class with singleton design pattern.
class Singleton {
private static Singleton instance;
private int x;
private Singleton() {
x = 5;
}
public static synchronized Singleton getInstance() {
if(instance == null) {
instance = new Singleton();
}
return instance;
}
public void doSomething() {
System.out.println("Hello");
}
}
I'm just wondering can I create this class with same variables and methods declared as static. Is it same as the singleton?
Singleton should be considered only if all three of the following criteria are satisfied:
Ownership of the single instance cannot be reasonably assigned
Lazy initialization is desirable
Global access is not otherwise provided for
Yes, It is the same.
If you really need to implement a singelton pattern I would recommend using an enum:
public enum MySingelton{
INSTANCE;
private final String[] variable = new String[]{"test", "test2};
public void randomFunction(){
}
}
Call it with:
MySingelton.INSTANCE.randomFunction();
With an enum implementation it's guaranteed that only one instance is created and that it's available all the time. Also it's possible to serialize and deserialize the singelton without creating multiple copies of it.
More information can be found here:
What is an efficient way to implement a singleton pattern in Java?
http://www.drdobbs.com/jvm/creating-and-destroying-java-objects-par/208403883?pgno=3
Since the purpose of the singleton pattern is to ensure that a single instance of a class exists, yes, you could use static members to achieve the same effect.
So instead of
public class Singleton {
private static Singleton theInstance = new Singleton();
private int aVar = 10;
public void aMethod() {
System.out.println(aVar);
}
public static Singleton getInstance() {
return theInstance;
}
}
you could do
public class FakeSingleton {
private static int aVar = 10;
public static void aMethod() {
System.out.println(aVar);
}
}
and have exactly the same functionality (instead of Singleton.getInstance().aMethod() you would write FakeSingleton.aMethod()).
Using the singleton pattern can be advantageous if you want lazy initialization, so that the singleton is only initialized when it is first needed, as follows:
public class Singleton {
private static Singleton theInstance = null;
private int aVar = 10;
public void aMethod() {
System.out.println(aVar);
}
public static Singleton getInstance() {
if (theInstance == null) {
theInstance = new Singleton();
}
return theInstance;
}
}
(Note that the above is not thread-safe, in multithreaded code you will need to add synchronization.)
I have a Singleton class like this:
class Singleton {
static class SingletonHolder {
static final Singleton INSTANCE = new Singleton();
}
public static Singleton getInstance() {
return SingletonHolder.INSTANCE;
}
}
Can I call like this from other class:
Singleton dummy = new Singleton();
if yes how can I disable it? if not, then why cannot i?
Sure. Just make the constructor private to avoid that: just add private Singleton() {} to prevent the default constructor from being public.
I suggest you make it an enum (since they can't be instantiated anyway);
enum Singleton {
INSTANCE;
public static Singleton getInstance() {
return INSTANCE;
}
}
I want to use the following pattern to create a singleton in java
public class Singleton {
// Private constructor prevents instantiation from other classes
private Singleton() { }
/**
* SingletonHolder is loaded on the first execution of Singleton.getInstance()
* or the first access to SingletonHolder.INSTANCE, not before.
*/
private static class SingletonHolder {
public static final Singleton INSTANCE = new Singleton();
}
public static Singleton getInstance() {
return SingletonHolder.INSTANCE;
}
}
But what happens when the private constructor I want to call is
private Singleton(Object stuff) {... }
How do I pass stuff to INSTANCE = new Singleton()? As in INSTANCE = new Singleton(stuff);
Rewriting the above snippet:
public class Singleton {
// Private constructor prevents instantiation from other classes
private Singleton(Object stuff) { ... }
/**
* SingletonHolder is loaded on the first execution of Singleton.getInstance()
* or the first access to SingletonHolder.INSTANCE, not before.
*/
private static class SingletonHolder {
public static final Singleton INSTANCE = new Singleton();
}
public static Singleton getInstance(Object stuff) {
return SingletonHolder.INSTANCE;//where is my stuff passed in?
}
}
EDIT:
for those of you claiming this pattern is not thread safe, read here: http://en.wikipedia.org/wiki/Singleton_pattern#The_solution_of_Bill_Pugh.
The object I am passing in is the android application context.
If you really want a singleton, there should be only one instance of it (duh!). If you add a parameter to getInstance you probably expect the returned instance to be different (otherwise there is no need for a parameter) which defeats the purpose.
If your goal is to add some configuration when the only instance is created, the simplest way would be to have your singleton query for the configuration information when it is instantiated:
public static final Singleton INSTANCE = new Singleton(getConfiguration());
where getConfiguration returns what is needed (whether by reading a file or forwarding some other variable for example).
Usual disclaimer: Singletons are evil.
Additional resource: Google guide to writing testable code (in case you were not convinced the first time).
You might want to read
a singleton with parameters is not a singleton
The first answer argues why a >>singleton with parameters<< is not a singleton, and doesn't come near a singleton.
public class Singleton {
private static Singleton singleton;
// Private constructor prevents instantiation from other classes
private Singleton() { }
public void addStuff(Object stuff){}
public static Singleton getInstance() {
if(singleton == null) singleton = new Singleton()
return singleton;
}
}
and use it as:
Singleton s = Singleton.getInstance();
s.addStuff(stuff);
or an alternative
public class Singleton {
private static Singleton singleton;
// Private constructor prevents instantiation from other classes
private Singleton() { }
public static void redefine(Object stuff){
singleton = new Singleton(stuff) // choose constructor based on parameters
}
public static Singleton getInstance() {
return singleton;
}
}
Why not get rid of SingletonHolder use factory pattern. You will have to decide what to do, when try to call getInstance twice, but with different 'stuff'.
public class Singleton {
private static Singleton singleton
private final Object stuff;
private Singleton(Object stuff) {
this.stuff = stuff;
}
public static synchronized Singleton getInstance(Object stuff) {
if (singleton == null) {
singleton = new Singleton(stuff);
return singleton;
}
return singleton; // or throw error because trying to re-init
}
}