Singleton
- Java course
- Advanced
- Lesson 37/82
- 5 min
Learn to write the Singleton pattern in Java.
The Singleton pattern
The Singleton guarantees that a class has only one instance. This instance becomes application-wide: it is used for settings, caches, or a single log point.
It has three core parts: a private constructor prevents external new; a private static field holds the single instance; a public static getInstance method returns it. The instance is created on the first call; later calls return the same one.
Lazy and thread-safe
The simplest Singleton creates the instance on the first getInstance call (lazy init). But if several threads call at once, two instances may form. To prevent this you use synchronized or double-checked locking.
In Java the most robust way is sometimes an enum: enum Singleton { INSTANCE; }. Enum constants are naturally single-instance and thread-safe, so modern code often prefers an enum-based Singleton.
Practice Guarantee a single instance with Singleton
Write a Logger Singleton class with a private static Logger instance field, a private constructor, and a public static Logger getInstance() method (create the instance on the first call). In main, call getInstance twice and print the result of a == b with System.out.println.
Starter code
// Define the Logger Singleton class
// Add a private constructor and getInstance
// Compare two instances in mainSample solution
class Logger {
private static Logger instance;
private Logger() { }
public static Logger getInstance() {
if (instance == null) instance = new Logger();
return instance;
}
}
public class SingletonDemo {
public static void main(String[] args) {
Logger a = Logger.getInstance();
Logger b = Logger.getInstance();
System.out.println(a == b);
}
}Expected output
true
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the Singleton parts
| Term | Meaning |
|---|---|
private constructor |
Blocks new |
private static field |
Holds the instance |
getInstance |
Returns the instance |
static |
Class-level access |
2 Which element prevents creating a new instance with new from outside in a Singleton?
- private constructor Correct answer
- public static field
- static getInstance
- final field
Explanation: A private constructor blocks external new calls.
3 Complete the getInstance call
Logger log = Logger.____();Logger log = Logger.getInstance();Explanation: getInstance returns the single instance.