☰
-
Php 95
-
Java 94
-
Javascript 37
-
Regex 18
-
Git 17
-
Security 16
-
Docker 07
-
Python 58
-
Machine Learning 14
-
Book Highlights
Java
/
Design Patterns
- 1 Basics 9
-
Classes S
-
Objects S
-
Arrays S
-
Variables S
-
Loops S
-
Numbers S
-
Strings S
-
Exceptions S
-
Regexp S
- 2 OOP 9
-
Inheritance
-
Polymorphism
-
Static S
-
Abstract
-
Interfaces
-
Constructors S
-
Packages
-
Nested Classes
-
Final
- 3 Compiler 2
-
Sublime Text S
-
Apache Ant
- 4 Collections 8
-
Lists
-
Comparable S
-
Sets
-
Maps
-
Generics
-
Properties
-
Streams
-
Json
- 5 Threads 4
-
Create Thread S
-
Sleep
-
Lock
-
Scheduler
- 6 Design Patterns 4
-
Singleton
-
Observer
-
Strategy
-
Mediator
- 7 Swing 12
-
Frame
-
Panel
-
Listener
-
Combo Box
-
Label
-
Image
-
Menu
-
Table
-
Layout
-
Drawing
-
Timer
-
Designer
- 8 I/O 7
-
Streams IO
-
Socket
-
Watching Files
-
Mail
-
Logger
-
Clipboard
-
Encrypt S
- 9 Effective 7
-
Constructors S
-
Dependency Injection
-
Composition
-
Interfaces Default
-
Import Static S
-
Enums
-
Lambdas
- 10 Junit 5
-
About Junit S
-
Test Case
-
Suite Test
-
Annotations
-
Exceptions
- 11 Lambdas 7
-
Expressions S
-
Functional Interfaces
-
Streams
-
Common Operations
-
Default Methods
-
Static Methods S
-
Single Responsibility
- 12 JavaFX 6
-
Openjfx
-
Scene Builder
-
First App
-
Jar Archive
-
On Action
-
Change Listener
- 13 Maven 4
-
Demo
-
Spring Boot
-
Junit
-
Guava
- 14 Spring Boot 8
-
Quick start S
-
Rest service
-
Consuming Rest
-
Templates
-
Security
-
Command Line
-
Scheduling Tasks
-
Ajax
R
Q
SINGLE INSTANCE
Singleton pattern restricts the instantiation of a class to a single instance. A private constructor hides the class for outside.
class LearningApp {
public static void main(String[] args) {
MyClass A = MyClass.getInstance();
MyClass B = MyClass.getInstance();
MyClass C = new MyClass();
// Error: The constructor MyClass() is not visible
}
}
class MyClass {
private static final MyClass INSTANCE = new MyClass();
private MyClass() {} // private constructor
public static MyClass getInstance() {
return INSTANCE;
}
}
The static keyword is used for unique initialization.
A static class is not automatically a Singleton class.
class LearningApp {
public static void main(String[] args) {
Obj o1 = new Obj();
System.out.println( o1.getInstances() );// count = 1
Obj o2 = new Obj();
System.out.println( o2.getInstances() );// count = 1
}
private static class Obj {
private int count = 0; // it should be declared static
private Obj() {
count++; // without static it is always 1
}
public int getInstances() {
return count;
}
}
}
➥ Questions
Last update: 463 days ago