/******************************************************************************
The Singleton design pattern is a creational design pattern that restricts the instantiation of a class
to a single instance and ensures that only one instance of the class exists in the system.
This pattern provides a global point of access to the instance of the class and is often used by applications
that need to control access to shared resources, such as a database connection object.
In practice, the Singleton pattern involves creating a private constructor, a private static instance variable
for the class, and a public static method called "getInstance" that returns the singleton instance.
The "getInstance" method includes a check to see if the private static instance variable has already been initialized,
and if it has not, it creates a new instance of the class.
The Singleton pattern should be used when we need exactly one instance of a class that should be accessible
from anywhere in the application. It is often used in cases where resources such as database connections,
network sockets are expensive to create and maintain, or to ensure that a shared resource is accessed in a thread-safe way.
A singleton instance can also be used to enforce a system-wide policy such as a logging policy or a security policy.
However, overuse of the Singleton pattern can lead to issues such as tight coupling, making the system harder to test and maintain, and making the code harder to reuse in other contexts. Therefore, it should be used judiciously and with careful consideration of the specific use case.
*******************************************************************************/
class Singleton {
private static final Singleton instance = new Singleton();
private String data;
private Singleton() {
data = "Bye, bye, Singleton!";
}
public static Singleton getInstance() {
return instance;
}
public String getData() {
return this.data;
}
public void setData(String data) {
this.data = data;
}
// Niezbędne do zablokowania możliwości klonowania
@Override
protected Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException("Cannot clone a singleton object");
}
// zablokowanie możliwości deserializacji
protected Object readResolve() {
return getInstance();
}
}
public class Main {
public static void main(String[] args) {
Singleton singleton = Singleton.getInstance();
System.out.println(singleton.getData()); // Output: Bye, bye, Singleton!
singleton.setData("Married and happy!");
System.out.println(singleton.getData()); // Output: Married and happy!
try {
Singleton clonedSingleton = (Singleton) singleton.clone(); // To spowoduje błąd uruchomienia
} catch (CloneNotSupportedException e) {
System.out.println("Cannot clone a singleton object");
}
// W ten sposób zapewniamy, że rzeczywiście istnieje tylko jedna instancja Singletona
}
}
Write, Run & Share Java code online using OneCompiler's Java online compiler for free. It's one of the robust, feature-rich online compilers for Java language, running the Java LTS version 17. Getting started with the OneCompiler's Java editor is easy and fast. The editor shows sample boilerplate code when you choose language as Java and start coding.
OneCompiler's Java online editor supports stdin and users can give inputs to the programs using the STDIN textbox under the I/O tab. Using Scanner class in Java program, you can read the inputs. Following is a sample program that shows reading STDIN ( A string in this case ).
import java.util.Scanner;
class Input {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.println("Enter your name: ");
String inp = input.next();
System.out.println("Hello, " + inp);
}
}
OneCompiler supports Gradle for dependency management. Users can add dependencies in the build.gradle file and use them in their programs. When you add the dependencies for the first time, the first run might be a little slow as we download the dependencies, but the subsequent runs will be faster. Following sample Gradle configuration shows how to add dependencies
apply plugin:'application'
mainClassName = 'HelloWorld'
run { standardInput = System.in }
sourceSets { main { java { srcDir './' } } }
repositories {
jcenter()
}
dependencies {
// add dependencies here as below
implementation group: 'org.apache.commons', name: 'commons-lang3', version: '3.9'
}
Java is a very popular general-purpose programming language, it is class-based and object-oriented. Java was developed by James Gosling at Sun Microsystems ( later acquired by Oracle) the initial release of Java was in 1995. Java 17 is the latest long-term supported version (LTS). As of today, Java is the world's number one server programming language with a 12 million developer community, 5 million students studying worldwide and it's #1 choice for the cloud development.
short x = 999; // -32768 to 32767
int x = 99999; // -2147483648 to 2147483647
long x = 99999999999L; // -9223372036854775808 to 9223372036854775807
float x = 1.2;
double x = 99.99d;
byte x = 99; // -128 to 127
char x = 'A';
boolean x = true;
When ever you want to perform a set of operations based on a condition If-Else is used.
if(conditional-expression) {
// code
} else {
// code
}
Example:
int i = 10;
if(i % 2 == 0) {
System.out.println("i is even number");
} else {
System.out.println("i is odd number");
}
Switch is an alternative to If-Else-If ladder and to select one among many blocks of code.
switch(<conditional-expression>) {
case value1:
// code
break; // optional
case value2:
// code
break; // optional
...
default:
//code to be executed when all the above cases are not matched;
}
For loop is used to iterate a set of statements based on a condition. Usually for loop is preferred when number of iterations is known in advance.
for(Initialization; Condition; Increment/decrement){
//code
}
While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.
while(<condition>){
// code
}
Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.
do {
// code
} while (<condition>);
Class is the blueprint of an object, which is also referred as user-defined data type with variables and functions. Object is a basic unit in OOP, and is an instance of the class.
class keyword is required to create a class.
class Mobile {
public: // access specifier which specifies that accessibility of class members
string name; // string variable (attribute)
int price; // int variable (attribute)
};
Mobile m1 = new Mobile();
public class Greeting {
static void hello() {
System.out.println("Hello.. Happy learning!");
}
public static void main(String[] args) {
hello();
}
}
Collection is a group of objects which can be represented as a single unit. Collections are introduced to bring a unified common interface to all the objects.
Collection Framework was introduced since JDK 1.2 which is used to represent and manage Collections and it contains:
This framework also defines map interfaces and several classes in addition to Collections.
| Collection | Description |
|---|---|
| Set | Set is a collection of elements which can not contain duplicate values. Set is implemented in HashSets, LinkedHashSets, TreeSet etc |
| List | List is a ordered collection of elements which can have duplicates. Lists are classified into ArrayList, LinkedList, Vectors |
| Queue | FIFO approach, while instantiating Queue interface you can either choose LinkedList or PriorityQueue. |
| Deque | Deque(Double Ended Queue) is used to add or remove elements from both the ends of the Queue(both head and tail) |
| Map | Map contains key-values pairs which don't have any duplicates. Map is implemented in HashMap, TreeMap etc. |