A widely used, object-oriented language for backend systems, JVM tooling, and enterprise applications.
Getting Started
Java is a statically typed, class-based language that runs on the Java Virtual Machine (JVM). It is commonly used for backend services, Android apps, and large-scale systems.
Installation
java -version
javac -version
Hello World
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, Java!");
}
}
Compile and run:
javac HelloWorld.java
java HelloWorld
---
## Classes & Objects
Java uses classes as the primary building blocks.
```java
public class User {
private int id;
private String name;
public User(int id, String name) {
this.id = id;
this.name = name;
}
public String getName() {
return name;
}
}
Inheritance
public class Animal {
public void speak() {
System.out.println("..." );
}
}
public class Dog extends Animal {
@Override
public void speak() {
System.out.println("Woof");
}
}
Collections & Streams
The Java Collections Framework provides lists, sets, maps, and more.
List<String> names = List.of("Alice", "Bob", "Charlie");
Map<String, Integer> ages = Map.of("Alice", 30, "Bob", 25);
Java Streams enable expressive, functional-style operations:
List<String> filtered = names.stream()
.filter(name -> name.startsWith("A"))
.map(String::toUpperCase)
.toList();
Exception Handling
Java uses exceptions for error handling.
try {
int result = divide(10, 0);
} catch (ArithmeticException e) {
System.err.println("Cannot divide by zero");
} finally {
System.out.println("Cleanup code");
}
Checked exceptions must be declared or handled.
public void loadFile(String path) throws IOException {
Files.readString(Path.of(path));
}
JVM & Tooling
Java code compiles to bytecode that runs on the JVM. Modern JVM tooling includes javac, jshell, and build systems like Maven and Gradle.
mvn clean install
# or
./gradlew build
