Java

Creating traditional Threads

There are multiple ways to create threads

• Implementing a Runnable Interface
• Extend Thread class
• Implement Callable Interface
• Use Executer Framework/Thread Pool
• Reactive Programming: e.g. SpringWebFlux

Virtual Threads

A lightweight threading model (Project Loom). They allow you to create many concurrent threads efficiently. Introduced in Java 21.

• Managed by the JVM, not the OS.
• Scheduled by the Java runtime, not the operating system kernel.
• Each virtual thread runs on a carrier platform thread, but the JVM can suspend and resume them as needed.

When to use

• Concurrent I/O-bound tasks (e.g., handling many HTTP requests).
• As a drop-in replacement for traditional thread where you need numerous concurrent tasks.
• Simplifying asynchronous code by writing it in a synchronous style.

How to use

Runnable

Runnable task = () -> {
    System.out.println("Running in: " + Thread.currentThread());
};

Thread.startVirtualThread(task);

Thread

Thread t = Thread.ofVirtual().start(() -> {
    System.out.println("Virtual thread says hi!");
});

Executor service

try (ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor()) {
    executor.submit(() -> {
        System.out.println("Hello from a virtual thread!");
    });
}

Difference between ClassNotFound ans NoClassDefFound Exception

ClassNotFound

A RuntimeException while loading a class with Class.forName or loadClass. It's a checked Exception

NoClassDefFound

Class exists while compiling, but not present to runtime It's an Error

Records

A record is a compact syntax for declaring classes. They are pure data classes. It helps to reduce boilerplate code. Introduced in Java 16.

Key Characteristics

• Immutable: Fields are final and set only once via the constructor.
• Reduces boilerplate by generating:
  • Constructor
  • Getters (named exactly like the field names)
  • equals(), hashCode(), toString()
  • Final: Records are implicitly final and cannot be extended.

Limitations

• Can't extend other classes.
• Can't define mutable fields.

Example

public record Guest(String name, int age) {}

Pattern matching for instanceof

With pattern matching for instanceof, Java combines the type check and the cast into one step. Introduced in Java 16.

Example

if (obj instanceof String s) {
    System.out.println(s.length());  // no need to cast
}

Instead of (pre Java 16)

if (obj instanceof String) {
    String s = (String) obj;  // explicit cast
    System.out.println(s.length());
}

Can be used in the same expression

if (obj instanceof String s && s.length() > 3) {
    System.out.println("Long string: " + s);
}