Java
Java Multithreading
Java Start() Thread

Java Thread.start() vs Thread.run() Method

In Java’s multi-threading concept, start() and run() are the two most important methods. Below are the main differences between Thread.start() and Thread.run() in Java.

Differences

Thread.start()Thread.run()
Creates a new thread and executes run() on the newly created thread.No new thread is created; run() executes on the calling thread.
Cannot be invoked more than once (throws java.lang.IllegalStateException).Can be invoked multiple times.
Defined in java.lang.Thread class.Defined in java.lang.Runnable interface and must be overridden in the implementing class.

New Thread Creation

When start() is called, a new thread is created, and run() is executed within it. If run() is called directly, no new thread is created; execution happens in the main thread.

Example 1: Using start()

class MyThread extends Thread {
    @Override
    public void run() {
        System.out.println("Current thread name: " + Thread.currentThread().getName());
        System.out.println("run() method called");
    }
}
 
class Geeks {
    public static void main(String[] args) {
        MyThread t = new MyThread();
        t.start();
    }
}

Output:

Current thread name: Thread-0
run() method called

Example 2: Using run() directly

class MyThread extends Thread {
    @Override
    public void run() {
        System.out.println("Current thread name: " + Thread.currentThread().getName());
        System.out.println("run() method called");
    }
}
 
class Coding75 {
    public static void main(String[] args) {
        MyThread t = new MyThread();
        t.run();
    }
}

Output:

Current thread name: main
run() method called

Multiple Invocation

Calling start() more than once throws an exception, while run() can be called multiple times.

Example 1: Multiple calls to start()

class MyThread extends Thread {
    @Override
    public void run() {
        System.out.println("Current thread name: " + Thread.currentThread().getName());
        System.out.println("run() method called");
    }
}
 
class Coding75 {
    public static void main(String[] args) {
        MyThread t = new MyThread();
        t.start();
        t.start();
    }
}

Output:

Current thread name: Thread-0
run() method called
Exception in thread "main" java.lang.IllegalThreadStateException

Example 2: Multiple calls to run()

class MyThread extends Thread {
    @Override
    public void run() {
        System.out.println("Current thread name: " + Thread.currentThread().getName());
        System.out.println("run() method called");
    }
}
 
class Coding75 {
    public static void main(String[] args) {
        MyThread t = new MyThread();
        t.run();
        t.run();
    }
}

Output:

Current thread name: main
run() method called
Current thread name: main
run() method called
Java Runnable InterfaceJava Sleep() Thread