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Invoking the Thread.start() method tells the Java runtime to start executing the thread's run() method using the started thread. Invoking a Thread object's run() method directly is incorrect. When a a Thread object's run() method is invoked directly, the statements in the run() method are executed by the current thread, rather than by the newly created thread. Furthermore, if the Thread object was constructed by instantiating a subclass of Thread that fails to override the run() method rather than being constructed from a Runnable object, calls to the subclass's run() method invoke Thread.run(), which does nothing.

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public final class Foo implements Runnable {
  @Override public void run() {
    // ...
  }

  public static void main(String[] args) {
    Foo foo = new Foo();
    new Thread(foo).start();
  }
}

Exceptions

THI00-EX0: The run() method may be directly invoked when during unit testing functionality. Note that this method cannot be used to test a class for multithreaded use.

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public void sampleRunTest(){

  Thread thread = new Thread(new Runnable() {
    @Override public void run() {
        // ...
      }
    });
  
  ((Runnable) thread).run();  // ExceptionTHI00-EX0: ThisDoes does not start a new thread

}

Casting a thread to Runnable before calling the run() method documents that the explicit call to Thread.run() is intentional. Adding an explanatory comment alongside the invocation is highly recommended.

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