创建一个新线程的方法有两种：

Method 1
声明一个实现Runnable接口的类，实现(implements) run方法。然后可以分配类的实例，在创建线程时作为参数传递，并启动。

package MyThread;

class RunnableDemo implements Runnable {
       private Thread t;
       private String threadName;
       
       RunnableDemo( String name) {
          threadName = name;
          System.out.println("Creating " +  threadName );
       }
       
       // 1 
       public void run() { 
          System.out.println("Running " +  threadName );
          try {
             for(int i = 100; i > 0; i--) {
                System.out.println("Thread: " + threadName + ", " + i);
                // Let the thread sleep for a while.
                Thread.sleep(10);
             }
          }catch (Exception e) {
             System.out.println("Thread " +  threadName + " interrupted.");
          }
          System.out.println("Thread " +  threadName + " exiting.");
       }
       
       public void start () { 
          System.out.println("Starting " +  threadName );
          if (t == null) {
             t = new Thread (this, threadName);  // 2
             t.start ();  // 3
          }
       }
    }
                    
    public class TestThread {

       public static void main(String args[]) {
          RunnableDemo R1 = new RunnableDemo( "Thread-1");
          R1.start();
          
          RunnableDemo R2 = new RunnableDemo( "Thread-2");
          R2.start();
       }   
    }

Method 2
声明一个Thread的子类，并覆盖重写(overrides)Thread类的run方法。

package MyThread;

class ThreadDemo extends Thread {
       private Thread t;
       private String threadName;
       
       ThreadDemo( String name) {
          threadName = name;
          System.out.println("Creating " +  threadName );
       }
       
       public void run() {
          System.out.println("Running " +  threadName );
          try {
             for(int i = 4; i > 0; i--) {
                System.out.println("Thread: " + threadName + ", " + i);
                // Let the thread sleep for a while.
                Thread.sleep(50);
             }
          }catch (InterruptedException e) {
             System.out.println("Thread " +  threadName + " interrupted.");
          }
          System.out.println("Thread " +  threadName + " exiting.");
       }
       
       public void start () {
          System.out.println("Starting " +  threadName );
          if (t == null) {
             t = new Thread (this, threadName);
             t.start ();
          }
       }
    }

    public class TestThread2 {

       public static void main(String args[]) {
          ThreadDemo T1 = new ThreadDemo( "Thread-1");
          T1.start();
          
          ThreadDemo T2 = new ThreadDemo( "Thread-2");
          T2.start();
       }   
    }

References:
[1]https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html
[2]https://www.tutorialspoint.com/java/java_multithreading.htm