兩種方法：

假設如下：

                如果有a,b，c三個執行緒，我們依次執行這三個執行緒。因為不保證有序性，可能執行出來是混亂的

解決如下：

第一種使用join()方法，這個方法的作用是，讓當前執行執行緒等待直到呼叫join方法的執行緒結束執行

程式碼如下：

class Thread1 extends Thread{
    public void run(){
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Threada");
    }
}


class Thread2 extends Thread{
    public void run(){
        try {
            Thread.sleep(20);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Threadb");
    }
}


class Thread3 extends Thread{
    public void run(){
        System.out.println("Threadc");
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
           Thread1 thread1=new Thread1();
               thread1.start();
               thread1.join();
           Thread2 thread2=new Thread2();
               thread2.start();
               thread2.join();
           Thread3 thread3=new Thread3();
           thread3.start();
//        ExecutorService executorService=Executors.newSingleThreadExecutor();
//        executorService.execute(thread1);
//        executorService.execute(thread2);
//        executorService.execute(thread3);
//        executorService.shutdown();
    }
}
 

第二種是通過執行緒池裡的方法：我們使用單執行緒池，這樣就能根據傳入的順序執行執行緒。

package didi;


import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

class Thread1 extends Thread{
    public void run(){
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Threada");
    }
}


class Thread2 extends Thread{
    public void run(){
        try {
            Thread.sleep(20);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Threadb");
    }
}


class Thread3 extends Thread{
    public void run(){
        System.out.println("Threadc");
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
           Thread1 thread1=new Thread1();
//               thread1.start();
//               thread1.join();
          Thread2 thread2=new Thread2();
//               thread2.start();
//               thread2.join();
          Thread3 thread3=new Thread3();
//           thread3.start();
        ExecutorService executorService=Executors.newSingleThreadExecutor();
        executorService.execute(thread1);
        executorService.execute(thread2);
        executorService.execute(thread3);
        executorService.shutdown();
    }
}