java线程面试手写题

1.设计4个线程，其中两个线程每次对j增加1，另外两个线程对j每次减少1。写出程序。

 public class Question1 {

     private int j = 0;
     /**
      * @param args
      */
     public static void main(String[] args) {
         Question1 q = new Question1();
         Dec dec = q.new Dec();
         Inc inc = q.new Inc();
         for(int i=0; i<2;i++){
             Thread t2 = new Thread(inc);
             t2.start();
             Thread t1 = new Thread(dec);
             t1.start();

         }
     }

     private synchronized void inc(){
         j++;
         System.out.println( Thread.currentThread().getName()+"-inc:"+j);
     }

     private synchronized void dec(){
         j--;
         System.out.println( Thread.currentThread().getName()+"-dec:"+j);
     }
     class Inc implements Runnable {

         @Override
         public void run() {
             inc();

         }
     }

     class Dec implements Runnable {

         @Override
         public void run() {
             dec();

         }

     }
 }

2.设计4个线程， 其中3个线程将一个长度为3的数组每项增加1，第4个线程输出数组3个数的和写出程序。

public class Question2 {
    int[] a = new int[3];
    /**
     * @param args
     */
    public static void main(String[] args) {
        Question2 q = new Question2();
        Inc inc = q.new Inc();
        Print p = q.new Print();
        for(int i=0; i<3; i++){
            Thread t1 = new Thread(inc);
            t1.start();
        }
        Thread t2 = new Thread(p);
        t2.start();
    }

    private synchronized void inc(){
        for(int i=0;i<a.length;i++){
            a[i] += 1;
        }

        System.out.println(Thread.currentThread().getName()+"-inc:"+Arrays.toString(a));
    }

    private synchronized void print(){
        int sum = 0;
        for(int i=0;i<a.length;i++){
            sum += a[i];
        }
        System.out.println(Thread.currentThread().getName()+"-print:"+Arrays.toString(a)+"和为："+sum);
    }

    class Inc implements Runnable {

        @Override
        public void run() {
            inc();

        }

    }

    class Print implements Runnable {

        @Override
        public void run() {
            print();

        }

    }
}