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。

 1 public class Question1 {
 2 
 3     private int j = 0;
 4     /**
 5      * @param args
 6      */
 7     public static void main(String[] args) {
 8         Question1 q = new Question1();
 9         Dec dec = q.new Dec();
10         Inc inc = q.new Inc();
11         for(int i=0; i<2;i++){
12             Thread t2 = new Thread(inc);
13             t2.start();
14             Thread t1 = new Thread(dec);
15             t1.start();
16             
17         }
18     }
19     
20     private synchronized void inc(){
21         j++;
22         System.out.println( Thread.currentThread().getName()+"-inc:"+j);
23     }
24     
25     private synchronized void dec(){
26         j--;
27         System.out.println( Thread.currentThread().getName()+"-dec:"+j);
28     }
29     class Inc implements Runnable {
30 
31         @Override
32         public void run() {
33             inc();
34             
35         }
36     }
37     
38     class Dec implements Runnable {
39 
40         @Override
41         public void run() {
42             dec();
43             
44         }
45         
46     }
47 }

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();
            
        }
        
    }
}