Java试题

1、不使用循环，等比数列输出整型 n、2n、4n、8n……当大于max时，反向输出8n、4n、2n、n。

例如 n=10，max=100。 输出：

10

20

40

80

80

40

20

10

解题思路： 递归实现。

public class Test {

    public static void main(String[] args) {
        multiply(10, 100);
    }
    public static void multiply(int n, int max) {
        if (n < max) {
            System.out.println(n);
            multiply(2 * n, max);
            System.out.println(n);
        }
    }
}

如果需要输出大于100后的数，则代码如下：

public class Test {

    public static void main(String[] args) {

        multiply(10, 100,10);

    }

    public static void multiply(int n, int max,int min) {
        if (n < max) {
            System.out.println(n);
            multiply(2 * n, max, min);
        } else{
            System.out.println(n);
            divide(n,min);
        }
    }

    public static void divide(int n, int min) {
        if (n >= min) {
            System.out.println(n);
            divide(n / 2, min);
        }
    }
}

2、Comparator 排序。 对某些对象 collection 进行整体排序。排序时将此接口的实现传递给Collections.sort方法或者Arrays.sort方法排序。实现 o1.compareTo(o2) 方法。如果是升序排序：o1小于o2，返回-1（负数），相等返回0，o1大于o2，返回1（正数）；反之降序。

代码如下：

public class Supermarket{

    private String name;
    private double distance; //距离
    private boolean isBusiness; // 是否营业
    private double money; // 营业额

    public Supermarket(String name, double distance, boolean isBusiness,
            double money) {
        super();
        this.name = name;
        this.distance = distance;
        this.isBusiness = isBusiness;
        this.money = money;
    }

    @Override
    public String toString() {
        return name + "\t" + distance + "\t" + (isBusiness ? "是" : "否") + "\t"
                + money + "\r\n";
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getDistance() {
        return distance;
    }

    public void setDistance(double distance) {
        this.distance = distance;
    }

    public boolean isBusiness() {
        return isBusiness;
    }

    public void setBusiness(boolean isBusiness) {
        this.isBusiness = isBusiness;
    }

    public double getMoney() {
        return money;
    }

    public void setMoney(double money) {
        this.money = money;
    }

}

public class Menu {

    public static void main(String[] args) {

        List<Supermarket> list = new ArrayList<Supermarket>();

        Supermarket s1 = new Supermarket("天天超市", 1.0, true, 5000);
        Supermarket s2 = new Supermarket("地地超市", 3.5, false, 2000);
        Supermarket s3 = new Supermarket("人人超市", 2.5, true, 6000);
        Supermarket s4 = new Supermarket("神神超市", 5.0, false, 1000);
        Supermarket s5 = new Supermarket("佛佛超市", 2.0, true, 3000);

        list.add(s1);
        list.add(s2);
        list.add(s3);
        list.add(s4);
        list.add(s5);

//        System.out.println(list);
        String choose ="";
        Scanner input = new Scanner(System.in);
        while(!"0".equals(choose)){
            System.out.println("1、店名\t2、距离\t3、是否营业\t4、营业额\t0、退出");
            System.out.println("请选择：");
            choose = input.next();
            switch (choose) {
            case "1":
                Collections.sort(list,new Comparator<Supermarket>() {
                    @Override
                    public int compare(Supermarket o1, Supermarket o2) {
                        return o1.getName().compareTo(o2.getName());
                    }
                });
                for (Supermarket supermarket : list) {
                    System.out.print(supermarket.toString());
                }
                break;
            case "2":
                Collections.sort(list, new Comparator<Supermarket>() {

                    @Override
                    public int compare(Supermarket o1, Supermarket o2) {
                        if(o1.getDistance()<o2.getDistance())
                            return -1;
                        return 1;
                    }
                });
                for (Supermarket supermarket : list) {
                    System.out.print(supermarket.toString());
                }
                break;
            case "3":
                Collections.sort(list, new Comparator<Supermarket>() {

                    @Override
                    public int compare(Supermarket o1, Supermarket o2) {
                        if(o1.isBusiness())
                            return -1;
                        return 1;
                    }
                });
                for (Supermarket supermarket : list) {
                    System.out.print(supermarket.toString());
                }
                break;
            case "4":
                Collections.sort(list, new Comparator<Supermarket>() {

                    @Override
                    public int compare(Supermarket o1, Supermarket o2) {
                        if(o1.getMoney()<o2.getMoney())
                            return -1;
                        return 1;
                    }
                });
                for (Supermarket supermarket : list) {
                    System.out.print(supermarket.toString());
                }
                break;
            case "0":
                System.out.println("程序结束。");
                break;
            default:
                System.out.println("你输入的有误，请重新输入！");
                break;
            }
        }
        input.close();
    }

}

3、SimpleDateFormat 时间转换工具类。用来格式化时间日期。

示例：显示当前时间，格式为 格式为 2017年7月15日 上午9点42分23秒， 每5秒显示一次时间。

解题思路：创建线程，格式化日期。 代码如下：

public class Timer extends Thread {

    @Override
    public void run() {

        SimpleDateFormat format = new SimpleDateFormat("yyyy年MM月dd日 a HH时mm分ss秒");    // a --> AM, PM
        try {
            while (true) {
                Date date = new Date();    //获取当前时间
                String time = format.format(date); //格式化时间
                System.out.println(time);
                Thread.sleep(5000);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

4、创建两个基于udp的应用程序，其中一个发送10次随机整数到另一个应用程序，另外一个应用程序将所有整数进行相加后，将所有数的累加（x1+x2+x3+...+x=y）发往第一个应用程序，第一个应用程序将结果打印出来。

代码如下：

//发送端：

public class Send extends Thread {

    DatagramSocket socket;

    public Send() {
        try {
            socket = new DatagramSocket(7777);
        } catch (SocketException e) {
            e.printStackTrace();
        }
    }

    public void sendNumber() {
        InetSocketAddress address = new InetSocketAddress("127.0.0.1", 8888);
        for (int i = 0; i < 10; i++) {
            int num = (int) (Math.random() * 100);
            String msg = num + "";
            DatagramPacket packet = new DatagramPacket(msg.getBytes(),
                    msg.getBytes().length, address);
            try {
                socket.send(packet);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    @Override
    public void run() {
        byte[] buf = new byte[1024];
        DatagramPacket packet = new DatagramPacket(buf, buf.length);
        try {
            socket.receive(packet);
            System.out.println("收到信息："+ new String(packet.getData(),0,packet.getLength()));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

//接收端

public class Receiver extends Thread {

    DatagramSocket socket;

    public Receiver() {
        try {
            socket = new DatagramSocket(8888);
        } catch (SocketException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void run() {
        byte[] buf = new byte[1024];
        DatagramPacket packet = new DatagramPacket(buf, buf.length);
        List<Integer> list = new ArrayList<Integer>();
        for (int i = 0; i < 10; i++) {
            try {
                socket.receive(packet);
                int number = Integer.parseInt(new String(packet.getData(), 0,
                        packet.getLength()));
                list.add(number);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        System.out.println(list);
        int sum = 0;
        StringBuffer sb = new StringBuffer();
        for (Integer integer : list) {
            sum += integer;
            sb.append(integer + "");
        }
        String str = sb.toString();
        str = str.substring(0, sb.length() - 1);
        str += "=" + sum;
        InetSocketAddress address = new InetSocketAddress("127.0.0.1", 7777);
        DatagramPacket sendPacket = new DatagramPacket(str.getBytes(),
                str.getBytes().length, address);
        try {
            socket.send(sendPacket);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

//Test测试
public class Test {

    public static void main(String[] args) throws IOException {

        new Receiver().start();
        Send send = new Send();
        send.start();
        send.sendNumber();

    }
}