public class text1 {

      public static void main(String[] args) {   //程序入口

          // TODO Auto-generated method stub

                 String s1="Hello!";            //定义了一个String，名为s1,并将字符串Hello赋值给s1

                 String s2="World";             //定义了一个String，名为s2，并将字符串World赋值给s2

                 String s3=s1+s2;               //定义了一个String，名为上，并将s1+s2的值赋给s3

                 System.out.println(s3);        //输出s3的值

     }

10 }

 public class text2 {

     public static void main(String[] args) {

         // TODO Auto-generated method stub

             StringBuilder s1 = new StringBuilder("Hello!");              //在字符串构建器StringBuilder中创建一个新的StringBuilder

             StringBuilder s2 = new StringBuilder("World");

             StringBuilder s3 = new StringBuilder().append(s1).append(s2);//用StringBuilder将s1和s2拼接起来

             System.out.println(s3);

      }

 }

 import java.io.File;

 import java.io.FileNotFoundException;

 import java.io.PrintWriter;

 import java.util.Scanner;

 import java.util.Arrays;

 public class WriteReadFileTest {

     /**

      * @param args

      * @throws FileNotFoundException

      */

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

         //写入文件演示

         PrintWriter out = new PrintWriter("myfile.txt");

         out.println("姓名 高数 Java 数据结构 平均成绩 总成绩");

         out.println("张三 20 30 40 0 0");

         out.println("李四 50 60 70 0 0");

         out.close();//切记，输出完毕，需要close

         //读入文件演示

         Scanner in = new Scanner(new File("myfile.txt"));//为myfile.txt这个File创建一个扫描器in

         int number = 1;//行号

         while(in.hasNextLine()){//判断扫描器是否还有下一行未读取，该循环把文件的每一行都读出

             String line = in.nextLine();//读出myfile.txt的下一行

             System.out.println("第"+(number++)+"行的内容" +"="+line);

             Scanner linescanner = new Scanner(line);//对每一行的内容建立一个扫描器

             linescanner.useDelimiter(" ");//使用空格作为分隔符

             String name = linescanner.next();

             String math = linescanner.next();

             String java = linescanner.next();

             String ds = linescanner.next();

             String avg = linescanner.next();

             String total = linescanner.next();

             System.out.println("name="+name+" math="+math+" java="+java+" ds="+ds+" avg"+avg+" total="+total);

         }

         in.close();//读入完毕，最后需要对其进行close。    

  }

 }

 import java.util.*;

 /**

  * This program demonstrates a <code>while</code> loop.

  * @version 1.20 2004-02-10

  * @author Cay Horstmann

  */

 public class Retirement

 {

    public static void main(String[] args)

    {

       // read inputs

       Scanner in = new Scanner(System.in);

       System.out.print("How much money do you need to retire? ");

       double goal = in.nextDouble();

       System.out.print("How much money will you contribute every year? ");

       double payment = in.nextDouble();

       System.out.print("Interest rate in %: ");

       double interestRate = in.nextDouble();

       double balance = 0;

       int years = 0;

       // update account balance while goal isn't reached

       while (balance < goal)

       {

          // add this year's payment and interest

          balance += payment;

          double interest = balance * interestRate / 100;

          balance += interest;

          years++;

       }

       System.out.println("You can retire in " + years + " years.");

    }

 }

 import java.util.*;

 /**

  * This program demonstrates a <code>do/while</code> loop.

  * @version 1.20 2004-02-10

  * @author Cay Horstmann

  */

 public class Retirement2

 {

    public static void main(String[] args)

    {

       Scanner in = new Scanner(System.in);

       System.out.print("How much money will you contribute every year? ");

       double payment = in.nextDouble();

       System.out.print("Interest rate in %: ");

       double interestRate = in.nextDouble();

       double balance = 0;

       int year = 0;

       String input;

       // update account balance while user isn't ready to retire

       do

       {

          // add this year's payment and interest

          balance += payment;

          double interest = balance * interestRate / 100;

          balance += interest;

          year++;

          // print current balance

          System.out.printf("After year %d, your balance is %,.2f%n", year, balance);

          // ask if ready to retire and get input

          System.out.print("Ready to retire? (Y/N) ");

          input = in.next();

       }

       while (input.equals("N"));

    }

 }

 import java.util.*;

 /**

  * This program demonstrates a <code>for</code> loop.

  * @version 1.20 2004-02-10

  * @author Cay Horstmann

  */

 public class LotteryOdds

 {

    public static void main(String[] args)

    {

       Scanner in = new Scanner(System.in);

       System.out.print("How many numbers do you need to draw? ");

       int k = in.nextInt();

       System.out.print("What is the highest number you can draw? ");

       int n = in.nextInt();

       /*

        * compute binomial coefficient n*(n-1)*(n-2)*...*(n-k+1)/(1*2*3*...*k)

        */

       int lotteryOdds = 1;

       for (int i = 1; i <= k; i++)

          lotteryOdds = lotteryOdds * (n - i + 1) / i;

       System.out.println("Your odds are 1 in " + lotteryOdds + ". Good luck!");

    }

 }