example0601_io.java

设计一个应用程序

/* 设计一个应用程序，从键盘输入一个整数n，
 * 然后在文件"data.dat"中写入所有比n小的素数，
 * 最后在屏幕上显示在每个素数中各数字（0--9）出现的次数
 */
 
 import java.io.*;
 
 public class Example0601_IO
 {
 	public static boolean isPrime(int n)
 	{
 		for (int i = 2 ; i <= n/2 ; i++)
 		{
 			if (n % i == 0) return false;
 		}
 		
 		return true;
 	}
 	
 	public String getDigitAppearanceCount(int n)
 	{
 		StringBuffer s = new StringBuffer("\n素数 " + n + " 中各数字出现的次数如下：\n");

 		int[] c = new int[10];
 		
 		while (n != 0)
 		{
 			c[n % 10]++;
 			n /= 10;
 		}
 		
 		for (int i = 0 ; i < 10 ; i++)
 		{
 			if (c[i] != 0) s.append("count(" + i + ") = " + c[i] + "\n");
 		}
 		
 		return s.toString();
 	}
 	
 	public static void main(String[] args)
 	{
 		try
 		{
 			System.out.print("Please input n = ");
 			
 			BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
	 		String strn = reader.readLine();
	 		
	 		int n = Integer.parseInt(strn);
	 		
	 		DataOutputStream output = new DataOutputStream(new FileOutputStream("data.dat"));
	 		for (int i = 2 ; i <= n ; i++)
	 		{
	 			if (isPrime(i))
	 			{
	 				System.out.println(new Example0601_IO().getDigitAppearanceCount(i));
	 				output.writeInt(i);
	 			}
	 		}
	 		output.close();
	 		
	 		/* 下面的程序段用于读取文件的内容并输出
 	 		DataInputStream input = new DataInputStream(new FileInputStream("data.dat"));
 	 		while (input.available() > 0)
	 		{
 				System.out.println(input.readInt());
	 		}
	 		input.close();
	 		**************************************/
	 		
	 		/* 下面的程序段使用PrintStream流类输出数据
	 		PrintStream output2 = new PrintStream(new FileOutputStream("data.txt"));
	 		for (int i = 2 ; i <= n ; i++)
	 		{
	 			if (isPrime(i))
	 			{
	 				output2.printf("%d ",i);
	 			}
	 		}
	 		output2.close();
	 		**************************************/
		}
 		catch (IOException e)
 		{
 			System.out.println(e.getMessage());
 		}
 		catch (NumberFormatException e)
 		{
 			System.out.println(e.getMessage());
 		}
 	}
 }