hostlookup.java

该程序是一个主机查找程序,运行后输入你想要查找的主机域名,它将会找到该主机的IP地址,

import java.net.*;
import java.io.*;
public class HostLookup {
	public static void main (String[] args) {
		if (args.length > 0) { // 使用命令行的方式
			for (int i = 0; i < args.length; i++) {
				System.out.println(lookup(args[i]));
			}
		}
		else {
			//使用BufferedReader类来装饰输入流
			BufferedReader in = new BufferedReader(
				new InputStreamReader(System.in));
			System.out.println(
				"Enter names and IP addresses. Enter \"exit\" to quit.");
			try {
				while (true) {
					String host = in.readLine( );
					if (host.equals("exit")) break;
					System.out.println(lookup(host));
				}
			}
			catch (IOException e) {
				System.err.println(e);
			}
		}
	} /*main的结尾*/
	private static String lookup(String host) {
		InetAddress thisComputer;
		byte[] address;
		// 获得IP地址的类型
		try {
			thisComputer = InetAddress.getByName(host);
			address = thisComputer.getAddress( );
		}
		catch (UnknownHostException e) {
			//返回一个字符串
			return "Cannot find host " + host;
		}
		if (isHostName(host)) {
			// 打印IP地址 
			String dottedQuad = "";
			for (int i = 0; i < address.length; i++) {
				int unsignedByte = address[i] < 0 ? address[i] + 256 :
				address[i];
				dottedQuad += unsignedByte;
				if (i != address.length-1) dottedQuad += ".";
			}
			return dottedQuad;
		}
		else { // 这是IP地址
			return thisComputer.getHostName( );
		}
	} //lookup的结尾
	private static boolean isHostName(String host) {
		char[] ca = host.toCharArray( );
		//当字符不是数字或句号
		// 这是一个主机名字
		for (int i = 0; i < ca.length; i++) {
			if (!Character.isDigit(ca[i])) {
				if (ca[i] != '.') return true;
			}
		}
		// 所有的是数字或句号
		// 所以主机名字像IP地址一样
		return false;
	}  
}