java - 为什么要测试args.length？

Question

我是一名 Java 初学者，我对在许多代码开头测试 args.length 感到困惑，为什么它在我的任何代码中永远不会高于 0？

import java.net.Socket;
import java.net.UnknownHostException;
import java.io.IOException;

public class LowPortScanner {
public static void main(String[] args) {
String host = "localhost";
if (args.length > 0) {
host = args[0];
}
for (int i = 1; i < 1024; i++) {
try {
Socket s = new Socket(host, i);
System.out.println("There is a server on port " + i + " of "
+ host);
}
catch (UnknownHostException ex) {
System.err.println(ex);
break;
}
catch (IOException ex) {}
} // end for
} // end main
} // end PortScanner

Answer 1

您必须从命令提示符输入main 方法。

如下所示

java LowPortScanner TEST1 TEST2