Java中的命令行如何读写文件？

Question

I have that code in Python, but I want to do the same thing in Java.

import sys

def main(args):
    # Showing the arguments of the program
    print("#ARGS = %i" %len((args)))
    print("PROGRAM = %s" %(args[0]))
    print("ARG1 = %s, ARG2 = %s" %(args[1], args[2]))
    # Open files
    input = open(sys.argv[1],'r')
    output = open(sys.argv[2],'w')
    # ...
    # Closing files
    input.close()
    output.close()
    # end of the program

if __name__ == '__main__':
    main(sys.argv)

The same code in C

#include <stdio.h>

int main(int argc, char* argv[]) {
    // Showing the arguments of the program
    printf("#ARGS = %i\n", argc);
    printf("PROGRAMA = %s\n", argv[0]);
    printf("ARG1 = %s, ARG2 = %s\n", argv[1], argv[2]);
    // Opening files
    FILE* input = fopen(argv[1], "r");
    FILE* output = fopen(argv[2], "w");
    // ...
    // Closing files
    fclose(input);
    fclose(output);
    // End of the program
    return 0;
}

How can I create a code like that in Java? Is there a command like that "input = open(sys.argv[1],'r')" or "FILE* input = fopen(argv[1], "r")" in Java?

Answer 1

We can use Scanner to read from file, and use PrintWriter to write to a file.

public class FileIo {
    public static void main(String[] args) throws FileNotFoundException {
        try (
                Scanner in = new Scanner(new File("filename.txt"));
                PrintWriter out = new PrintWriter("anotherFileName.txt")
        ) {

            while (in.hasNextLine()) {
                String str = in.nextLine();
                out.println(str);
            }
        }
    }
}

In java, the try(resources) {} block will automatically close its resources.

Use args[0] and args[1] to replace the in and out file name.

Answer 2

java 9 or above

java provide repl env what is repl jshell since java version 9

C:\Users\User>jshell
| Welcome to JShell -- Version 9.0.4
| For an introduction type: /help intro

jshell> Math.round(34.543)
$1 ==> 35

read/write file with FileReader/FileWriter example

jshell> import java.io.*;

jshell> BufferedWriter writer = new BufferedWriter(new FileWriter("/Users/xxx/local/tempFiles/gids", true));
   ...> writer.write("appended text");
   ...> writer.close();
   ...>
writer ==> java.io.BufferedWriter@66a29884

jshell>

jshell> BufferedReader reader = new BufferedReader(new FileReader("/Users/xxx/local/tempFiles/gids"));
   ...> String currentLine = reader.readLine();
   ...> reader.close();
   ...>
reader ==> java.io.BufferedReader@17a7cec2
currentLine ==> "appended textappended text"

jshell> currentLine
currentLine ==> "appended textappended text"

jshell>

java 8

1. put a while(true) loop in main method, and get user input, and process things accordingly. For example, q for exit, r /User/xxx/file for read file '/User/xxx/file', and it is not great because you have to consider all the requirements and needs and it is set, you can not change the code at runtime.

2. groovysh , you can run java code in it