尝试从 Java 中的控制台读取

Question

I have just started learning Java with IntelliJ IDE. I know a bit C# so the logic makes some sense, however there is one thing so far I couldn't get over it.

How do I read from the console? In C#, you could easily read what the human typed into it, using Console.ReadLine() . In Java, System.console().readLine(); does not work for me and throws a NullPointerException .

What am I missing here?

Answer 1

NOTE: problem doesn't appear when we run your code from console/terminal via java [options] [MainClass] so if this is valid solution for you you can stop reading here. Rest of this answer is for people who are using some IDEs to run their code.

Problem

Most IDEs are using javaw.exe instead of java.exe to run Java code (see image below).

Difference between these two programs is that javaw runs Java code without association with current terminal/console (which is useful for GUI applications), and since there is no associated console window System.console() returns null . Because of that System.console().readLine() ends up as null.readLine() which throws NullPointerException since null doesn't have readLine() method (nor any method/field).

But just because there is no associated console, it doesn't mean that we can't communicate with javaw process. This process still supports standard input/output/error streams, so IDEs process (and via it also we) can use them via System.in , System.out and System.err .

This way IDEs can have some tab/window and let it simulate console.

For instance when we run code like in Eclipse:

package com.stackoverflow;

public class Demo {

    public static void main(String[] args) throws Exception {
        System.out.println("hello world");
        System.out.println(System.console());
    }

}

we will see as result

which shows that despite javaw.exe not having associated console ( null at the end) IDE was able to handle data from standard output of the javaw process System.out.println("hello world"); and show hello world .

General solution

To let user pass information to process use standard input stream ( System.in ). But since in is simple InputStream and Stream s are meant to handle binary data it doesn't have methods which would let it easily and properly read data as text (especially if encoding can be involved). That is why Readers and Writers ware added to Java.

So to make life easier and let application read data from user as text you can wrap this stream in one of the Reader s like BufferedReader which will let you read entire line with readLine() method. Unfortunately this class doesn't accept Streams but Reader s, so we need some kind of adapter which will simulate Reader and be able to translate bytes to text. But that is why InputStreamReader exists.

So code which would let application read data from input stream could look like

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.print("Hello. Please write your name: ");
String name = br.readLine();
System.out.println("Your name is: "+name);

Preferred/simplest solution - Scanner

To avoid this magic involving converting Stream to Reader you can use Scanner class, which is meant to read data as text from Streams and Readers.

This means you can simply use

Scanner scanner = new Scanner(System.in);
//...
String name = scanner.nextLine();

to read data from user (which will be send by console simulated by IDE using standard input stream).

Answer 2

If you realy need to Console object you can compile your class from command line. Firstly in my java file first statement is package com.inputOutput;

Go in your project " src " folder and compile it like : " javac com/inputOutput/Password.java " ' com ' and ' inputOutput ' are folder(package). Run your class file in src folder
java com.inputOutput.Password ". It had worked work for me.

Answer 3

You could use an Jframe.

JFrame frame = new JFrame("titile");

// prompt the user to enter their code
String code = JOptionPane.showInputDialog(frame, "promt here");

Answer 4

java.util.Scanner API 正是您要找的。