Is it possible that after introducing the nth array character into the console, the program will automatically start (without Enter).

Since it is not entirely clear what exactly you mean by launching the program, I will answer two possible interpretations of the question.

Terminal management before launch

If you assume that the operating system will start the JVM at the moment when you write the last character of the following command:

$ java com.example.Main 4532 

then unfortunately it is incredibly difficult to do and strongly depends on the operating system’s terminal settings provided and you cannot do it from your program, since control of your program will not be transferred until it is launched, and you want to change the startup behavior.

Terminal management inside the program

Now consider another option. Suppose you run your application of arguments, after which the user sees a sentence to enter a four-digit number:

 $ java com.example.Main Enter 4-digit number: 

And in this case, we consider the task to start processing the input data as soon as the user enters 4 digits.

The short answer is "Yes, it is possible." In principle, in programming, any behavior is possible to implement. However, the answer will not be cross-platform until you take care of it, or do not use a library that already cares about it. For example, jCurses will provide the necessary functionality.

Why do I even speak about the need to provide a specific platform? The fact is that System.in connects to the system terminal, which in most cases, by default, implements input buffering, that is, it sends input data to the program after entering user input.

If a healthy curiosity of a programmer leads you to realize the desired behavior, let's write a naive (and, therefore, the shortest) working version from which you can begin to satisfy curiosity.

Immediately, I can write with you a program that will implement the behavior you need on unix-like systems. I apologize in advance for ignoring the canonical error handling rules of I / O systems: I am sure that you are able to independently supplement the program to a safe option.

Let's start with the implementation of the method of reading n digits from the stream:

 private static char[] readInRawMode(int numberOfCharacters, InputStream stream) throws IOException { char [] result = new char[numberOfCharacters]; InputStreamReader is = new InputStreamReader(stream); int characterCount = 0; while (characterCount < numberOfCharacters) { char aChar = (char) is.read(); if (aChar < '0' || aChar > '9') { //проверим, что это цифра continue; } result[characterCount] = aChar; characterCount++; } return result; } 

If we now use this method in a program, for example readInRawMode(4, System.in) , then the is.read() call is.read() blocked until the user presses enter . In order to avoid this behavior, we need to switch the current system terminal to raw mode before calling the method and return it to cooked mode after the processing method completes. In raw mode, the terminal does not repeat the characters entered by the user on the screen (as when entering a password in the terminal) and we must do it ourselves. Moreover, the raw terminal will no longer respond to special characters including the carriage return character and the user’s enter will also be ignored and not displayed until he has entered the required number of digits. This is exactly the behavior that suits us.

Putting it all together we get something like this:

 package com.alex; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStream; import java.io.InputStreamReader; import java.util.Arrays; public class Main { public static void main(String[] args) throws IOException, InterruptedException { System.out.print("Enter 4-digit number : "); char [] input = interactiveRead(); System.out.println(); System.out.println("User input: " + Arrays.toString(input)); } private static char[] interactiveRead() throws IOException, InterruptedException { String[] raw = {"/bin/sh", "-c", "stty raw </dev/tty"}; //команда перевода текущего терминала в режим raw String[] cooked = {"/bin/sh", "-c", "stty cooked </dev/tty"}; //команда перевода текущего терминала в режим cooked Process p = Runtime.getRuntime().exec(raw); p.waitFor();//ожидаем перевода терминала try { return readInRawMode(4, System.in); } finally { p = Runtime.getRuntime().exec(cooked); //возвращаем терминал к изначальному виду вне зависимости от ошибок p.waitFor(); } } private static char[] readInRawMode(int numberOfCharacters, InputStream stream) throws IOException { char [] result = new char[numberOfCharacters]; InputStreamReader is = new InputStreamReader(stream); int characterCount = 0; while (characterCount < numberOfCharacters) { char aChar = (char) is.read(); if (aChar < '0' || aChar > '9') { continue; } System.out.print(aChar); // повторяем "правильный" символ на экране, чтобы пользователь понял что он все делает правильно result[characterCount] = aChar; characterCount++; } return result; } } 

Discussion and comments

In fact, in most cases it makes no sense to write the complex logic of the "rich" user interaction with the terminal. This gives a strong complication to the application logic (what we just described is not enough: you need to take care of execution errors and control commands such as ^ C in the terminal), and the profit is minimal: in the modern world, users prefer graphical interfaces, and people used to working with the terminal and so they will understand what is happening and what exactly you need to enter: these are people of a technical mentality.

I also want to note that the written application is sharpened to start from the terminal by hand and only then it will work correctly. In the case of launching through IDE, its native terminal will not be transferred to the required mode, since it uses not a standard system terminal, but its wrapper, which also implements buffering. I hope this makes it even more understandable to write a universal solution for your problem.