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🧠 8.4 Interacting With Users

  • We shall lookk at the java.io.Console class which is an evolved form of the System.in/System.out methods,

🟥 8.4.1 The Old Way

  • Suppose we want to obtain input from the user
  • We need to use System.in and wrap it with InputStreamReader class
  • We then use BufferedReader so that the user can enter multiple characters and use termination (enter):
try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
  System.out.println("Write something:");
  String userInput = reader.readLine();
  System.out.println("This is what you wrote: "+userInput);
} catch (IOException e) {
  
}
  • Running the program:

🟥 8.4.2 The New Way

  • Java 6 introduced java.io.Console which is a singleton which is accessed using System.console() - this can potentially return null
  • Here is the previous program rewritten:
Console console = System.console();
if (console!=null) {
  System.out.println("Write something:");
  String userInput = console.readLine();
  System.out.println("This is what you wrote: "+userInput);
}

🟡 reader() and writer()

  • The Console instance can also give us an instance of Reader and PrintWriter:
Console console = System.console();
Reader reader = console.reader();
PrintWriter printWriter = console.writer();

🟡 format() and printf()

  • We can use the format(String, Objects...) method directly. This method only has one signature which does NOT take a locale variable
public class ConsoleSamplePrint {
  public static void main(String[] args) throws NumberFormatException, IOException {
    Console console = System.console();
    if (console == null) {
      throw new RuntimeException();
    } else {
      console.writer().println("Welcome to Our Zoo!");
      console.format("Our zoo has %f animals and employs %f people.",201, 25);
    }
  }
}

🟡 flush()

  • The flush() method forces all buffered output to be written immediately. This ensures no data is lost when calling readLine() or readPassword()
  • Failure to call this method can result in no text pormpt before obtaining user input.

🟡 readLine()

  • This method takes user input terminated by the enter key
  • We also have an overloaded method which displays formatted text before reading input readLine(String format, Objects...)
Console console = System.console();
console.writer().println("Enter your name: ");
String userName = console.readLine();
console.format("Hi %s, your name has %d characters", userName, userName.length());
/*
  Enter your name: 
  shiv
  Hi shiv, your name has 4 characters
*/

🟡 readPassword()

  • This method is likek readLine, except it will not echo the text the user is typing
  • This method returns a char[] instead of string, because this can be garbage collected rather than kept in String pool
Console console = System.console();
if (console == null) {
  throw new RuntimeException("Console unavailable");
} else {
  char[] password = console.readPassword("Enter your password: ");
  // immediately clear password from memory
  for (int i=0;i<password.length;i++)
    password[i]='x';
}