Console

Java programs often write to the console window. We use System.out—we read input, and write output, with this stream. And this fills many program requirements.

We can use println with no argument for a blank line. Different argument types, like arrays, strings, and ints are supported. The methods are versatile.

Println

This example uses the println and print methods. The println method adds a newline to the end of any output. It can even be used with no argument—this outputs only a newline.

public class Program {
    public static void main(String[] args) {
        System.out.println("Print line");
        System.out.print("A");
        System.out.print("B");
        System.out.println();
        System.out.println("C");
    }
}
Print line
AB
C

Char array

The print and println methods on System.out can receive a char array. This can avoid conversions—if we have a char array, we can print it directly.

public class Program {
    public static void main(String[] args) {

        char[] array = new char[3];
        array[0] = 'a';
        array[1] = 'b';
        array[2] = 'c';

        // Print a char array.
        System.out.println(array);
    }
}
abc

Ints

For primitive types like ints, chars and doubles we do not need to convert before using print and println. There are overloads that handle these types directly.

public class Program {
    public static void main(String[] args) {

        // Print ints directly, no conversion needed.
        int value = 10;
        System.out.println(value);
        System.out.println(value * 10);
    }
}
10
100

Append

System.out references a PrintStream. With this stream, we can append data. A CharSequence is accepted by the append method, so we can pass a string.

public class Program {
    public static void main(String[] args) {

        // Append strings to output stream.
        System.out.append("cat");
        System.out.append("\n");

        // Append range of CharSequence.
        System.out.append("java program", 5, 5 + 7);
    }
}
cat
program

Read line input

We wrap a BufferedReader around System.in to read strings from the console. The syntax is complicated, but once we get an available BufferedReader, it is easy to use.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Program {
    public static void main(String[] args) throws IOException {

        // Get BufferedReader for System.in.
        BufferedReader in = new BufferedReader(
            new InputStreamReader(System.in));

        // Read line from console.
        String line = in.readLine();
        System.out.println("You typed " + line);
    }
}
something
You typed something

Interactive program

This program uses a while-true loop to infinitely ask the user for input. It then parses the input into an int, using 0 for invalid values.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Program {
    public static void main(String[] args) throws IOException {

        // Read input with BufferedReader.
        BufferedReader in = new BufferedReader(
            new InputStreamReader(System.in));

        while (true) {
            // Read line and try to call parseInt on it.
            String line = in.readLine();
            int result;
            try {
                result = Integer.parseInt(line);
            } catch (NumberFormatException exception) {
                result = 0;
            }

            // Handle user input in a switch case.
            switch (result) {
            default:
                System.out.println("Default");
                break;
            case 1:
                System.out.println("One");
                break;
            case 2:
                System.out.println("Two");
                break;
            }
        }
    }
}
cat
Default
2
Two
1
One
Default

Summary

Java provides robust console support. It recognizes the utility of these programs. Console programs are often harder to use, but easier to develop.