Input and Output

Programs need to communicate with users. The most basic interactions are printing output and reading keyboard input.

System.out.println()

System.out.println() displays text or values and then ends the line.

Some characters have special meaning in code, so they need escape sequences.

• \\: backslash
• \': single quote
• \": double quote
• \n: newline
• \t: tab

System.out.println("\"Hello\nWorld\"");

Output:

"Hello
World"

Formatted output

You can also insert values into a string with placeholders.

• %d: integer
• %f: floating-point number
• %s: string

int length = 10;
int width = 5;
double area = length * width;
System.out.printf("Area = %d * %d = %.2f%n", length, width, area);

Output:

Area = 10 * 5 = 50.00

The placeholders correspond to the following arguments in order; %.2f formats a floating-point value with two decimal places.

System.out.printf("Area = %d * %d = %.2f%n", length, width, area);

Controlling the line ending

System.out.println() ends with a newline, while System.out.print() leaves the cursor on the same line.

int num1 = 1;
int num2 = 2;
int num3 = 4;
int num4 = 8;

System.out.print(num1 + ", ");
System.out.print(num2 + ", ");
System.out.print(num3 + ", ");
System.out.print(num4 + "...");

Output:

1, 2, 4, 8...

Scanner input

Scanner reads keyboard input. Create one for System.in, then call next(), nextInt(), or nextDouble().

next() returns a String; nextInt() and nextDouble() parse numeric input directly.

Scanner scanner = new Scanner(System.in);
System.out.print("Radius: ");
double r = scanner.nextDouble();
double area = Math.PI * r * r;
System.out.printf("Area = %.2f%n", area);

One possible run:

Radius: 5
Area = 78.54

Here, scanner.nextDouble() reads a double directly, while Math.PI is Java’s standard-library constant for pi.