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Chapter 1

Input and output

Every program in this chapter does the same thing: it reads something, calculates with it and prints a result. That's all you need for the first tasks.

The frame is always the same. Your code goes between the curly braces of main, and Java runs it line by line from top to bottom. What public class and static mean exactly comes in later chapters. Until then, just leave the frame as it is.

Output and variables

System.out.println(...) prints a line. A variable is a named place that stores a value. When you create one, you put its type in front:

  • int for whole numbers, e.g. int age = 19;
  • double for decimal numbers, e.g. double price = 4.99; (with a dot, not a comma)
  • String for text in double quotes, e.g. String name = "Ada";

+ joins texts and values together. Every statement ends with a semicolon.

You can change the code and run it again.

Try it: change the name or the age and run the example again.

Reading input and calculating

A Scanner reads what someone types into the console. nextInt() gets the next whole number, nextDouble() the next decimal number and nextLine() a whole line of text. Spaces and line breaks between numbers don't matter.

Java calculates with +, -, *, / and %. Two things are new:

  • If you divide two int values, the decimal places are dropped: 17 / 5 gives 3.
  • % (modulo) gives the remainder of the division: 17 % 5 gives 2.

As soon as a decimal number is involved, Java keeps the decimal places, for example / 2.0 instead of / 2.

You can change the code and run it again.

The “Input” field holds what the program reads. Change the numbers and watch the result and the remainder change.

Nice formatting with printf

Decimal numbers often have lots of decimal places. System.out.printf lets you decide how a value looks. The text contains placeholders that are replaced, in order, by the values after it:

  • %.2f: decimal number with exactly two decimal places (rounded)
  • %d: whole number
  • %s: text
  • %n: line break (printf doesn't add one by itself)
You can change the code and run it again.

Common mistakes

  • Integer division

    double average = (2 + 3) / 2; gives 2.0, not 2.5. The division happens with two int values, and only then is the result stored in a double. Fix: divide by 2.0.

  • Plus joins text

    Once there's a text on the left, + just appends everything:

    System.out.println("Sum: " + 2 + 3);   // Sum: 23
System.out.println("Sum: " + (2 + 3)); // Sum: 5

    So put calculations in the output in parentheses.

  • Typos the compiler reports

    Java is case-sensitive: it doesn't know system.out.println or string. If a semicolon is missing, the compiler reports ';' expected. The line number in the message usually points to that spot or the line before it.

  • The output has to match exactly

    The tests compare your output character by character. A missing space, an extra comma or print instead of println is enough to fail a test. Compare your output carefully with the task's example.