Input and Output in Java

A program needs two ways to communicate with a person:

• Output means the program shows information, such as a result or instruction, on the screen.
• Input means the person types information and the program reads it.

In a simple console program, Java uses System.out to show output and Scanner to read what the user types.

Show text and values on the screen

You already used System.out.println. Java gives three common output methods. They all show something on the screen; the difference is what happens after the text is printed.

Run this example to see its output.

Read this program line by line:

1. print("Java ") displays the word but keeps the cursor on the same line.
2. println("Basics") immediately adds Basics, then moves to a new line. Together the first two lines show Java Basics on one line.
3. printf(...) formats a value before showing it. %.1f means “show a decimal number with one digit after the decimal point.” %n moves to a new line.

For most early programs, use println. Use print before waiting for input, and use printf when decimal places matter.

Read what a user types with Scanner

When a program needs a name, age, score, or menu choice, it cannot know the value in advance. Scanner reads the text typed into the console after the user presses Enter.

Before using Scanner, add this line above your class:

This import tells Java that your program wants to use the ready-made Scanner class. Now run the complete program in a terminal. The live editor cannot ask for interactive input, so this particular example is designed for Command Prompt or PowerShell.

Run this example to see its output.

Here is what happens:

1. new Scanner(System.in) connects scanner to the keyboard input in the terminal.
2. print("Your name: ") asks the user a question without moving to the next line.
3. scanner.nextLine() waits for the user to type a full line and press Enter. That text is saved in name.
4. scanner.nextInt() waits for a whole number and saves it in lessons.
5. The final println joins the entered values into a sentence.
6. scanner.close() closes the input reader because the program has finished using it.

Try entering your own name and a number. The values printed by the program should be the values you typed, not fixed values from the code.

Different Scanner methods read different kinds of input

Choose a Scanner method based on what the user is expected to type.

Use nextLine() for names and sentences because it keeps spaces. next() would read only Asha from Asha Patel.

Why nextInt() and nextLine() can confuse beginners

nextInt() reads the number, but it leaves the Enter key press waiting in the input. If nextLine() comes immediately after it, that method reads the leftover Enter instead of waiting for new text.

Run this example to see its output.

The extra scanner.nextLine() after nextInt() removes that leftover Enter. Then the next nextLine() can wait for the feedback sentence. Whenever you read a number and then need a full line of text, use this pattern.

Format decimals with printf

printf is useful when a value should always have a fixed number of decimal places.

Run this example to see its output.

%s is replaced by text. %.2f is replaced by a decimal number rounded for display to two digits after the decimal point. %% prints one percent sign. %n moves to a new line. This does not change the original average variable; it only controls how Java displays it.

Common input mistake

If a program uses nextInt() and the person types ten instead of 10, Java cannot turn that word into a number. The program stops with an input error.

For now, enter the kind of value the prompt asks for. In a later lesson, you will learn validation so a program can show a friendly message instead of stopping.

Try it yourself

Create a program named AverageMarks. Ask the user for their name and two marks. Calculate the average, then print the learner's name and average with two decimal places.

Hint

Use nextLine() for the name, nextDouble() for each mark, and (firstMark + secondMark) / 2 for the average. Use printf with %.2f to display it.

One working answer

Run this example to see its output.

Run this in a terminal, enter different marks, and check whether the printed average changes correctly.