Scanner & Interactive Input

In Plain English

The `Scanner` class from the `java.util` package is the standard tool for reading user input in console applications. It parses primitive types and strings using regular expressions from `System.in`.

The Mental Model

"Scanner is like an attentive stenographer listening to System.in. When you ask for an integer with .nextInt(), it listens until a number is typed and delivers it into your variable."

Deep Dive: How It Works

Import required: `import java.util.Scanner;` at the top of your file.

Instantiation: `Scanner scanner = new Scanner(System.in);`

Methods: `.nextLine()` (reads full line text), `.next()` (reads single word), `.nextInt()`, `.nextDouble()`, `.nextBoolean()`.

Closing: Call `scanner.close()` when finished to release system resources.

Syntax Blueprint

import java.util.Scanner;

Scanner sc = new Scanner(System.in);
String name = sc.nextLine();
int age = sc.nextInt();
sc.close();

Always import Scanner from java.util and instantiate with System.in.

Core Rules to Remember

Import Required: Must have import java.util.Scanner; at top of source file.

Type-Specific Readers: Use .nextInt() for numbers, .nextLine() for full string lines.

Standard Input: System.in represents the keyboard or standard input stream.

Common Beginner Traps & How to Fix Them

Mistake:Calling nextLine() immediately after nextInt()

Why it happens: nextInt() consumes the number but leaves the newline character in the buffer.

How to fix: Call an extra scanner.nextLine() to consume the leftover newline before reading text.

Pro Tip: In ZenCompiler, you can simulate input streams or construct Scanner with hardcoded test strings: new Scanner("Alice\n25").

Live Interactive Example

Hit Run Code to see it live

Reading and Processing Input

Java 13

Output Console

Click "Run Code" to view the rendered output.

How it works: Scanner reads the first line as String and the next token as an integer.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Read and double an input number

Read the integer from the scanner.
Multiply it by 2 and print "Double: " + (num * 2).

Sandbox Output

Click "Run & Check" to test your solution.

Finished reading and practicing?

Mark this lesson as completed to update your course progress.

Deep Dive: How It Works

Import required: `import java.util.Scanner;` at the top of your file.

Instantiation: `Scanner scanner = new Scanner(System.in);`

Methods: `.nextLine()` (reads full line text), `.next()` (reads single word), `.nextInt()`, `.nextDouble()`, `.nextBoolean()`.

Closing: Call `scanner.close()` when finished to release system resources.