Reading integer input in Java - Step-by-Step Execution

Concept Flow - Reading integer input

Start Program

↓

Create Scanner

↓

Wait for User Input

↓

Read Input as Integer

↓

Store Integer

↓

Use Integer

↓

End Program

The program starts, creates a Scanner to read input, waits for user input, reads it as an integer, stores it, and then uses it.

Execution Sample

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int num = sc.nextInt();
        System.out.println(num);
    }
}

This code reads an integer from the user and prints it.

Execution Table

Step	Action	Input/Value	Variable State	Output
1	Create Scanner object	N/A	sc = Scanner(System.in)	N/A
2	Wait for user input	User types '42' and presses Enter	N/A	N/A
3	Read integer input	42	num = 42	N/A
4	Print the integer	N/A	num = 42	42
5	Program ends	N/A	num = 42	N/A

💡 Program ends after printing the integer input.

Variable Tracker

Variable	Start	After Step 3	Final
sc	null	Scanner(System.in)	Scanner(System.in)
num	undefined	42	42

Key Moments - 3 Insights

Why do we need to create a Scanner object before reading input?

The Scanner object connects the program to the keyboard input stream. Without it, the program cannot read what the user types. See execution_table step 1.

What happens if the user types something that is not an integer?

The program will throw an InputMismatchException because nextInt() expects an integer. This is not shown in the table but is important to handle in real programs.

Why do we store the input in a variable?

Storing the input in a variable like 'num' lets us use the number later in the program. See execution_table step 3 where 'num' gets the value 42.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'num' after step 3?

A undefined

B null

C 42

D 0

💡 Hint

Check the 'Variable State' column at step 3 in the execution_table.

At which step does the program print the user's input?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for the 'Print the integer' action in the execution_table.

If the user types '100' instead of '42', what changes in the variable tracker?

A 'num' changes to 100 after step 3

B 'sc' changes to 100 after step 3

C 'num' remains undefined

D 'sc' becomes null

💡 Hint

The variable 'num' stores the input value, see variable_tracker.

Concept Snapshot

Reading integer input in Java:
1. Import Scanner: import java.util.Scanner;
2. Create Scanner: Scanner sc = new Scanner(System.in);
3. Read int: int num = sc.nextInt();
4. Use 'num' as needed.
Remember: nextInt() reads only integers and throws error on invalid input.

Full Transcript

This example shows how to read an integer input from the user in Java. First, the program creates a Scanner object connected to the keyboard. Then it waits for the user to type a number and press Enter. The nextInt() method reads the typed number and stores it in the variable 'num'. Finally, the program prints the number back to the screen. The execution table traces each step, showing how variables change and when output happens. Key points include creating the Scanner before reading input and storing the input in a variable for later use. If the user types something not an integer, the program will throw an error. This simple flow helps beginners understand how input works in Java.