Study Notes on Java Scanner and User Input

Building Java Programs

Chapter 3: Parameters and Objects

Overview

The chapter emphasizes the importance of parameters and objects in programming with Java.
Introduces interactive programming through user input.
Focuses on the Scanner class for reading input from various sources.

Interactive Programs with Scanner

Definition of Interactive Programs

An interactive program is designed to read input from the console, allowing users to provide data.
It utilizes the Scanner class to facilitate input collection.

Characteristics of User Input

User input can be unpredictable, leading to potential misbehavior in programs.
Despite the challenges, interactive programs offer engaging behaviors and experiences for users.

Scanner Class in Java

Input and System.in

System.in: Used for receiving input, functioning opposite to System.out, which is used for output.
The Scanner class can also read data from various sources, including files, web pages, and databases (referenced in Chapter 6).
Scanner: An object that reads input from specified sources, typically the console.

Syntax of Using Scanner

Importing the Scanner utility package
- Necessary to utilize the Scanner class in the program.
- Example syntax:
```
 import java.util.*;
```
Constructing a Scanner Object
- To initialize a Scanner object for reading console input, use:
  java Scanner name = new Scanner(System.in);
- Example instantiation:
  java Scanner console = new Scanner(System.in);

Methods of Scanner Class

Common Scanner Methods

nextInt(): Reads an int from user input and returns it.
nextDouble(): Reads a double from user input and returns it.
next(): Reads a one-word String from user input and returns it.
nextLine(): Reads a full line of text from user input and returns it.
Each method execution waits for the user to press Enter, after which the specified value is returned.
Prompt: A message displayed to guide the user on what information to provide.

Examples Demonstrating Scanner Usage

Example 1: Age Calculation

import java.util.*;
public class UserInputExample  {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("How old are you? ");
        int age = console.nextInt();
        int years = 65 - age;
        System.out.println(years + " years to retirement!");
    }
}

User input: 36
Output: 29 years to retirement!

Example 2: Multiplication of Two Numbers

import java.util.*;
public class ScannerMultiply {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("Please type two numbers: ");
        int num1 = console.nextInt();
        int num2 = console.nextInt();
        int product = num1 * num2;
        System.out.println("The product is " + product);
    }
}

User input: 8 and 6
Output: The product is 48

Input Tokens

Token definition: A unit of user input as interpreted by the Scanner.
Tokens are separated by whitespace (spaces, tabs, new lines).
If the user input does not match the expected token type, the program will crash.
Example of potential error:

System.out.print("What is your age? ");
int age = console.nextInt(); // User inputs non-integer value
Output: What is your age? Timmy
java.util.InputMismatchException

Additional Examples of Scanner Usage

Example 3: Age and Favorite Food

import java.util.Scanner;
public class Greeting  {
    public static void main(String[] args)  {
        int age;
        String food;
        Scanner console = new Scanner(System.in);
        System.out.print("How old are you?");
        age = console.nextInt();
        // Clear scanner buffer to read the next line correctly
        console.nextLine();
        System.out.print("What's your favorite food?");
        food = console.nextLine();
        System.out.println("Your age= " + age + " and your favorite food = " + food + ".");
    }
}

Example 4: Calculating Tip

import java.util.Scanner;
public class FindTip {
    public static void main(String[] args)  {
        Scanner console = new Scanner(System.in);
        System.out.print("Please type in the total: ");
        double total = console.nextDouble();
        double subtotal = total * (1 + 0.075);
        System.out.println("The subtotal is " + subtotal);
    }
}

Example 5: Variable Tip Calculation

import java.util.Scanner;
public class FindTip {
    public static void main(String[] args)  {
        Scanner keyboard = new Scanner(System.in);
        System.out.print("Please type in the total: ");
        double total = keyboard.nextDouble();
        System.out.print("Please type in the tip: ");
        int tip = keyboard.nextInt();
        double subtotal = total * (1 + tip/100.0);
        System.out.println("The subtotal is " + subtotal);
    }
}

Example 6: Generating a Cool Name

import java.util.Scanner;
public class MakeCoolName {
    public static void main(String[] args)  {
        Scanner keyboard = new Scanner(System.in);
        System.out.print("Please type in your Name: ");
        String name = keyboard.next();
        String coolName = name + "izzle";
        System.out.println("Your cool name would be " +  coolName);
    }
}

In-Class Practice

Task Description

Ask users for their name and current age.
Calculate the grade that corresponds to their age using the formula:
- Grade = Age - 5
- For example, a 6-year-old corresponds to Grade 1, while a 43-year-old corresponds to Grade 38.
Example output statement:
- Hello Lily, as a person of age 55 you should be in Grade 50.

Conclusion

With the Scanner class, interactive Java programs can effectively read and process user input, enhancing the overall usability and engagement of applications. This chapter serves as a foundational understanding of utilizing user input in programming.