Module 4: Strings, User Input, and Comments

Working with Strings in Java

Strings are sequences of characters and are one of the most commonly used data types in Java. The String class provides many useful methods for manipulating text.

Creating Strings

// String literal - most common way
String greeting = "Hello, World!";

// Using the String constructor
String message = new String("Welcome to Java");

// Empty string
String empty = "";

String Immutability

Strings in Java are immutable, which means once a String object is created, it cannot be changed. When you perform operations on a String, a new String object is created.

String name = "John";
name = name + " Smith";  // Creates a new String object "John Smith"
                         // The original "John" string remains unchanged in memory

Common String Methods

• length() - Returns the number of characters in the string
• charAt(int index) - Returns the character at the specified index (0-based)
• substring(int beginIndex) - Returns a new string starting from the specified index to the end
• substring(int beginIndex, int endIndex) - Returns a new string from beginIndex (inclusive) to endIndex (exclusive)
• indexOf(String str) - Returns the index of the first occurrence of the specified substring
• lastIndexOf(String str) - Returns the index of the last occurrence of the specified substring
• toLowerCase() - Returns a new string with all characters converted to lowercase
• toUpperCase() - Returns a new string with all characters converted to uppercase
• trim() - Returns a new string with leading and trailing whitespace removed
• replace(char oldChar, char newChar) - Returns a new string with all occurrences of oldChar replaced with newChar
• contains(CharSequence s) - Returns true if the string contains the specified sequence
• startsWith(String prefix) - Checks if the string starts with the specified prefix
• endsWith(String suffix) - Checks if the string ends with the specified suffix
• equals(Object obj) - Compares this string to another object
• equalsIgnoreCase(String anotherString) - Compares strings, ignoring case differences

Example: Using String Methods

String text = "Java Programming";

// Basic information
int length = text.length();  // 16
char firstChar = text.charAt(0);  // 'J'
boolean containsJava = text.contains("Java");  // true

// Extracting substrings
String sub1 = text.substring(5);  // "Programming"
String sub2 = text.substring(0, 4);  // "Java"

// Finding positions
int indexOfP = text.indexOf("P");  // 5
int indexOfr = text.indexOf("r");  // 7
int lastIndexOfr = text.lastIndexOf("r");  // 9

// Transforming
String upper = text.toUpperCase();  // "JAVA PROGRAMMING"
String lower = text.toLowerCase();  // "java programming"
String replaced = text.replace('a', 'A');  // "JAvA ProgrAmming"

String Concatenation

There are multiple ways to concatenate (join) strings in Java:

// Using the + operator
String firstName = "John";
String lastName = "Smith";
String fullName = firstName + " " + lastName;  // "John Smith"

// Using concat() method
String greeting = "Hello, ";
String message = greeting.concat(firstName);  // "Hello, John"

// When concatenating with non-String values, Java automatically converts them to strings
int age = 30;
String info = "Age: " + age;  // "Age: 30"
double price = 19.99;
String priceTag = "Price: $" + price;  // "Price: $19.99"

StringBuilder for Efficient Concatenation

When you need to concatenate many strings (especially in a loop), using the + operator can be inefficient. StringBuilder provides a more efficient way:

// Inefficient way for many concatenations
String result = "";
for (int i = 0; i < 10; i++) {
    result = result + i;  // Creates a new String object each time
}

// Efficient way using StringBuilder
StringBuilder builder = new StringBuilder();
for (int i = 0; i < 10; i++) {
    builder.append(i);  // Modifies the same StringBuilder object
}
String result = builder.toString();  // Convert to String when finished

String Formatting

Java provides powerful string formatting capabilities:

// Using String.format()
String name = "Alice";
int age = 25;
double height = 5.75;
String formatted = String.format("Name: %s, Age: %d, Height: %.2f feet", name, age, height);
// Result: "Name: Alice, Age: 25, Height: 5.75 feet"

// Common format specifiers
// %s - String
// %d - Decimal integer
// %f - Floating point
// %.2f - Floating point with 2 decimal places
// %n - Platform-specific line separator

// You can also use printf for direct output
System.out.printf("Name: %s, Age: %d%n", name, age);

Gathering User Input in Java

The Scanner class is the most common way to read user input from the console in Java.

Setting Up Scanner

// Import the Scanner class
import java.util.Scanner;

// Create a Scanner object to read from System.in (keyboard)
Scanner scanner = new Scanner(System.in);

Reading Different Types of Input

// Reading a string (whole line)
System.out.print("Enter your name: ");
String name = scanner.nextLine();  // Reads the entire line

// Reading a word (reads until whitespace)
System.out.print("Enter a word: ");
String word = scanner.next();

// Reading numbers
System.out.print("Enter your age: ");
int age = scanner.nextInt();  // Reads an integer

System.out.print("Enter your height: ");
double height = scanner.nextDouble();  // Reads a double

// Reading a boolean
System.out.print("Are you a student? (true/false): ");
boolean isStudent = scanner.nextBoolean();  // Reads a boolean

Important Considerations

When mixing calls to nextLine() with other methods like nextInt(), be aware of this common issue:

// This can cause problems
System.out.print("Enter your age: ");
int age = scanner.nextInt();  // Reads the number but leaves the newline character

System.out.print("Enter your name: ");
String name = scanner.nextLine();  // This immediately reads the leftover newline character!

// The solution: add an extra nextLine() call after nextInt()
System.out.print("Enter your age: ");
int age = scanner.nextInt();
scanner.nextLine();  // Consume the leftover newline

System.out.print("Enter your name: ");
String name = scanner.nextLine();  // Now this works as expected

Handling Input Errors

When reading user input, it's important to handle potential errors. For example, if the user enters text when you're expecting a number:

// Basic error handling with try-catch
Scanner scanner = new Scanner(System.in);
int number = 0;

try {
    System.out.print("Enter a number: ");
    number = scanner.nextInt();
    System.out.println("You entered: " + number);
} catch (Exception e) {
    System.out.println("That's not a valid number!");
}

Reading from a File

Scanner can also be used to read from files:

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

try {
    File file = new File("data.txt");
    Scanner fileScanner = new Scanner(file);
    
    while (fileScanner.hasNextLine()) {
        String line = fileScanner.nextLine();
        System.out.println(line);
    }
    
    fileScanner.close();
} catch (FileNotFoundException e) {
    System.out.println("File not found!");
}

Closing the Scanner

It's good practice to close the Scanner when you're done with it to free up resources:

scanner.close();

Complete Example: Basic Calculator

import java.util.Scanner;

public class Calculator {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        System.out.print("Enter first number: ");
        double num1 = scanner.nextDouble();
        
        System.out.print("Enter an operator (+, -, *, /): ");
        scanner.nextLine();  // Consume newline
        char operator = scanner.nextLine().charAt(0);
        
        System.out.print("Enter second number: ");
        double num2 = scanner.nextDouble();
        
        double result = 0;
        
        switch (operator) {
            case '+':
                result = num1 + num2;
                break;
            case '-':
                result = num1 - num2;
                break;
            case '*':
                result = num1 * num2;
                break;
            case '/':
                if (num2 != 0) {
                    result = num1 / num2;
                } else {
                    System.out.println("Error: Division by zero");
                    scanner.close();
                    return;
                }
                break;
            default:
                System.out.println("Error: Invalid operator");
                scanner.close();
                return;
        }
        
        System.out.println(num1 + " " + operator + " " + num2 + " = " + result);
        scanner.close();
    }
}

Code Comments in Java

Comments are used to explain code, make it more readable, and provide additional information that might not be immediately evident from the code itself. They are ignored by the compiler.

Types of Comments in Java

Java supports three types of comments:

1. Single-line Comments

Used for brief explanations. Everything after // to the end of the line is a comment.

// This is a single-line comment
int count = 10;  // Initialize count to 10

2. Multi-line Comments

Used for longer explanations that span multiple lines. Everything between /* and */ is a comment.

/* This is a multi-line comment
   that spans multiple lines
   and can provide detailed explanations */
int result = calculateTotal();

3. JavaDoc Comments

Special documentation comments that can be used to generate API documentation. They start with /** and end with */.

/**
 * Calculates the sum of two integers.
 *
 * @param a The first integer
 * @param b The second integer
 * @return The sum of a and b
 */
public int add(int a, int b) {
    return a + b;
}

Best Practices for Comments

• Be Clear and Concise: Write comments that clearly explain what the code is doing without being overly verbose.
• Comment Why, Not What: Focus on explaining why something is done a certain way, rather than what the code is doing (which should be clear from the code itself).
• Update Comments: Keep comments up-to-date when you change the code.
• Use JavaDoc for Public APIs: Use JavaDoc comments for classes, methods, and fields that are part of your public API.
• Don't Over-Comment: Too many comments can clutter the code and make it harder to read.

Common JavaDoc Tags

• @param - Describes a method parameter
• @return - Describes what a method returns
• @throws or @exception - Describes exceptions that might be thrown
• @see - Provides a reference to another class or method
• @since - Specifies when this feature was added
• @deprecated - Indicates that a method is deprecated and should no longer be used

Using Comments to Temporarily Disable Code

Comments can be used to "comment out" code, temporarily disabling it without deleting it:

// This code is currently disabled
// System.out.println("This won't be executed");

/* Multiple lines of code can be disabled this way
int a = 5;
int b = 10;
System.out.println(a + b);
*/

Example: Well-Commented Code

/**
 * A simple Student class to represent student information.
 * 
 * @author Your Name
 * @version 1.0
 * @since 2023-04-15
 */
public class Student {
    // Student's name
    private String name;
    
    // Student's ID (unique identifier)
    private int id;
    
    // GPA on a 4.0 scale
    private double gpa;
    
    /**
     * Constructs a new Student with the specified name, ID, and GPA.
     * 
     * @param name The name of the student
     * @param id The unique ID of the student
     * @param gpa The GPA of the student on a 4.0 scale
     */
    public Student(String name, int id, double gpa) {
        this.name = name;
        this.id = id;
        
        // Ensure GPA is within valid range
        if (gpa < 0.0 || gpa > 4.0) {
            throw new IllegalArgumentException("GPA must be between 0.0 and 4.0");
        }
        this.gpa = gpa;
    }
    
    /**
     * Returns whether the student is on the honor roll.
     * A student is on the honor roll if their GPA is 3.5 or higher.
     * 
     * @return true if the student is on the honor roll, false otherwise
     */
    public boolean isOnHonorRoll() {
        return gpa >= 3.5;  // Honor roll threshold is 3.5
    }
    
    /* This method was replaced with a more efficient implementation
    public String getFullInfo() {
        return "Name: " + name + ", ID: " + id + ", GPA: " + gpa;
    }
    */
    
    /**
     * Returns a string representation of the student.
     * 
     * @return A string containing the student's name, ID, and GPA
     */
    @Override
    public String toString() {
        return String.format("Student[name=%s, id=%d, gpa=%.2f]", name, id, gpa);
    }
}

Project Overview

In this guided project, you'll apply your knowledge of strings, user input, and comments to create interactive programs:

Secret Strings

This project focuses on string manipulation and involves:

• Encrypting and decrypting messages using string operations
• Working with substring extraction and concatenation
• Using string methods to analyze and modify text
• Implementing proper code documentation with comments

Chess Strings

This project demonstrates practical applications of strings in a chess game context:

• Representing chess positions using string notation
• Parsing user input to make chess moves
• Validating input strings against expected patterns
• Using StringBuilder for efficient string operations
• Adding comprehensive documentation using JavaDoc comments

By completing these projects, you'll gain practical experience with the core concepts of this module and see how they can be applied to solve real problems.