instance-methods

Instance Methods Exercises

Welcome to the Instance Methods exercises! This topic focuses on understanding what instance methods are, how they work, and how to write effective methods for your classes.

What You'll Learn

• Understanding the difference between instance methods and static methods
• Writing methods that operate on object data (fields)
• Creating methods with different return types and parameters
• Using methods to encapsulate behavior and maintain object state
• Method naming conventions and best practices

Exercise Overview

You'll work with several classes to practice writing instance methods. Each exercise builds on the previous one, introducing new concepts and complexity.

Exercises

Exercise 1: Basic Instance Methods

Class: Calculator Create a Calculator class with basic arithmetic methods that work with internal state.

Requirements:

• Fields: double result (to store the current result)
• Constructor: Initialize result to 0.0
• Methods:
  • add(double value) - Add value to result
  • subtract(double value) - Subtract value from result
  • multiply(double value) - Multiply result by value
  • divide(double value) - Divide result by value (check for division by zero)
  • getResult() - Return the current result
  • clear() - Reset result to 0.0

Exercise 2: String Manipulation Methods

Class: TextProcessor Create a class that processes text using various instance methods.

Requirements:

• Fields: String text (the text to process)
• Constructor: Takes initial text as parameter
• Methods:
  • setText(String newText) - Update the text
  • getText() - Return the current text
  • getLength() - Return length of text
  • toUpperCase() - Return text in uppercase (don't modify original)
  • toLowerCase() - Return text in lowercase (don't modify original)
  • reverse() - Return reversed text
  • contains(String substring) - Check if text contains substring
  • getWordCount() - Return number of words in text

Exercise 3: State Management Methods

Class: Counter Create a counter class with methods that manage internal state.

Requirements:

• Fields: int count, int step (increment/decrement amount)
• Constructor: Takes initial count and step values
• Methods:
  • increment() - Increase count by step
  • decrement() - Decrease count by step
  • reset() - Set count back to 0
  • getCount() - Return current count
  • setStep(int newStep) - Change the step value
  • getStep() - Return current step value
  • isPositive() - Return true if count > 0
  • isNegative() - Return true if count < 0

Exercise 4: Validation and Business Logic

Class: Temperature Create a temperature class with conversion methods and validation.

Requirements:

• Fields: double celsius (store temperature in Celsius)
• Constructor: Takes temperature value and unit ("C", "F", or "K")
• Methods:
  • getCelsius() - Return temperature in Celsius
  • getFahrenheit() - Return temperature in Fahrenheit
  • getKelvin() - Return temperature in Kelvin
  • setCelsius(double temp) - Set temperature in Celsius
  • setFahrenheit(double temp) - Set temperature in Fahrenheit
  • setKelvin(double temp) - Set temperature in Kelvin
  • isFreezingWater() - Return true if water would freeze
  • isBoilingWater() - Return true if water would boil
  • getTemperatureCategory() - Return "Cold", "Mild", "Hot", or "Extreme"

Exercise 5: Object Interaction Methods

Class: ShoppingCart Create a shopping cart that manages items and calculates totals.

Requirements:

• Fields: ArrayList<String> items, ArrayList<Double> prices
• Constructor: Initialize empty lists
• Methods:
  • addItem(String item, double price) - Add item with price
  • removeItem(String item) - Remove first occurrence of item and its price
  • getItemCount() - Return total number of items
  • calculateTotal() - Return sum of all prices
  • calculateAverage() - Return average price per item
  • getMostExpensive() - Return name of most expensive item
  • getCheapest() - Return name of cheapest item
  • containsItem(String item) - Check if item exists in cart
  • clear() - Remove all items

Exercise 6: Complex State Methods

Class: BankAccount Create a bank account with comprehensive transaction methods.

Requirements:

• Fields: String accountNumber, double balance, ArrayList<String> transactionHistory
• Constructor: Takes account number, sets balance to 0, initializes history
• Methods:
  • deposit(double amount) - Add money (validate amount > 0)
  • withdraw(double amount) - Remove money if sufficient funds
  • transfer(BankAccount toAccount, double amount) - Transfer money to another account
  • getBalance() - Return current balance
  • getAccountNumber() - Return account number
  • getTransactionHistory() - Return copy of transaction history
  • getTransactionCount() - Return number of transactions
  • hasInsufficientFunds(double amount) - Check if withdrawal would overdraft
  • calculateInterest(double rate) - Return interest amount for given rate

Exercise 7: Advanced Method Design

Class: StudentGradeBook Create a grade book that manages student scores and calculations.

Requirements:

• Fields: String studentName, ArrayList<Double> grades
• Constructor: Takes student name, initializes empty grades list
• Methods:
  • addGrade(double grade) - Add grade (validate 0-100 range)
  • removeLowestGrade() - Remove the lowest grade
  • getGradeCount() - Return number of grades
  • calculateAverage() - Return average of all grades
  • getHighestGrade() - Return highest grade
  • getLowestGrade() - Return lowest grade
  • getLetterGrade() - Return letter grade based on average
  • isPassingGrade() - Return true if average >= 60
  • getGradesSummary() - Return string with key statistics
  • hasGradeAbove(double threshold) - Check if any grade exceeds threshold

Getting Started

1. Open the topic.jsh file
2. Find the class template for Exercise 1 (Calculator)
3. Implement the required methods according to the specifications
4. Run your code in jshell to test: jshell topic.jsh
5. Verify all tests pass before moving to the next exercise
6. Repeat for each exercise

Key Concepts to Remember

• Instance methods operate on specific object instances and can access instance fields
• Methods should have clear, descriptive names that indicate what they do
• Return types should match what the method actually returns
• Parameter validation helps prevent errors and maintains data integrity
• Encapsulation means controlling access to object data through methods
• Methods can modify object state or return information about the object

Testing Your Code

Each class has test code at the bottom of the file. Make sure:

• All tests pass without errors
• Methods return the expected values
• Object state changes correctly when methods are called
• Validation works as expected (e.g., division by zero, invalid grades)

Good luck with your instance methods practice!