Foundations of Class Definitions

Topics

Class Definitions in the Objects-First Approach
Testing a class with Tester class
Setting Field Values Directly
Practice Questions

Class Definitions in the Objects-First Approach

1. Introduction to Class Definitions

Class as a Blueprint: In Java, a class serves as a blueprint for creating objects. It defines the properties (attributes) and behaviors (methods) that the objects created from it will have.
Objects-First Approach: This approach emphasizes understanding objects and their interactions early in the learning process. By starting with classes that model real-world entities, you can build a solid foundation in object-oriented programming.

2. Starting with a Simple Structure: `BankAccount` Class

Class Name: BankAccount

This will be the first class in the Bank Application project. The BankAccount class will represent the essential attributes and behaviors of a bank account.

Step 1: Define the Class

Class Declaration: The class starts with a declaration. In Java, this is done using the class keyword followed by the class name.
```
 public class BankAccount {
    // class body
 }
```

Step 2: Define Attributes (Fields)

Attributes: These are the properties of the class, typically representing the state of an object. In the BankAccount class, common attributes might include accountNumber, accountHolderName, and balance.
Data Types: For each attribute, choose an appropriate data type. For example, accountNumber might be a String, balance might be a double, and accountHolderName could also be a String.
```
 public class BankAccount {
    String accountNumber;
    String accountHolderName;
    double balance;
 }
```

Tester Class with `main` Method

1. Introduction to the `main` Method

Purpose: The main method is the entry point of any Java application. It is where the execution of the program begins.
Structure: The main method is always written as public static void main(String[] args).

2. Creating a Tester Class

Class Name: BankAccountTester

This class will be used to create instances of the BankAccount class and test its functionality.

   public class BankAccountTester {
      public static void main(String[] args) {
         // Execution starts here
      }
   }

3. Testing the `BankAccount` Fields

Step 1: Create an Instance of `BankAccount`

Object Creation: To test the BankAccount class, you first need to create an instance (object) of it. This is done using the new keyword followed by a call to the constructor of the BankAccount class.
```
 BankAccount account1 = new BankAccount();
```

Step 2: Access and Test the Fields

Accessing Fields: Initially, you can access and print the values of the fields in the BankAccount object to verify that they are initialized correctly.

Printing Field Values: Use System.out.println to display the values of accountNumber, accountHolderName, and balance.

 System.out.println("Account Number: " + account1.accountNumber);
 System.out.println("Account Holder Name: " + account1.accountHolderName);
 System.out.println("Balance: " + account1.balance);

4. Full Code Example for `BankAccountTester`

   public class BankAccountTester {
      public static void main(String[] args) {
         // Creating an instance of BankAccount
         BankAccount account1 = new BankAccount();

         // Accessing and printing the field values
         System.out.println("Account Number: " + account1.accountNumber);
         System.out.println("Account Holder Name: " + account1.accountHolderName);
         System.out.println("Balance: " + account1.balance);
      }
   }

Setting Field Values Directly

When fields are defined with default access, you can set their values directly in any class within the same package. This is a simple way to interact with the data in the object.

Here’s how you can set field values directly in Tester class:

1. Creating an Instance of the Class

First, you need to create an instance of the BankAccount class using the new keyword.
```
   BankAccount account1 = new BankAccount();
```

2. Setting Field Values

After creating the object, you can assign values to its fields directly by accessing them through the object reference (account1 in this case).

   account1.accountNumber = "123456789";           // Setting the account number
   account1.accountHolderName = "John Doe";        // Setting the account holder's name
   account1.balance = 1000.50;                     // Setting the initial balance

Explanation:
- account1.accountNumber = "123456789";: This line sets the accountNumber field of the account1 object to the string "123456789".
- account1.accountHolderName = "John Doe";: This line sets the accountHolderName field to "John Doe".
- account1.balance = 1000.50;: This line sets the balance field to 1000.50.

3. Testing and Verifying

After setting these values, you can print them out using System.out.println to verify that they have been correctly assigned.

   System.out.println("Account Number: " + account1.accountNumber);
   System.out.println("Account Holder Name: " + account1.accountHolderName);
   System.out.println("Balance: " + account1.balance);

4. Full Code Example for `BankAccountTester` with setting values and printing

   public class BankAccountTester {
      public static void main(String[] args) {
         // Creating an instance of BankAccount
         BankAccount account1 = new BankAccount();

         // Setting field values directly
         account1.accountNumber = "123456789";           // Setting the account number
         account1.accountHolderName = "John Doe";        // Setting the account holder's name
         account1.balance = 1000.50;                     // Setting the initial balance

         // Accessing and printing the field values
         System.out.println("Account Number: " + account1.accountNumber);
         System.out.println("Account Holder Name: " + account1.accountHolderName);
         System.out.println("Balance: " + account1.balance);
      }
   }

Practice Writing Similar Code

Go to Practice and find exercises there.