Defining Classes in Java — Syntax, Structure & Best Practices

A class is the fundamental building block of Java. Let’s learn how to define one properly.

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Basic Class Structure

public class Employee {
    // Fields (instance variables)
    private String name;
    private String department;
    private double salary;

    // Constructor
    public Employee(String name, String department, double salary) {
        this.name = name;
        this.department = department;
        this.salary = salary;
    }

    // Methods
    public String getName() {
        return name;
    }

    public void giveRaise(double percentage) {
        this.salary += this.salary * (percentage / 100);
    }

    public void displayInfo() {
        System.out.printf("%s | %s | $%,.2f%n", name, department, salary);
    }
}

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Anatomy of a Class

[access modifier] class ClassName {
    // 1. Fields (state)
    // 2. Constructors (initialization)
    // 3. Methods (behavior)
    // 4. Inner classes, enums (optional)
}

Conventional Ordering

Most Java teams follow this order:

1. static constants and fields
2. Instance fields
3. Constructors
4. Public methods
5. Private/helper methods
6. toString(), equals(), hashCode()
7. Inner classes

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Naming Conventions

Element	Convention	Example
Class	PascalCase, noun	Employee, BankAccount
Interface	PascalCase, adjective/noun	Serializable, PaymentProcessor
Method	camelCase, verb	calculatePay(), getName()
Field	camelCase, noun	firstName, totalAmount
Constant	UPPER_SNAKE	MAX_SALARY, DEFAULT_TAX_RATE

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One Public Class Per File

Java requires the filename to match the public class name:

Employee.java → public class Employee { }
BankAccount.java → public class BankAccount { }

You can have multiple non-public classes in one file, but don’t — it’s bad practice.

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The this Keyword

this refers to the current object:

public class Employee {
    private String name;

    public Employee(String name) {
        this.name = name;  // this.name = field, name = parameter
    }

    public Employee getThis() {
        return this;  // returns the current object
    }
}

Method Chaining with this

public class Employee {
    private String name;
    private String department;

    public Employee setName(String name) {
        this.name = name;
        return this;  // enables chaining
    }

    public Employee setDepartment(String dept) {
        this.department = dept;
        return this;
    }
}

// Usage — fluent API
Employee emp = new Employee()
    .setName("Alice")
    .setDepartment("Engineering");

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Complete Example

public class Product {
    // Fields
    private String name;
    private double price;
    private int quantity;

    // Constructor
    public Product(String name, double price, int quantity) {
        this.name = name;
        this.price = price;
        this.quantity = quantity;
    }

    // Business logic
    public double getTotalValue() {
        return price * quantity;
    }

    public boolean isInStock() {
        return quantity > 0;
    }

    public void sell(int amount) {
        if (amount > quantity) {
            throw new IllegalArgumentException("Not enough stock");
        }
        quantity -= amount;
    }

    // Display
    @Override
    public String toString() {
        return String.format("%s ($%.2f) x %d", name, price, quantity);
    }
}

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Quick Exercises

1. Create a Rectangle class with width and height. Add methods for area(), perimeter(), and isSquare().
2. Add method chaining to a PersonBuilder class with setName(), setAge(), setEmail().
3. What error do you get if the filename doesn’t match the public class name?

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Summary

• Classes define fields (state) + methods (behavior)
• Follow naming conventions: PascalCase for classes, camelCase for methods
• One public class per file, filename must match class name
• Use this to refer to the current object
• Order: fields → constructors → public methods → private methods

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What’s Next?

Now let’s learn how to Create Objects (Instances) using the new keyword and constructors.