2 · Method Declaration Structure

A method is a reusable block of code designed to perform a specific task. Declaring methods enforces modular design, making programs cleaner and reducing duplicates. Java method headers contain: 方法是为执行特定任务而设计的可重用代码块。声明方法可以强制执行模块化设计，使程序更清晰并减少重复代码。Java 方法头包含：

modifier returnType methodName(parameterList) {{
    // method body
}}

Example: public static double calculateTax(double amount). Here, public static are modifiers, double is the return type, calculateTax is the method name, and double amount is a formal parameter. 示例：public static double calculateTax(double amount)。这里，public static 是修饰符，double 是返回类型，calculateTax 是方法名称，而 double amount 是形参。

3 · Parameters & Return Statements

Java passes parameters to methods by value, meaning the method receives a copy of the argument values. A method can output values back to callers using the return keyword: Java 按值传递参数给方法，这意味着方法收到的是实参值的副本。方法可以使用 return 关键字将值输出返回给调用者：

public static int multiply(int a, int b) {{
    return a * b; // returns value product to caller
}}

If a method does not return a value, its return type must be declared as void. 如果方法不返回值，则必须将其返回类型声明为 void。

4 · Local Variable Scope

A variable declared inside a method is called a local variable. Its scope is bounded by the enclosing curly braces {{ ... }}. It cannot be accessed or modified from outside the method: 在方法内声明的变量称为局部变量。它的作用域受到闭合大括号 {{ ... }} 的限制。不能从方法外部访问或修改它：

Applied Case Study: Modular Calculations Engine应用案例研究：模块化计算引擎

Company: Kompas Calculator Engine.
We encapsulate calculation logic into modular static methods. The main routine prompts user, passes arguments, and prints formatted output values. 公司： Kompas Calculator Engine。
我们将计算逻辑封装到模块化静态方法中。主方法提示用户输入、传递参数，并打印格式化的输出值。

import java.util.Scanner;

public class CalcEngine {{
    
    // Method to calculate subtotal
    public static double calculateSubtotal(double price, int qty) {{
        return price * qty;
    }}

    // Method to calculate tax
    public static double calculateTax(double subtotal, double taxRate) {{
        return subtotal * taxRate;
    }}

    public static void main(String[] args) {{
        Scanner input = new Scanner(System.in);
        
        System.out.print("Enter item price: RM ");
        double price = input.nextDouble();
        
        System.out.print("Enter item quantity: ");
        int qty = input.nextInt();

        double sub = calculateSubtotal(price, qty);
        double tax = calculateTax(sub, 0.06); // 6% tax rate
        double total = sub + tax;

        System.out.printf("Subtotal: RM %.2f%n", sub);
        System.out.printf("Tax (6%%): RM %.2f%n", tax);
        System.out.printf("Grand Total: RM %.2f%n", total);
        
        input.close();
    }}
}}

Enter item price: RM 20.00
Enter item quantity: 5
Subtotal: RM 100.00
Tax (6%): RM 6.00
Grand Total: RM 106.00

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