1 · The Software Development Method

A program development method is a repeatable, disciplined procedure that takes you from a real-world need to a working, maintainable program. Writing code is only one step — and usually not the first one. The most common model is a five-stage cycle:

The cycle, visually

       ┌──────────────────┐
       │ 1. Analyze       │
       └────────┬─────────┘
                ▼
       ┌──────────────────┐
       │ 2. Design        │◀──┐
       └────────┬─────────┘   │
                ▼             │
       ┌──────────────────┐   │
       │ 3. Code          │   │
       └────────┬─────────┘   │
                ▼             │
       ┌──────────────────┐   │
       │ 4. Test & Debug  │───┘  (loop back when tests fail)
       └────────┬─────────┘
                ▼
       ┌──────────────────┐
       │ 5. Maintain      │
       └──────────────────┘
      

2 · Applying the Software Development Method

Let's run the method on a real business problem. Suppose a café owner says: "I want to know how much each customer's order costs, including a 6% service tax, and I want a grand total at the end of the day."

1. Analyze. Inputs: list of items (name + price). Process: sum prices, add 6% tax per order. Output: order subtotal, tax, total, and a daily grand total. Constraints: handle empty input gracefully.
2. Design. We need a loop reading items, accumulators for subtotal/tax/grand-total, and formatted output. Decide on double for money (we'll discuss BigDecimal later) and Scanner for input.
3. Code. Translate the design into Java. See the snippet below.
4. Test & Debug. Try: 0 items, 1 item, 100 items, items with decimals, sentinel value to stop.
5. Maintain. Later: support discounts, multiple tax rates, file I/O.

The Java code for this problem

import java.util.Scanner;

public class CafeBilling {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        final double TAX_RATE = 0.06;
        double dailyTotal = 0.0;

        System.out.print("Number of orders today: ");
        int n = sc.nextInt();

        for (int i = 1; i <= n; i++) {
            double subtotal = 0.0;
            System.out.println("\nOrder #" + i + " — enter prices (0 to finish):");
            while (true) {
                double price = sc.nextDouble();
                if (price == 0) break;
                if (price < 0) {
                    System.out.println("Price cannot be negative, ignored.");
                    continue;
                }
                subtotal += price;
            }
            double tax      = subtotal * TAX_RATE;
            double total    = subtotal + tax;
            dailyTotal          += total;

            System.out.printf("Subtotal: %.2f  Tax: %.2f  Total: %.2f%n",
                              subtotal, tax, total);
        }
        System.out.printf("%nDaily Grand Total: %.2f%n", dailyTotal);
        sc.close();
    }
}

3 · Introduction to Algorithms

An algorithm is a finite, well-defined sequence of steps that solves a class of problems or computes a result. Three properties matter:

• Definite — every step is unambiguous.
• Finite — it must eventually stop.
• Effective — each step can actually be carried out.

Algorithm vs. Program

Algorithm: FindMax(numbers)
  max ← numbers[0]
  for each n in numbers
    if n > max then max ← n
  return max
          

static int findMax(int[] a) {
    int max = a[0];
    for (int n : a)
        if (n > max) max = n;
    return max;
}
          

Two ways to describe an algorithm

            ┌───────────────┐
            │     Start     │
            └──────┬────────┘
                   ▼
            ┌───────────────┐
            │  max ← a[0]   │
            └──────┬────────┘
                   ▼
            ┌───────────────┐     ┌──────┐
            │  more items?  │─No─▶│ Print│
            └──────┬────────┘     │ max  │
                   │Yes           └──┬───┘
                   ▼                 ▲
            ┌───────────────┐        │
            │  n > max ?    │──Yes──▶┤
            └──────┬────────┘        │
                   │ No              │
                   ▼                 │
            ┌───────────────┐        │
            │  next item    │────────┘
            └───────────────┘
        

# Pseudocode: Linear Search
function linearSearch(list, target):
    for i from 0 to length(list)-1:
        if list[i] == target:
            return i
    return -1

4 · Steps in Algorithm Development

Algorithms are not just written — they are developed. A reliable flow looks like this:

1. Understand the problem. Restate it in your own words. What is given? What is the result? Any edge cases?
2. Identify inputs and outputs. Name them, give them types, decide on units.
3. Work through examples by hand. Trace small cases. This often reveals the algorithm itself.
4. Choose an approach. Brute force? Divide & conquer? Greedy? Iteration vs. recursion?
5. Write the algorithm. Use pseudocode or a flowchart first — not Java.
6. Test the algorithm on paper. Walk through it step-by-step with multiple inputs, including edge cases.
7. Translate to code. Only now — with a clean plan in hand — implement it in Java.
8. Refine for clarity & efficiency. Reduce duplication, improve variable names, then look at Big-O complexity.

Example: Average of N numbers — full walkthrough

5 · Live Code Examples

Three classic starter problems solved with the method above. Click any tab.

import java.util.Scanner;
public class LargestOfThree {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter three integers: ");
        int a = sc.nextInt(), b = sc.nextInt(), c = sc.nextInt();
        int max = a;
        if (b > max) max = b;
        if (c > max) max = c;
        System.out.println("Largest = " + max);
    }
}

import java.util.Scanner;
public class EvenOdd {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter an integer: ");
        int n = sc.nextInt();
        System.out.println(n + " is " + (n % 2 == 0 ? "Even" : "Odd"));
    }
}

import java.util.Scanner;
public class Sum1ToN {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter N: ");
        long n = sc.nextLong();
        if (n < 0) { System.out.println("N must be ≥ 0"); return; }
        long sum = n * (n + 1) / 2;   // O(1) Gauss
        System.out.println("Sum 1.." + n + " = " + sum);
    }
}

6 · Algorithm Visualizer — Bubble Sort

Watch an algorithm run step by step. Yellow = comparing, red = swapping, green = sorted.

7 · Worked Problems

Try the SDM on each one before peeking at the solution.

if (score < 0 || score > 100) System.out.println("Invalid");
else if (score >= 90) System.out.println("A");
else if (score >= 80) System.out.println("B");
else if (score >= 70) System.out.println("C");
else if (score >= 60) System.out.println("D");
else                      System.out.println("F");

int digitSum(int n) {
    n = Math.abs(n);
    int sum = 0;
    while (n > 0) {
        sum += n % 10;
        n   /= 10;
    }
    return sum;
}

String reverse(String s) {
    char[] a = s.toCharArray();
    int i = 0, j = a.length - 1;
    while (i < j) {
        char t = a[i]; a[i] = a[j]; a[j] = t;
        i++; j--;
    }
    return new String(a);
}

8 · Check Your Understanding