Week 3 Cheat Sheet - Collections & File I/O

1. try/catch — handling exceptions

Wrap risky code in try { }
Catch the exception in catch (Exception e) { }
Without a try/catch, an uncaught exception halts the program
Always catch IOException around file operations

try {
  String[] names = { "Ezra", "Ian" };
  System.out.println(names[99]);
}
catch (Exception e) {
  System.out.println("Out of range!");
  e.printStackTrace();
}

Rule of thumb: catch the specific exception (IOException, NumberFormatException) when you know what can go wrong. Catch generic Exception as a backstop.

2. Reading files — BufferedReader

try {
  FileReader fr = new FileReader("data.txt");
  BufferedReader br = new BufferedReader(fr);
  String line;
  while ((line = br.readLine()) != null) {
    System.out.println(line);
  }
  br.close();
}
catch (IOException e) { e.printStackTrace(); }

readLine() returns null at end of file
Always close() when done
Requires import java.io.*;

3. Writing files — BufferedWriter

try {
  FileWriter fw = new FileWriter("out.txt");
  BufferedWriter bw = new BufferedWriter(fw);
  bw.write("Hello\n");
  bw.write("World\n");
  bw.close();  // flushes buffer
}
catch (IOException e) { e.printStackTrace(); }

new FileWriter("f.txt") — overwrites
new FileWriter("f.txt", true) — appends
BufferedWriter holds writes in memory; close() flushes

4. Date types — java.time

LocalDate	date only (YYYY-MM-DD)
LocalTime	time only (HH:mm:ss)
LocalDateTime	both together

LocalDate today = LocalDate.now();
LocalTime time = LocalTime.now();
LocalDateTime stamp = LocalDateTime.now();

today.getYear();      // 2026
today.getMonthValue(); // 1-12
today.getDayOfMonth();
today.getDayOfWeek();  // WEDNESDAY

5. DateTimeFormatter — format & parse

DateTimeFormatter fmt =
  DateTimeFormatter.ofPattern("MM/dd/yyyy");

// date → string
String s = today.format(fmt);

// string → date
LocalDate d = LocalDate.parse("04/15/2026", fmt);

y / yyyy	year
M / MM / MMM / MMMM	month
d / dd	day of month
E / EEEE	day of week
H / HH	hour (0-23)
m / mm	minute
s / ss	second
a	AM/PM

Watch out: MM = month, mm = minute. Easy mix-up.

6. ArrayList

import java.util.ArrayList;

ArrayList<String> kids = new ArrayList<>();

kids.add("Ezra");      // append
kids.add(0, "Ian");    // insert at index
kids.get(0);             // "Ian"
kids.set(0, "Eli");    // replace
kids.remove(0);          // shifts others down
kids.size();              // count
kids.clear();             // empty it
Collections.sort(kids);

Dynamic size — grows as you add
Access by index with get(i), not [i]
Only stores objects — use Integer not int

7. HashMap

import java.util.HashMap;

HashMap<String, String> states = new HashMap<>();

states.put("TX", "Austin");
states.get("TX");          // "Austin"
states.get("OK");          // null (not found)
states.containsKey("TX");  // true
states.remove("TX");
states.size();

Iterating

for (String v : states.values()) { ... }
for (String k : states.keySet()) { ... }
for (Map.Entry<String,String> e : states.entrySet()) {
  e.getKey(); e.getValue();
}

Null danger: get(key) returns null for missing keys. Always check before using the result.

8. ArrayList or HashMap?

ArrayList: need to preserve order, iterate in sequence, or access by position (0, 1, 2…)
HashMap: need to find items by a unique key (id, SKU, username)

Quick test: can I ask "given X, find Y"? If yes → HashMap. Otherwise → ArrayList.

list of students	ArrayList
employee by id	HashMap
log of events	ArrayList
state → capital	HashMap

9. Agile — the basics

Waterfall: plan once, build once, ship once. Hard to adapt.
Agile: plan → build → ship, every 2-3 weeks. Repeat.
Scrum is one Agile framework. Others: Kanban, XP.

Scrum roles

Product Owner — prioritizes the backlog
Scrum Master — facilitates, removes blockers
Dev Team — builds the increment

Scrum events

Sprint Planning — day 1 of the sprint
Daily Standup — 15 min each morning
Sprint Review — demo on the last day
Retrospective — reflect and improve

10. User Stories

As a [role], I want [action],
so that [benefit].

Role = a real user (customer, admin, store owner — not a developer)
Action = something they want to do in the product
Benefit = why it matters (the "so that")

Not a user story: "As a developer, I want to set up the project structure…" That's a technical task. Keep user stories for real users.

11. GitHub Project Board

Columns: Todo → In Progress → QA / Testing → Done
New cards go into Todo (the backlog)
Move to In Progress when you start
QA/Testing = reviewed by someone else
Done = verified and shipped

Limit work in progress. Focus on finishing cards before starting new ones. Finishing is what delivers value.

12. MVP — Minimum Viable Product

The smallest version of your product that a real user can use
Ship the MVP first — don't wait for "everything"
Collect feedback, then decide what to build next
Reduces risk — you find out early if you're building the right thing