Checkpoint: Java Basics

You've learned data types, type casting, Scanner input, the Math class, StringBuilder, ArrayList and HashMap — let's combine them. In this checkpoint you'll build one small but complete program that reads input, tallies it with a HashMap, and prints a formatted report.

Learn Checkpoint: Java Basics in our free Java course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Java course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

8 primitives, their ranges, overflow, and why double is wrong for money.

Widening is automatic, narrowing needs (type) , casts truncate, and parsing turns text into numbers.

nextLine / next / nextInt / nextDouble , the newline trap, and hasNextInt validation.

abs , max / min , pow , sqrt , rounding, and random numbers.

Efficient text building and printf / String.format specifiers.

Resizable lists, key→value maps, iteration, and the tally pattern.

Write a program that reads spending entries and prints a per-category report. Each entry is a category word followed by an amount, for example food 12.50 . Reading stops when the user types done .

Start from this scaffold — fill in the TODOs, then run it. Try it yourself before revealing the solution!

Give it a real attempt first. When you're ready, expand the full solution and walk through how each skill from this section fits together.

Test yourself across the whole section. Try to answer before expanding each one.

1. Why does the tally use HashMap<String, Double> rather than HashMap<String, double> ?

Collections store objects, not primitives, so the value type must be the wrapper class Double . Autoboxing converts between double and Double automatically.

2. What does (int) 19.99 evaluate to, and how would you instead get 20?

19 — casting truncates, it doesn't round. To get 20 use Math.round(19.99) (which returns a long ), then cast to int if needed.

3. After int n = scanner.nextInt(); the next scanner.nextLine() returns an empty string. Why, and what's the fix?

nextInt() leaves the trailing newline in the buffer, which the next nextLine() consumes immediately. Fix it by adding an extra scanner.nextLine(); after the numeric read.

4. With an ArrayList<Integer> holding [10, 20, 30], what does list.remove(1) leave, and why is this a famous trap?

[10, 30] . remove(1) deletes the element at index 1 (the value 20). To remove the value 1 you'd write remove(Integer.valueOf(1)) .

5. Why prefer StringBuilder over + when assembling a long report in a loop?

Strings are immutable, so each + creates a new String object; in a large loop that's quadratic work. StringBuilder.append writes into one growable buffer, so it stays linear.

6. What does map.getOrDefault("x", 0) return when "x" is not in the map, and why is that useful for counting?

It returns 0 instead of null . That lets the tally pattern add 1 to a not-yet-seen key without a NullPointerException or a special first-time check.

You combined data types, casting, Scanner input, validation, the HashMap tally pattern, and printf formatting into one complete program. That's a real, useful skill — the same loop-read-tally-report shape powers countless tools.

Next up: Switch Statement & Switch Expressions — a cleaner way to branch on many values than long if/else chains.

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