Using Objects and Methods

Name: AP Computer Science A Unit 1: Using Objects and Methods
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15–25% of the exam4 lessons · 53 min

What this unit covers

The topics below follow the published CS A course framework for Unit 1. This unit is worth 15–25% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Objects & classesMethod callsString methodsMath class & wrappers

Lessons in this unit

Objects & Classes13 min · 3 objectivesDistinguish a class (the blueprint) from an object (an instance built from it) · Create an object with the `new` keyword and a constructor · Call a method on an object using dot notation to read its state
Method Calls14 min · 3 objectivesDifferentiate methods that return a value from `void` methods that do not · Pass arguments to a method and predict the returned result · Chain method calls, evaluating them left to right
String Methods14 min · 3 objectivesUse core String methods: `length`, `substring`, `indexOf`, and `equals` · Compare Strings correctly with `.equals` rather than `==` · Predict the result of substring and index operations by counting positions
Math Class & Wrapper Classes12 min · 3 objectivesCall static Math methods such as `abs`, `max`, `min`, and `pow` · Predict results of integer division and how it differs from double division · Recognize the Integer and Double wrapper classes for boxing primitives

Formulas in Unit 1

Constructing and using an object

ClassName ref = new ClassName(args); ref.method(args);

`new` builds the object and returns a reference; dot notation then sends messages to it.

substring(from, to)

s.substring(a, b) → characters at indices a through b − 1

The start index is included, the end index is excluded, so the result has length b − a. `s.substring(a)` alone returns from index a to the end.

indexOf

s.indexOf(t) → first index where t begins, or −1 if absent

Searching left to right, it reports the position of the first match. A return of −1 means the substring is not present.

Integer vs. double division

7 / 2 = 3 (int) 7.0 / 2 = 3.5 (double)

If both operands are int, the result is truncated to an int. One double operand promotes the whole operation to double.

What examiners penalize here

On the exam, `new` always appears with a class that has a matching constructor. If the arguments do not match any constructor’s parameter list, the code fails to compile — check the number and types of arguments carefully.
When you see chained calls, rewrite them as separate steps on scratch paper: compute the leftmost call, replace it with its result, and repeat. This prevents mis-reading `s.trim().length()` as calling `length()` on the original spaced string.
For any `indexOf` or `substring` question, write the string out and label each character with its index starting at 0. Counting on paper prevents off-by-one mistakes with the excluded end index.
`Math` methods are static: always call them as `Math.name(...)`, never on an object. And watch the return type — `Math.pow` returns a `double`, so `Math.pow(2, 3)` is `8.0`, not `8`.

Practice CS A

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

42 practice questionsEvery one with a written explanation FRQ trainerWritten answers scored against the rubric CS A study guideAll units, weights and score data in one place All 4 CS A unitsThe full course

Questions about this unit

How much of the AP Computer Science A exam is Unit 1?

Unit 1, Using Objects and Methods, is worth 15–25% of the CS A multiple-choice section according to the published course framework. Across all 4 units that makes it one of the heaviest units on the exam, and worth front-loading.

What topics are covered in CS A Unit 1?

Using Objects and Methods covers Objects & classes, Method calls, String methods and Math class & wrappers.

How should I study CS A Unit 1?

Read the 4 lessons below first — about 55 minutes — then work practice questions on these topics. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.

All 4 units of AP Computer Science A

Unit names, topics and exam weights follow the published College Board course framework for AP Computer Science A. AP® is a trademark registered by the College Board, which does not endorse this site.