Using Objects: every key term you need

Objects

Instance of a class; contains data (fields) and behavior (methods). Created with new keyword: Dog myDog = new Dog();

Classes

Blueprint for objects; defines fields (data) and methods (behavior). Example: class Dog { int age; void bark() {} }

References vs Primitives

Primitive: stores actual value. Reference (object): stores memory address pointing to object. Different behavior for assignment/comparison.

null Reference

Reference pointing to no object. Causes NullPointerException if you try to use it. Check with if (obj != null).

String Class

Immutable sequence of characters. Methods: length(), charAt(index), substring(start, end), indexOf(char), equals(), compareTo().

String Concatenation

+ operator joins strings: \"Hello\" + \"World\" = \"HelloWorld\". Also converts primitives: \"Age: \" + 25 = \"Age: 25\".

String Methods

toUpperCase(), toLowerCase(), trim(), split(delimiter), startsWith(), endsWith(), replace(). Each returns new String (immutable).

ArrayList

Resizable array; stores object references. ArrayList<Integer> list = new ArrayList<>(); Access with get(index), add(element), remove(index).

ArrayList vs Array

Array: fixed size, faster. ArrayList: dynamic size, slower. ArrayList more convenient for unknown sizes.

Method Calling

object.method(arguments). Example: myString.length() returns length. Methods act on object (implicit 'this').

Constructor

Special method creating objects; same name as class. Can have parameters. Default no-arg constructor provided if none written.

toString() Method

Returns String representation of object; automatically called when printing. Override in custom classes for meaningful output.

Unit 2 Summary

Objects contain data (fields) and methods. References store memory addresses. Strings immutable; ArrayList resizable. Constructors initialize objects.

More AP Computer Science A guides

• Primitives
• Boolean Expressions
• Loops
• Writing Classes
• Arrays