AP CSA Unit 1 Study Notes: Using Objects and Methods (Objects and Methods)

Object

A single usable entity in a Java program that you can ask for information (via methods) and tell to perform actions.

Class

A blueprint/definition that describes what kinds of objects can exist and what they can do (constructors, methods, and often fields).

Instance

An object created from a particular class; a specific realization of that class.

Constructor

Special code used to set up a new object; has the same name as the class and has no return type.

Method

A named block of code that performs an action or computes a result; called on an object (or sometimes on a class if static).

Dot operator (member access)

The '.' used to access an object’s methods (and other members): objectReference.methodName(…).

Type (of a reference)

The class name on a variable declaration (e.g., String) that determines what methods you are allowed to call using dot notation.

Reference variable

A variable that stores a reference to an object (not the entire object itself).

Reference assignment

Setting one reference variable equal to another (e.g., b = a) so both refer to the same object rather than copying the object.

Instantiation

Creating a new object, typically using the keyword new with a constructor call.

Initialization

Storing an initial value in a variable; for reference variables, storing a reference to an object (often right after instantiation).

new (keyword)

Java keyword used to create objects by calling a constructor (e.g., new String("hi")).

null

A special reference value meaning “no object”; the variable does not refer to any object.

NullPointerException

A runtime error that occurs when you try to use dot notation (call a method/access a member) on a null reference.

Identity (object identity)

Whether two references point to the exact same object in memory; for objects, checked with ==.

Equality (content/value equality)

Whether two objects represent the same content/value; for Strings, typically checked with .equals(…).

== (with objects)

Compares identity for objects (whether two references refer to the same object), not content.

equals(…)

A method commonly used to compare object content/value (e.g., String content), rather than identity.

String literal

A shorthand way to create a String object using quotes (e.g., "hello"), without explicitly writing new String(…).

Immutable

Describes an object whose contents do not change after creation; String is immutable, so many methods return a new String instead of modifying the original.

Void method

A method with return type void; performs an action but returns no value to the caller.

Non-void method

A method that returns a value (e.g., int, double, boolean, String), so the call can be used as an expression.

Parameter

A variable in a method definition that receives a value when the method is called.

Argument

The actual value or expression passed into a method call to match a parameter.

Method overloading

Having multiple methods/constructors with the same name but different parameter lists; Java chooses the version whose parameters match the provided arguments.