In Java, you may notice keywords in front of variables, especially when looking at object-oriented programming. For example, 

 is one of the most important keywords, especially in object-oriented programming and within class structures. 

When a variable is "private", it means that the value of the variable can only be accessed from inside the class the variable is declared in. 

You cannot directly access or change the value of the value from outside the class.

 A private method is the same, it can only be called from inside the class the method is written in.

public class Student
{
   private String name;

   public Student (String newName)
   {
      name = newName;
   }

   public void setName (String newName)
   {
      name = newName;
   }
 
   public String getName ()
   {
      return name;
   }
}

public class Athlete extends Student
{
   public void printName ()
   {
      System.out.println(name); // THIS WILL NOT WORK
   }

   // There may be instance variables, constructors, and other methods not
   // shown.
}

The "System.out.println(name)" would only work if 

 is a private variable, the code in the Athlete class does not inherit 

 or change what's stored in it? You'll have to use 

 (the "setName(...)" and "getName()" methods in the Student class). For more information on using these, visit the article on getters and setters.

cannot be accessed from outside its class

On FRQs, you'll need to use getters and setters to access the values of private variables

AP Computer Science A

AP Computer Science A unit 1 covers primitive types, variables, data types, expressions, assignments and so much more to prepare you for your exam and make sure you understand everything in unit 1.

Unit 1 – Primitive Types

To help you understand unit 2 of AP Computer Science A take a look at all of our resources. We cover using objects, void methods, non-void methods, string objects, string methods, and so much more! With helpful examples and study guides you will understand unit 2 in no time!

Unit 2 – Using Objects

Take a look at these resources on AP Computer Science A unit 3 to help you prepare. We cover boolean expressions with examples of compound, equivalent, and complex boolean expressions, along with if-statements, else-if statements and comparing objects. All of these resources will make sure you have a better understanding of unit 3.

Unit 3 – Boolean Expressions & if Statements

Learn everything you need to know about AP Computer Science A unit 4 using these resources. We cover the role of iteration, along with nested iteration and code analysis. There are also resources explaining, with examples, while loops and for loops to ensure you know everything about unit 4!

Unit 4 – Iteration

These resources will make sure you understand AP Computer Science A unit 5. Study guides, explanations and examples are available for all the important topics including constructors, the mutator method, writing methods, accessor methods, along with so much more. 

Unit 5 – Writing Classes

Resources to help you review and understand unit 6 of AP Computer Science A. We cover arrays, with an overview, how to create them, and more, as well as detailed descriptions and examples for traversing arrays, array loops, and algorithms for arrays. 

Unit 6 – Array

Take a look at all of these resources for unit 7 of Computer Science A. They cover an introduction into an arraylist, along with helpful examples and explanations for methods, traversing arraylists, sorting arraylists, and the ethical dilemmas of data collection, along with so much more. These resources will make sure you understand unit 7!

Unit 7 – ArrayList

Make sure to check ou these resources for AP Computer Science A unit 8. We provide helpful resources and examples so you can gain a better understanding of 2D arrays and traversing 2D arrays, along with other important topics. These resources will make sure you fully understanding unit 8 of AP Computer Science A!

Unit 8 – 2D Array 

A complete overview of the important topics for unit 9 of AP Computer Science A. We cover inheritance, constructors with subclasses, super keyword, polymorphism, and object superclasses to help you study and prepare. 

Unit 9 – Inheritance

Learn everything you need to know about unit 10 of AP Computer Science A. We cover recursion and how to use recursion with sorting and searching, including linear and sequential searches, binary searches, insertion sorts, selection sorts and merge sorts. These examples and explanations will make sure you understand this final unit!

Unit 10 – Recursion

Frequently Asked Questions

Exam Reviews

Check out these resources that will cover all the important topics of AP Computer Science A so you can prepare for your exam. With study guides, review sheets and review videos with sample problems and helpful tips, these resources will make sure you are ready for your exam!

Big Reviews: Finals & Exam Prep

AP Cram Sessions 2021

First Unit

Overview

1.1 Why Programming? Why Java?

1.2 Variables and Primitive Data Types

Input

1.3 Expressions and Assignment Statements

1.4 Compound Assignment Operators

1.5 Casting and Ranges of Variables

Unit 1 Overview: Primitive Types

Why Programming? Why Java?

Intro to Programming

The Anatomy of a Java Program

System.out.println() vs System.out.print()

Variables and Data Types

Primitive Data Types

Variables

Input Using the Scanner Class

Variables and Primitive Data Types

Expressions and Assignment Statements

Basic Math in Java

Order of Operations in Java

Basic Math Practice Problems in Java

Compound Assignment Operators

Compound Operators

Incrementing and Decrementing

Code Tracing Practice

Casting

Integers to Doubles

Doubles to Integers

Rounding

Casting and Ranges of Variables

Multiple Choice Practice for Primitive Types

Multiple Choice Questions

Answers and Review for Multiple Choice Practice on Primitive Types

MC Answers and Review


The Big Takeaway Of This Unit

Unit Overview

Exam Weighting

Enduring Understanding

Building Computational Thinking

Main Ideas for This Unit

2.1: Objects: Instances of Classes


2.2 Creating and Storing Objects (Initialization)

2.3 Calling a Void Method

2.4 Calling a Void Method with Parameters


2.5 Calling a Non-Void Method


2.6 String Objects: Concatenation, Literals, and More


2.7 String Methods


2.8 Using the Math Class

Using Objects Overview

What are Objects?

What is a Class?

Objects are Instances of Classes

Real-world Example

Objects: Instances of Classes

Multiple Choice Practice for Using Objects

Answers and Review for Multiple Choice Practice on Using Objects

Constructors and Initialization

Constructor Overloading

Null Objects

Creating and Storing Objects (Instantiation)

Introduction to Methods

Void Methods vs. Non-Void Methods

Static vs. Non-Static Methods

Methods Practice Problems

Calling a Void Method

How To Write and Call Methods With Parameters

Overloading Methods

Calling a Void Method With Parameters

Returning Integers/Doubles

Returning Booleans

Returning Strings

Returning Objects and Other Reference Types

Non-Void Methods Practice Problems

Calling a Non-Void Method

A Technical Look At Strings

Escape Characters

String Concatenation

String Concatenation Practice Problems

Summary of String Objects

String Objects: Concatenation, Literals, and More

Intro to External Libraries and APIs

About the String Class

String Methods: Accessing Substrings

String Methods: Equality and Comparison

AP CS A Java Quick Reference Sheet

String Method Practice Problems

indexOf Practice Problems

Length Practice Problems

Substring Practice Problems

compareTo Practice Problems

String Methods

Introduction

Integer Wrapper Class

Double Wrapper Classes

Autoboxing

Practice Problems

Wrapper Classes: Integer and Double

Absolute Values

Exponents and Square Roots

Random Numbers

Using the Math Class

The Big Takeaway Of This Unit


Unit Overview


Building Computational Thinking

Potential Unit 3 Topics

Unit 3 Key Concepts

Unit 3 Overview: Boolean Expressions and if Statements

Boolean Operators

Boolean Expressions

Introduction to Conditional Statements

If Statements

Example: Even Number Checker

If Statements and Control Flow

If-Else Statements

Example: Number Rounding

Else If Statements

Example: Divisibility Counter

Example: Leap Year Decider

Nested Conditionals

Boolean Logical Operators

Compound Conditional Statements

Compound Boolean Expressions

Proof By Simplification

Basic Theorems


Consensus Theorems (Nice to know, but not required)

Commutative Law (boolean AND and OR are commutative)

Associative Law (boolean AND and OR are associative)

Distributive Law (boolean AND distributes over boolean OR)

DeMorgan's Theorems (Important to know) (boolean NOT does not distribute over boolean OR or AND)

Proof By Testing All Cases

Truth Table Example 1

Truth Table Example 2

Equivalent Boolean Expressions

What it Means When Objects Are Equal

The Equals Method

Comparing Objects

Multiple Choice Practice for Boolean Expressions and if Statements

Answers and Review for Multiple Choice Practice on Boolean Expressions and if Statements

Private Variables

AP Computer Science A 💻

What Does "Private" Mean?

Example:

This code will not work:

Summary:

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