7 min read•Last Updated on June 18, 2024
Avanish Gupta
Avanish Gupta
Welcome to AP Computer Science A, or AP CSA for short! In this guide, you will learn the basics of Java, a programming language that is widely used today.
Almost all the technology we use today, from everything on your computer to your smartwatch to your television, is made possible by programming. Programming is what makes these devices respond to your actions and commands. What is programming anyway? In short, it is giving a device instructions to respond to different actions and run. There's a lot more complexity to this involving computer hardware, but that is beyond the scope of this course. In essence, our computer is "run" using bits of 0s and 1s, which represent on and off electrical signals. Some programming languages, such as Assembly, interact almost directly with those bits. However, Assembly is inconvenient to program in and is incredibly hard to read.
Thus, a need for higher-level programming languages that are easier to code in and understand was born. First came languages like Pascal and FORTRAN, then C, C#, and C++, common mainstays in the programming world. The most popular programming languages of Javascript, Python, and the focus of this course: Java, have only come about recently.
These newer languages are much easier to understand and look like everyday English to an extent. For machines to understand this code, however, the program must be converted into machine code, which is what computers understand. As you progress, you'll learn that Each language has a different way of doing this.
Courtesy of MalwareBytes.
Java is one of the high-level programming languages mentioned earlier. It is also the second most used programming language in the world, behind Python. There are several reasons that Java is so popular, and here's a list from Cay Horstmann's *Core Java *****based on the original Java developers' claims:
Before we start programming, we have to actually set up our coding environment. To do this, we use an Integrated Development Environment (IDE). Your teacher will usually have a preference, but if they don't, any of these will work perfectly fine. A few well-known ones for Java include:
However, in these guides, code will either be embedded directly onto the site or be from repl.it, an online IDE that takes advantage of remote compilers so that your computer's RAM isn't overwhelmed.
Courtesy of Repl.it.
The white half is the code editor, where you enter your code that is compiled and later executed. The blue half is the console, which shows the output of the program and any errors that occur. On the left of the screen is the file explorer (not visible above), and this is where your different code files (which are your Java classes, which we’ll discuss later) are located on your computer.
Below is the first program many students learning Java will see. There seems to be a lot going on, but here, we'll break it down!
public class Main {
public static void main(String[] args) {
System.out.println("Hello world!");
}
}
Do you see the indentation? Indentation isn't needed but it can make your code much easier to read! I recommend proper indentation when coding by either using a tab or 2 spaces. Just remember to be consistent!
If you hit run, the console will return:
> javac -classpath .:/run_dir/junit-4.12.jar:target/dependency/* -d . Main.java
> java -classpath .:/run_dir/junit-4.12.jar:target/dependency/* Main
Hello world!
There is a lot that's going on in these snippets, and we'll learn these more in-depth in later units. In the console, we first see javac, which is the Java compiler compiling Main.java, and then a java command executing the Main bytecode. The third line (and beyond, depending on the program) has the result of the code. In future instances, we won't show the first two lines.
Meanwhile, on the code, let's decode it line by line! The java files are always in the name ClassName.java, and in this case, the file in question is Main.java thus the class Main. The public means that it can be accessed from any other class. The {} denotes the contents inside the class and the main method after it—in other programming languages, they are called functions—with the method header in line 2. Every program, no matter how many classes there are, must have a main method in only one of the classes. The main method is "the master switch," the method that determines what other methods are called. The static means that we don't need an instance of the "Main" class (we'll learn more about instantiations and objects in Unit 2). The void signifies that the method does not return anything directly to another function. Then, the method name is next, and any parameters in parentheses. In this case, the parameters (String[] args) is an array of strings named args (we'll learn more about arrays in Unit 6). The third line is what does some work. The System.out.println("Hello world!"); prints Hello world!, and sets the cursor to a new line if there is anything else to print. Note the semicolon after the line; this is necessary as it shows the end of an action. If it isn't there, the compiler returns a syntax error. "Hello world!" is what is known in Java as a string literal, a string for short. A string is an object in Java that sets up a template for strings. Any time there is a statement enclosed in double-quotes, that is a string.
There are two ways to print text to the consoles that we'll use in this course. They are both methods of the System.out object, which is a part of the System class. System.out.println() prints whatever is in the parentheses to the console and sets the cursor on a new line. On the other hand, System.out.print() prints whatever is in the parentheses, but doesn't set the cursor on a new line.
public class Main {
public static void main(String[] args) {
System.out.print("Hi! ");
System.out.println("Hello!");
System.out.println("Bye!");
}
}
Hi! Hello!
Bye!
AP CSA stands for Advanced Placement Computer Science A. It is an advanced level course offered in high schools that focuses on teaching students the fundamentals of computer science and programming.
Term 1 of 14
AP CSA stands for Advanced Placement Computer Science A. It is an advanced level course offered in high schools that focuses on teaching students the fundamentals of computer science and programming.
Term 1 of 14
AP CSA stands for Advanced Placement Computer Science A. It is an advanced level course offered in high schools that focuses on teaching students the fundamentals of computer science and programming.
Term 1 of 14
AP CSA stands for Advanced Placement Computer Science A. It is an advanced level course offered in high schools that focuses on teaching students the fundamentals of computer science and programming.
Java Programming Language: A popular programming language used in AP CSA to write code and develop software.
Algorithms: Step-by-step instructions or procedures used to solve problems or perform tasks in computer science.
Data Structures: The way data is organized, stored, and accessed in a computer program.
Java is a high-level, object-oriented programming language that is widely used for developing applications and software. It is known for its platform independence, meaning that programs written in Java can run on any device or operating system with a Java Virtual Machine (JVM).
Object-Oriented Programming (OOP): A programming paradigm that organizes data and behavior into reusable objects. OOP focuses on creating modular and maintainable code.
JVM (Java Virtual Machine): The runtime environment where Java programs are executed. It translates bytecode into machine-specific instructions.
AP Computer Science A: An advanced placement course that covers fundamental computer science concepts, including programming in languages like Java.
Machine code is the lowest level of instructions that can be executed directly by a computer's hardware. It consists of binary digits (0s and 1s) representing specific operations performed by the processor.
Assembly Language: A low-level programming language that uses mnemonic codes to represent machine instructions. It is a human-readable version of machine code.
Compiler: A software tool that translates high-level programming languages into machine code or assembly language so that computers can execute the program.
Instruction Set: The collection of all the instructions that a particular processor can execute. Each instruction performs a specific operation on data.
High-level programming languages are languages that are designed to be easily understood by humans. They use natural language elements and abstract away low-level details, making it easier for programmers to write code.
Syntax: The set of rules that define how programs written in a particular programming language should be structured.
Variables: Containers used to store data values in a program. They can hold different types of data such as numbers, text, or boolean values.
Control Structures: Statements or blocks of code that control the flow of execution within a program, such as loops and conditional statements.
Classes are user-defined data types in object-oriented programming. They serve as blueprints for creating objects by defining their attributes and methods.
Objects: Objects are instances created from classes. They represent individual entities with their own unique characteristics.
Inheritance: Inheritance is a mechanism in which one class inherits properties (attributes and methods) from another class, allowing for code reuse.
Encapsulation: Encapsulation is the concept of bundling data (attributes) and methods together within a class to hide implementation details from outside access.
Objects are instances of a class that represent real-world entities or concepts. They encapsulate data (attributes) and behavior (methods) into a single entity.
Classes: Classes are user-defined data types in object-oriented programming. They serve as blueprints for creating objects by defining their attributes and methods.
Attributes: Attributes are variables within an object that store data specific to that object.
Methods: Methods are functions within an object that define its behavior and allow it to perform actions.
Methods are functions defined within a class that perform specific tasks or actions when called upon by an object. They can manipulate data, interact with other objects, or return values.
Parameters: Parameters are variables used in method declarations to receive input values when the method is called. They allow methods to accept and work with different data.
Return Type: The return type of a method specifies the type of value that the method will return after its execution. It can be void (no return value) or any other data type.
Overloading: Method overloading is a feature that allows multiple methods within a class to have the same name but different parameters, enabling flexibility and code reusability.
The Java Virtual Machine (JVM) is a virtual machine that executes Java bytecode. It provides the runtime environment for running Java applications and translates bytecode into machine code that can be executed by the computer's operating system.
Bytecode: Bytecode is a low-level representation of Java source code that is platform-independent. It is generated by the Java compiler and can be executed by the JVM.
Object-oriented programming: Object-oriented programming (OOP) is a programming paradigm that organizes data and behavior into reusable objects. Java, which runs on the JVM, is an object-oriented programming language.
Garbage collection: Garbage collection is an automatic memory management process in which the JVM identifies and frees up memory occupied by objects that are no longer needed in order to prevent memory leaks.
IntelliJ is an integrated development environment (IDE) specifically designed for Java programming. It provides a user-friendly interface and a wide range of features to help developers write, debug, and test their code efficiently.
IDE: An Integrated Development Environment (IDE) is a software application that provides comprehensive tools for software development. It includes features like code editor, debugger, compiler, and build automation tools.
Java: Java is a popular object-oriented programming language used for developing various applications. It is known for its platform independence and extensive libraries.
Debugging: Debugging refers to the process of identifying and fixing errors or bugs in computer programs. It involves analyzing the program's behavior, finding the cause of the issue, and making necessary corrections.
Visual Studio Code (VS Code) is a lightweight yet powerful source code editor developed by Microsoft. It supports multiple programming languages and offers various extensions to enhance productivity during coding.
Source Code Editor: A source code editor is a text editor specifically designed for writing or editing source code of computer programs. It provides syntax highlighting, auto-completion, indentation support, and other features tailored for coding.
Extensions: In the context of VS Code, extensions are add-ons or plugins that enhance its functionality by providing additional features or support for specific programming languages or frameworks.
Productivity: Productivity refers to the efficiency and effectiveness with which tasks are completed. In the context of coding, productivity can be improved by using tools, shortcuts, and techniques that streamline the development process.
BlueJ is an integrated development environment (IDE) primarily used for teaching introductory Java programming. It provides a simplified interface and visual tools to help beginners understand object-oriented concepts.
Object-Oriented Programming (OOP): Object-Oriented Programming is a programming paradigm that organizes code around objects, which are instances of classes. It emphasizes concepts such as encapsulation, inheritance, and polymorphism.
IDE: An Integrated Development Environment (IDE) is a software application that provides comprehensive tools for software development. It includes features like code editor, debugger, compiler, and build automation tools.
Visual Tools: In the context of BlueJ, visual tools refer to graphical representations or interactive elements that help visualize object-oriented concepts such as class diagrams or object interactions. These visual aids assist learners in understanding complex relationships between objects.
A syntax error is a mistake in the structure or format of a program's code that prevents it from being compiled or executed properly. It occurs when the code violates the rules and grammar of the programming language.
Logic Error: A logic error refers to a mistake in the algorithm or flow of a program that causes incorrect output or unexpected behavior.
Runtime Error: A runtime error occurs during program execution when something unexpected happens, such as dividing by zero or accessing an invalid memory location.
Compiler: A compiler is a software tool that translates human-readable source code into machine-executable instructions. It checks for syntax errors and generates an executable file if there are no errors.
System.out.print() is a method in Java that allows you to display text or values on the console. It does not add a new line after printing.
System.out.println(): This is similar to System.out.print(), but it adds a new line after printing. It's like hitting the enter key after typing each line.
String concatenation: This refers to combining multiple strings together using the "+" operator. It's like gluing different pieces of text together to form one longer piece.
Escape sequences: These are special characters used within strings to represent non-printable characters or formatting instructions. For example, "\n" represents a new line and "\t" represents a tab space.