CS 177 Recitation Week 1 Intro to Java
Questions?
Computers Computers can do really complex stuff. How? By manipulating data according to lists of instructions. Fundamentally, this is all that a computer can actually do. Different kinds of computers: supercomputers, desktops, laptops, embedded. Different kinds of computer have different sizes, computing powers, uses, etc.
Computers What is hardware? What are some examples? The physical components of the computer (stuff you can touch). monitor, CPU, keyboard, memory, hard drive, etc. What is software? What are some examples? Programs that run on the hardware and make it do stuff. Windows, Firefox, MS Powerpoint, World of Warcraft, etc
Computers What are some important (hardware) parts of a computer? CPU does calculations (the brain) Memory stores data (cache, RAM, hard drive, etc) I/O devices allows the user and the computer to communicate (monitor, mouse, keyboard, etc)
Programming Computers only manipulate data according to a list of instructions. Computers are stupid, but really fast. Computers can only do very simple things (like adding two numbers), but they can do millions or billions of simple things a second. Programming the process of creating lists of instructions for the computer to follow.
Programming The process of software development: 1) Understand the problem that you need to solve. 2) Plan a solution to the problem. 3) Implement the solution (write the program). 4) Test the program (compile and run it), and fix any errors.
Programming Languages We have to use programming languages to give computers instructions because human languages (like English) are too vague. We are going to write programs in a programming language called Java. There are lots of other programming languages too.
Programming Languages We classify programming languages as high or low level. Low level languages are closer to (or are) what the computer actually understands, and have very simple commands. ex) machine code, assembly languages High level languages are easier for humans to understand and have more complex commands. Programs written in high level languages have to be translated to a low level language before they can be run. ex) Java, Python
Compiling What special program translates a program written in a high-level language into a low-level language? a compiler How things usually work: The compiler translates the program directly into machine code and the program can be run.
Compiling Java is weird: The Java compiler translates the program into bytecode, a sort of intermediate language. When the Java program is run, the Java Virtual Machine compiles/translates the bytecode into machine code, and then the program runs. The machine code produced is different depending on the computer. The bytecode will be the same no matter what computer the program is compiled on. The Whatever.class file you get after compiling your Whatever.java file contains the bytecode.
Java Java programs are just text files, and can be written in any text editor or DrJava. Java is a language, and therefore has rules that you have to learn and obey. If you do not follow Java s rules, your program will not compile or won t run correctly.
Anatomy of a Java Program public class NameOfClass { public static void main(string[] args) { } } Every Java program must be inside a class. The keyword class tells Java that you are making a new class. Replace NameOfClass with what you want to call your program. The program will need to be saved in a file called NameOfClass.java. For now, the rest of the stuff is magic words. Just remember to type them in, or your program won t work.
Simple Program public class HelloWorld { public static void main(string[] args) { System.out.print( Hello, world! ); System.out.println(100.34); System.out.println( Programming is fun! ); } } This program just prints some stuff on the screen. You can use System.out.println() to print anything in quotes ( fish, foobar ) or numbers (5, 20.6). System.out.print() does the same thing, but does not put a newline after the printed stuff.
Running a Program Let s run our HelloWorld program and see it in action! There are two different ways to do it. In the terminal or command prompt: Change to the directory where your Java file is saved. To compile, type javac HelloWorld.java If you don t see any messages, your program compiled successfully. Compiling creates a class file with the same name as your Java file (in this case, HelloWorld.class ). To run, type java HelloWorld.
Running a Program In DrJava: To compile, push the Compile button (upper right corner of the window). If it says Compilation completed, everything worked. To run, push the Run button (near the Compile button). NOTE: You don t have to use DrJava if you don t want to, especially at home. It will probably make your life easier, though.
Some Java Details All statements in Java must end with a semicolon (;). Think of it like the period at the end of an English sentence. All classes and methods must have curly braces ({ and }) around their contents. Java is case sensitive ( class is not the same as Class or CLASS or class ).
Comments Programs can be confusing to read. Use comments to leave notes for yourself or anyone else who reads your code. Comments do not affect how the program runs (they re ignored by the computer). Two ways to make comments: // I m a comment Use for single-line comments /* I m a comment. */ Use for comments that are multiple lines long
Coding with Style! Java ignores whitespace, so use whitespace to make your code more readable. Indent all the lines of code between a class or method s curly brackets. This makes your code MUCH easier to read. Really. It does. We WILL take points off on assignments if you do not indent your code nicely!
Indentation Examples Good public class NameOfClass { public static void main(string[] args) { System.out.println( Hello, world! ); } } Bad public class NameOfClass { public static void main(string[] args) { System.out.println( Hello, world! ); } } Horrible! public class NameOfClass { public static void main(string[] args) { System.out.println( Hello, world! ); } }
Questions?