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Week 1 - Friday. CS 121. Last time. What did we talk about last time? Our first Java program. Questions?. Java refresh. The full Hello World program Remember that everything is in a class The class name must match the file name ( Hello.java )

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Presentation Transcript
last time
Last time
  • What did we talk about last time?
  • Our first Java program
java refresh
Java refresh
  • The full Hello World program
  • Remember that everything is in a class
  • The class name must match the file name (Hello.java)
  • The main()method is where the program starts
  • The print statement outputs information on the screen

publicclass Hello


  • public static void main(String[] args)


System.out.println("Hello, world!");



  • In Java, like C, C++, and many other languages, we separate different statements with a semicolon ( ; )
  • If we want to do a number of statements, we just type them in order, with a semicolon after each one
  • For example, instead of one print statement, we can have several:
  • Each statement is an instruction to the computer
  • They are printed in order, one by one

System.out.println("Hello, world!");

System.out.println("Hello, galaxy!");

  • System.out.println("Goodbye, world!");
case sensitivity
Case Sensitivity
  • Java is a case sensitive language
  • Class is not the same as class
  • System.out.println("Word!"); prints correctly
  • system.Out.Println("Word!"); does not compile
  • Java generally ignores whitespace (tabs, newlines, and spaces)

is the same as:

  • You should use whitespace effectively to make your code readable

System.out.println("Hello, world!");


println( "Hello, world!");

  • Programs can be confusing
  • Sometimes you want to leave notes for yourself or anyone else who is reading your code
  • The standard way to do this is by using comments
  • Although comments appear in the code, they do not affect the final program
  • There are two kinds of comments (actually 3)
  • Single line comments use //
  • Multi-line comments start with a /* and end with a */

System.out.println("Hi!"); // this is a comment

System.out.println("Hi!"); /* this is

a multi-line

comment */

what we know
What we know
  • Java is a large, complex language
  • Even so, there are only a few tasks that you can ask it to do
  • You have already learned:
    • Sequencing
    • Basic output
where we are headed
Where we are headed
  • There are not that many other things you can tell Java to do
    • Storing numbers and text
    • Basic mathematical operations
    • Choosing between several options
    • Doing a task repetitively
    • Storing lists of things
    • More complicated input and output
    • Naming a task so that you can use it over and over again
  • That’s basically it
what is programming again
What is programming again?
  • The process of giving computers very detailed instructions about what to do
  • How do we do that exactly?
  • First, we need a programming language like Java
  • How do we turn a set of instructions written so that a human can read them into a set of instructions that a computer can read?
  • Magic, of course!
first let s talk about languages
First, let’s talk about languages
  • There are many different programming languages:
    • Java
    • C/C++
    • ML
    • …thousands more
  • Each has different advantages in different situations
high vs low
High vs. low
  • We classify languages as high or low level
  • High level languages allow you to give more abstract commands that are more like human thought processes or mathematics
  • Low level languages are closer to the computer world and give explicit instructions for the hardware to follow



  • We use a program called a compiler to turn a high level language into a low level language
  • Usually, the low level language is machine code
  • With, Java it's a little more complex
how does that work in general
How does that work in general?

Source Code

Machine Code


what s the issue with java
What’s the issue with Java?
  • Java is a more complicated
  • Java runs on a virtual machine, called the JVM
  • Java is compiled to an intermediate stage called bytecode, which is platform independent
  • Then, the JVM runs a just-in-time compiler whenever you run a Java program, to turn the bytecode into platform dependent machine code
compilation and execution for java
Compilation and execution for Java




Machine Code

Java Bytecode






let s review the steps we ll use
Let’s review the steps we’ll use
  • Write a program in Java
  • Compile the program into bytecode
  • Run the bytecode using the JVM (which automatically compiles the bytecode to machine code)
software development1
Software development

Often goes through phases similar to the following:

  • Understand the problem
  • Plan a solution to the problem
  • Implement the solution in a programming language
  • Test the solution
  • Maintain the solution and do bug fixes

Factor of 10 rule!

next time
Next time…
  • We'll talk about data representation
  • Reading Chapter 3
  • Look at Project 1
  • No class on Monday!