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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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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!