02 Java Programming Basics

8/2/2019 02 Java Programming Basics

1/31

Java Programming BasicsCSE 110: Introduction to Computer Science

SUNY at Stony Brook


2/31

Outline of Topics

The Java Environment

Java Grammar

A First Program

Parts of a Java Program

Compiling and Executing Java Code

Experimenting With Code


3/31

The Java Development Cycle

Standard edit-compile-execute cycle

Compilation translates source code into a form that the

computer can understand

With Java, this is slightly more complicated than with

other languages...


4/31

Two Faces of Java

The name Java actually refers to two things:

The programming language

The Java Virtual Machine (JVM)

In most languages, a compiled program runs directly onthe hardware (it speaks the CPUs language)

Java programs execute on a simulated CPU (the JVM).

The real CPU runs a program that simulates this CPU.


5/31

Desktop PCSource Code

is compiled by

executesprogram on

The Java Compilation Process


6/31

Java Compilation (Part 1)


is compiled by

executesprogram on

Source Code

100111

101010

110011

000110

Java Bytecode

Java compiler(javac)


7/31

Java Compilation (Part 2)


is compiled by

executesprogram on

100111

101010

110011

000110

Java Bytecode

Java VirtualMachine (JVM) Desktop PC

executeson

which issimulated by


8/31

Why Is It So Complicated?

With most languages, a program must be compiledseparately for every type of CPU

A new type of CPU requires another compilation

Java programs are compiled to a fake CPU (the JVM)

After one compilation, a Java program can run on any

CPU that can run the JVM

Write once, run anywhere reduces programmer effort


9/31

Outline of Topics


Java Grammar

A First Program





10/31

Formal Languages

A computer language is a set of notation for givinginstructions to a computer

Different languages may express the same concepts in

different ways, just like human languages

Like any language, Java has rules of grammar

We need to follow these rules in order to produce well-

formed Java programs


11/31

Parts of Speech

English has different parts of speech: nouns, verbs,

adjectives, etc.

Java has something similar: variables, statements,

modifiers, expressions, etc.

Just like with English, these parts of speech can only be

combined in certain ways


12/31

Statements and Expressions

Statements are commands

They tell Java what action to perform

Expressions are values

They dont do anything on their own

Expressions are manipulated by statements


13/31

Syntax and Semantics

Syntax refers to grammatical structure

Semantics refers to meaning

These are independent qualities; you can have one

without the other (just as in English)

Java programs need both to work effectively, but Java

only checks syntax for you


14/31

Outline of Topics


Java Grammar

A First Program





15/31

My First Java Program

public class MyProgram

{ public static void main (String [ ] args) { System.out.println(Hello, world!); }}


16/31

Line-by-Line Breakdown


{

public static void main (String [ ] args) { System.out.println(Hello, world!); }}

Every Java program

consists of one or more

classes

Aclass is a block of

Java code

Classes are indicated

by the reserved word

class

public is a modifier that

tells Java that the class can

be seen/used all over

Curly braces surround the

contents of the class


17/31



{


Amethod is a block of

Java statements

main() is a special method

It tells the JVM how to

start the program

Every Java program

(not every class) must

have a main() method

main() always has the

same header (first line)

The body of a method is

surrounded by curly braces


18/31



{


System.out.println() is

another Java method

It tells the program to

print something to the

console (screen)

The information to print

goes inside the

parentheses

Here, were printing a

string of characters

This is a method call

main() was a method

definition


19/31

Outline of Topics


Java Grammar

A First Program





20/31

Compiling Your Java Code1. Write and save your source code using a text editor

The source code MUST be a plain text file

The file should be named after the class it contains (e.g., a

class named Foo must live in a file named Foo.java)

2. Run the Java compiler (javac) to compile your source code fileinto a Java bytecode file:

javac myFile.java

This will produce a file named myFile.class


21/31

Executing Your Java Code

To execute your program, run the JVM with yourcompiled .class file:

java myFile

Note that the .class extension is OMITTED (The JVMfills that in automatically)

You can only call java on a class file that contains a

main() method


22/31

Outline of Topics


Java Grammar

A First Program





23/31

Learning by Tinkering Most methods change their behavior based on the input

that you give them

System.out.println() will print whatever you tell it to

Character sequences MUST be enclosed in double

quotes

Numbers can be supplied as-is

Use the plus sign (+) to combine (concatenate)

expressions of different types


24/31

Notes on Printing Behavior System.out.println() automatically adds a newline

character at the end, to move to the next line

System.out.print() (note the lack of ln in the name) does

not do this it stays on the same line

To go to the next line at any point in a print statement,insert the character sequence \n

\ indicates that the next character has special meaning

\n = newline, \t = tab, \\ = a single \ character


25/31

Introduction to BlueJ


26/31

What is BlueJ?

BlueJ is a simple IDE (integrated developmentenvironment) designed for new programmers

An IDE combines a source code editor and the compiler

BlueJ is written in Java, and runs on all platforms

You can download BlueJ for free from

http://www.bluej.org


27/31

Projects

Aproject is the set of files that make up a program

Before you can do anything in BlueJ, you must create a

new project (or open an existing one)

BlueJ creates a new folder on disk with the same nameas the project

A BlueJ project contains all of your .java and .class files,

along with a few special BlueJ-specific files


28/31

The Main BlueJ Window The project window has

two main sections:

The top area shows all

of the classes in your

current project (arrowsindicate relationships)

The object workbench is

at the bottom


29/31

Editing Source Code To open a source

code file for editing,

double-click on the

box for that class

BlueJ includes a basictext editor

BlueJ has auto-indent

and syntax coloring


30/31

Executing Your Program

To execute your program, right-click on the class in themain window

Select the main() method from the menu that appears

Unlike most IDEs, BlueJ actually allows you to startexecution from any method in your program

This lets you test a specific method without having to run

your entire program


31/31

The Object Workbench

BlueJ allows you to work with your classes withoutactually running your entire program

Right-click on a class and select New... to create a

new instance of that class

Instances appear in the object workbench at the bottom

Right-click on an object to call its methods

Double-click on an object to inspect its data

02 Java Programming Basics

Documents

Transcript of 02 Java Programming Basics