References and Primitive Data Types

References and Primitive Data Types:

•Java distinguishes two kinds of entities

•Primitive types

•Objects

•Primitive-type data is stored in primitive-type variables

•Reference variables store the address of an object

•No notion of “object (physically) in the stack”

•No notion of “object (physically) within an object”

Primitive Data Types:

•Represent numbers, characters, boolean values

•Integers: byte, short, int, and long

•Real numbers: float and double

•Characters: char

A Little Example of import and main:

A Little Example of import and main:

import javax.swing.*;    // all classes from javax.swing public class HelloWorld {  // starts a class  public static void main (String[] args) {  // starts a main method  // in: array of String; out: none (void)  }}•public = can be seen from any package•static = not “part of” an object

Processing and Running HelloWorld:

•javac HelloWorld.java

•Produces HelloWorld.class (byte code)

•java HelloWorld

•Starts the JVM and runs the main method.

Classes and Objects:

Classes and Objects:

•The class is the unit of programming

•A Java program is a collection of classes

•Each class definition (usually) in its own .java file

•The file name must match the class name

•A class describes objects (instances)

•Describes their common characteristics: is a blueprint

•Thus all the instances have these same characteristics

•These characteristics are:

•Data fields for each object

•Methods (operations) that do work on the objects

Grouping Classes: The Java API,

•API = Application Programming Interface

•Java = small core + extensive collection of packages

•A package consists of some related Java classes:

•Swing: a GUI (graphical user interface) package

•AWT: Application Window Toolkit (more GUI)

•util: utility data structures (important to CS 187!)

•The import statement tells the compiler to make available classes and methods of another package

•A main method indicates where to begin executing a class (if it is designed to be run as a program)

Java Processing and Execution

chapter:2

Java Processing and Execution:

•Begin with Java source code in text files: Model.java

•A Java source code compiler produces Java byte code

•Outputs one file per class: Model.class

•May be standalone or part of an IDE

•A Java Virtual Machine loads and executes class files

•May compile them to native code (e.g., x86) internally

Compiling and Executing a Java Program:

Intro to java

      Introduction to Java

Topics of the Review:

•Essentials of object-oriented programming, in Java

•Java primitive data types, control structures, and arrays

•Using some predefined classes:

•Math

•JOptionPane, I/O streams

•String, StringBuffer, StringBuilder

•StringTokenizer

•Writing and documenting your own Java classes

Some Salient Characteristics of Java:

•Java is platform independent: the same program can run on any correctly implemented Java system

•Java is object-oriented:

•Structured in terms of classes, which group data with operations on that data

•Can construct new classes by extending existing ones

•Java designed as

•A core language plus

•A rich collection of commonly available packages

•Java can be embedded in Web pages