Java Basics

 Java Basics  

Topics in this section include:

What makes Java programs portable, secure, and robust

The structure of Java applets and applications

How Java applications are executed

How applets are invoked and executed

The Java Language, Part I

Comments

Declarations

Expressions

Statements

Garbage collection

Java Semantics

Portability

Java programs are portable across operating systems and hardware environments.

Portability is to your advantage because:

You need only one version of your software to serve a broad market.

The Internet, in effect, becomes one giant, dynamic library.

You are no longer limited by your particular computer platform.

Three features make Java String programs portable:

1. The language. The Java language is completely specified; all data-type sizes and

formats are defined as part of the language. By contrast, C/C++ leaves these

"details" up to the compiler implementor, and many C/C++ programs therefore 

  are not portable.

2. The library. The Java class library is available on any machine with a Java

runtime system, because a portable program is of no use if you cannot use the

same class library on every platform. Window-manager function calls in a Mac

application written in C/C++, for example, do not port well to a PC.

3. The byte code. The Java runtime system does not compile your source code

directly into machine language, an inflexible and nonportable representation of

your program. Instead, Java programs are translated into machine-independent

byte code. The byte code is easily interpreted and therefore can be executed on

any platform having a Java runtime system. (The latest versions of the Netscape

Navigator browser, for example, can run applets on virtually any platform).

Security

The Java language is secure in that it is very difficult to write incorrect code or

viruses that can corrupt/steal your data, or harm hardware such as hard disks.

There are two main lines of defense:

Interpreter level:

No pointer arithmetic

Garbage collection

Array bounds checking

No illegal data conversions

Browser level (applies to applets only):

No local file I/O

Sockets back to host only

No calls to native methods

Robustness

The Java language is robust. It has several features designed to avoid crashes

during program execution, including:

Garbage collection--no bad addresses

Array and string bounds checking

No jumping to bad method addresses

Interfaces and exceptions

Java Program Structure

A file containing Java source code is considered a compilation unit. Such a

compilation unit contains a set of classes and, optionally, a package definition to

group related classes together. Classes contain data and method members that

specify the state and behavior of the objects in your program.

Java programs come in two flavors:

Standalone applications that have no initial context such as a pre-existing main

window

Applets for WWW programming

The major differences between applications and applets are:

Applets are not allowed to use file I/O and sockets (other than to the host

platform). Applications do not have these restrictions.

An applet must be a subclass of the Java Applet class. Aplications do not need to

subclass any particular class.

Unlike applets, applications can have menus.

Unlike applications, applets need to respond to predefined lifecycle messages

from the WWW browser in which they're running.

Java Program Execution

The Java byte-code compiler translates a Java source file into machine-

independent byte code. The byte code for each publicly visible class is placed in a

separate file, so that the Java runtime system can easily find it. If your program

instantiates an object of class A, for example, the class loader searches the

directories listed in your CLASSPATH environment variable for a file called A.class

that contains the class definition and byte code for class A.

There is no link phase for Java programs; all linking is done dynamically at

runtime.

The following diagram shows an example of the Java compilation and execution

sequence for a source file named A.java containing public class A and non-public

class B:

Java programs are, in effect, distributed applications. You may think of them as a

collection of DLLs (dynamically loadable libraries) that are linked on demand at

runtime. When you write your own Java applications, you will often integrate

your program with already-existing portions of code that reside on other

machines.

A Simple Application

Consider the following trivial application that prints "hi there" to standard

output:

public class TrivialApplication {

 // args[0] is first argument

 // args[1] the second

 public static void main(String args[]) {

 System.out.println("hi there");

 }

}

The command java TrivialApplication tells the Java runtime system to begin

with the class file TrivialApplication.class and to look in that file for a

method with the signature:

public static void main(String args[]);

The main() method will always reside in one of your class files. The Java

language does not allow methods outside of class definitions. The class, in effect,

creates scoped symbol StartingClassName.main for your main() method.

Applet Execution

An applet is a Java program that runs within a Java-compatible WWW browser or

in an appletviewer. To execute your applet, the browser:

Creates an instance of your applet

Sends messages to your applet to automatically invoke predefined lifecycle

methods

The predefined methods automatically invoked by the runtime system are:

init(). This method takes the place of the Applet constructor and is only called

once during applet creation. Instance variables should be initialized in this method.

GUI components such as buttons and scrollbars should be added to the GUI in

this method.

start(). This method is called once after init() and whenever your applet is

revisited by your browser, or when you deiconify your browser. This method

should be used to start animations and other threads.

paint(Graphics g). This method is called when the applet drawing area needs

to be redrawn. Anything not drawn by contained components must be drawn in

this method. Bitmaps, for example, are drawn here, but buttons are not because

they handle their own painting.

stop(). This method is called when you leave an applet or when you iconify

your browser. The method should be used to suspend animations and other

threads so they do not burden system resources unnecessarily. It is guaranteed to

be called before destroy().

destroy(). This method is called when an applet terminates, for example, when

quitting the browser. Final clean-up operations such as freeing up system

resources with dispose() should be done here. The dispose() method of Frame

removes the menu bar. Therefore, do not forget to call super.dispose() if you

override the default behavior.

The basic structure of an applet that uses each of these predefined methods is:

import java.applet.Applet;

// include all AWT class definitions

import java.awt.*;

public class AppletTemplate extends Applet {

 public void init() {

 // create GUI, initialize applet

 }

 public void start() {

 // start threads, animations etc...

 }

 public void paint(Graphics g) {

 // draw things in g

 }

 public void stop() {

 // suspend threads, stop animations etc...

 }

 public void destroy() {

 // free up system resources, stop threads

 }

}

All you have to do is fill in the appropriate methods to bring your applet to life. If

you don't need to use one or more of these predefined methods, simply leave them

out of your applet. The applet will ignore messages from the browser attempting

to invoke any of these methods that you don't use.

A Simple Applet

The following complete applet displays "Hello, World Wide Web!" in your

browser window:

import java.applet.Applet;

import java.awt.Graphics;

public class TrivialApplet extends Applet {

 public void paint(Graphics g) {

 // display a string at 20,20

 // where 0,0 is the upper-left corner

 g.drawString("Hello, World Wide Web!", 20, 20);

 }

}

An appletviewer may be used instead of a WWW browser to test applets. For

example, the output of TrivialApplet on an appletviewer looks like:

HTML/Applet Interface

The HTML applet tag is similar to the HTML img tag, and has the form:

<applet code=AppletName.class width=w height=h>

[parameters]

</applet>

where the optional parameters are a list of parameter definitions of the form:

<param name=n value=v>

An example tag with parameter definitions is:

<applet code=AppletName.class width=300 height=200>

<param name=p1 value=34>

<param name=p2 value="test">

</applet>

where p1 and p2 are user-defined parameters.

The code, width, and height parameters are mandatory. The parameters

codebase, alt, archives, align, vspace, and hspace are optional within the

<applet> tag itself. Your applet can access any of these parameters by calling:

Applet.getParameter("p")

which returns the String value of the parameter. For example, the applet:

import java.applet.Applet;

public class ParamTest extends Applet {

 public void init() {

 System.out.println("width is " + getParameter("width"));

 System.out.println("p1 is " + getParameter("p1"));

 System.out.println("p2 is " + getParameter("p2"));

 }

}

prints the following to standard output:

width is 300

p1 is 34

p2 is test

Comments

Java comments are the same as C++ comments, i.e.,

/* C-style block comments */

where all text between the opening /* and closing */ is ignored, and

// C++ style single-line comments

where all text from the opening // to the end of the line is ignored.

Note that these two comments can make a very useful combination. C-style

comments (/* ... */) cannot be nested, but can contain C++ style comments.

This leads to the interesting observation that if you always use C++-style

comments (// ...), you can easily comment out a section of code by surrounding

it with C-style comments. So try to use C++ style comments for your "normal"

code commentary, and reserve C-style comments for commenting out sections of

code.

The Java language also has a document comment:

/** document comment */

These comments are processed by the javadoc program to generate

documentation from your source code. For example,

/** This class does blah blah blah */

class Blah {

 /** This method does nothing

 /**

 * This is a multiple line comment.

 * The leading * is not placed in documentation.

 */

 public void nothing() {;}

}

Declarations

A Java variable may refer to an object, an array, or an item of primitive type.

Variables are defined using the following simple syntax:

TypeName variableName;

For example,

int a; // defines an integer

int[] b; // defines a reference to array of ints

Vector v; // reference to a Vector object

Primitive Types

The Java language has the following primitive types:

Primitive Types

Primitive Type Description

boolean true/false

byte 8 bits

char 16 bits (UNICODE)

short 16 bits

int 32 bits

long 64 bits

float 32 bits IEEE 754-1985

double 64 bits IEEE 754-1985

Java int types may not be used as boolean types and are always signed.

Objects

A simple C++ object or C struct definition such as "Button b;" allocates

memory on the stack for a Button object and makes b refer to it. By contrast, you

must specifically instantiate Java objects with the new operator. For example,

// Java code

void foo() {

 // define a reference to a Button; init to null

 Button b;

 // allocate space for a Button, b points to it

 b = new Button("OK");

 int i = 2;

}

As the accompanying figure shows, this code places a reference b to the Button

object on the stack and allocates memory for the new object on the heap.

The equivalent C++ and C statements that would allocate memory on the heap

would be:

// C++ code

Button *b = NULL; // declare a new Button pointer

b = new Button("OK"); // point it to a new Button

/* C code */

Button *b = NULL; /* declare a new Button pointer */

b = calloc(1, sizeof(Button)); /* allocate space for a Button */

init(b, "OK"); /* something like this to init b */

All Java objects reside on the heap; there are no objects stored on the stack.

Storing objects on the heap does not cause potential memory leakage problems

because of garbage collection.

Each Java primitive type has an equivalent object type, e.g., Integer, Byte,

Float, Double. These primitive types are provided in addition to object types

purely for efficiency. An int is much more efficient than an Integer.

Java Basics -10 © 1996-2003 jGuru.com. All Rights Reserved.

Strings

Java string literals look the same as those in C/C++, but Java strings are real

objects, not pointers to memory. Java strings may or may not be null-terminated.

Every string literal such as

"a string literal"

is interpreted by the Java compiler as

new String("a string literal")

Java strings are constant in length and content. For variable-length strings, use

StringBuffer objects.

Strings may be concatenated by using the plus operator:

String s = "one" + "two"; // s == "onetwo"

You may concatenate any object to a string. You use the toString() method to

convert objects to a String, and primitive types are converted by the compiler.

For example,

String s = "1+1=" + 2; // s == "1+1=2"

The length of a string may be obtained with String method length(); e.g.,

"abc".length() has the value 3.

To convert an int to a String, use:

String s = String.valueOf(4);

To convert a String to an int, use:

int a = Integer.parseInt("4");

Array Objects

In C and C++, arrays are pointers to data in memory. Java arrays are objects that

know the number and type of their elements. The first element is index 0, as in

C/C++.

Generic Array Object

# elements

element type

element 0

element 1

...

element n-1

The syntax for creating an array object is:

TypeName[] variableName;

This declaration defines the array object--it does not allocate memory for the array

object nor does it allocate the elements of the array In addition, you may not

specify a size within the square brackets.

To allocate an array, use the new operator:

int[] a = new int[5]; // Java code: make array of 5 ints

new int[5]

5

int

0

0

0

0

0

In C or C++, by contrast, you would write either

/* C/C++ code: make array of 5 ints on the stack */

int a[5];

or

/* C/C++ code: make array of 5 ints on the heap */

int *a = new int[5];

An array of Java objects such as

// Java code: make array of 5 references to Buttons

Button[] a = new Button[5];

creates the array object itself, but not the elements:

new Button[5]

5

Button

null pointer

null pointer

null pointer

null pointer

null pointer

You must use the new operator to create the elements:

a[0] = new Button("OK");

a[3] = new Button("QUIT");

In C++, to make an array of pointers to objects you would write:

// C++: make an array of 5 pointers to Buttons

Button **a = new Button *[5]; // Create the array

a[0] = new Button("OK"); // create two new buttons

a[3] = new Button("QUIT");

In C, code for the same task would look like:

/* C: make an array of 5 pointers to structs */

/* Allocate the array */

Button **a = calloc(5, sizeof(Button *));

/* Allocate one button */

a[0] = calloc(1, sizeof(Button));

/* Init the first button */

setTitle(a[0], "OK");

/* Allocate another button */

a[3] = calloc(1, sizeof(Button));

/* Init the second button */

setTitle(a[3], "QUIT");

Multi-dimensional Java arrays are created by making arrays of arrays, just as in

C/C++. For example,

T[][] t = new T[10][5];

makes a five-element array of ten arrays of references to objects of type T. This

statement does not allocate memory for any T objects.

Accessing an undefined array element causes a runtime exception called

ArrayIndexOutOfBoundsException.

Accessing a defined array element that has not yet been assigned to an object

results in a runtime NullPointerException.

Initializers

Variables may be initialized as follows:

Primitive types

int i = 3;

boolean g = true;

Objects

Button b = null;

Employee e = new Employee();

Arrays

int[] i = {1, 2, 3, 4};

or in Java 1.1

int[] i;

i = new int[] {1, 2, 3, 4};

Constants

Variables modified by the static final keywords are constants (equivalent to

the const keyword in C++; no equivalent in C). For example,

// same as "const int version=1;" in C++

static final int version = 1;

static final String Owner = "Terence";

Expressions

Most Java expressions are similar to those in C/C++.

Constant Expressions

Item Examples or Description

id i, nameList

qualified-id Integer.MAX_VALUE, obj.member,

id[e][f]...[g] a[i], b[3][4]

String literal "Jim", delimited by ""

char literal 'a', '\t', delimited by ''

Unicode character constant \u00ae

boolean literal true, false (not an int)

int constant 4

float constant 3.14f, 2.7e6F, f or F suffix

double constant 3.14, 2.7e6D, (default) / d or D suffix

hexadecimal constant 0x123

octal constant 077

null the null object (note lowercase!)

this the current object

super the superclass view of this object

npackage.class, package.obj

General Expressions

Item Examples or Description

id i, nameList

obj.method(args) instance method call

class.method(args) class method call

( expr ) (3+4)*7

new T(constructor-args) instantiates a new object or class T

new T[e][f]...[g] allocates an array object

Operators

The Java language has added the >>> zero-extend right-shift operator to the set of

C++ operators. (C++ operators include instanceof and new, which are not

present in C. Note that sizeof has been removed, as memory allocation is

handled for you.) The operators, in order of highest to lowest priority, are:

new

.

-- ++ + - ~ ! (TypeName)

* / %

+ -

<< >> >>>

< > <= >= instanceof

== !=

&

^

|

&&

||

?:

= *= /= %= += -= <<= >>= >>>= &= ^= |=

Note that the precedence of the new operator and the '.' operator bind

differently than in C++. A proper Java statement is:

// Java code

new T().method();

In C++, you would use:

// C++ code

(new T)->method();

Statements

Java statements are similar to those in C/C++ as the following table shows.

Forms of Common Statements

Statement Examples

if if (boolean-expr) stat1

switch

if (boolean-expr) stat1 else stat2

switch (int-expr) {

case int-const-expr : stat1

case int-const-expr : stat2

default : stat3

}

for for (int i=0; i<10; i++) stat

while while (boolean-expr) stat

do-while do { stats } while (boolean-expr)

return return expr;

The Java break and continue statements may have labels. These labels refer to

the specific loop that the break or continue apply to. (Each loop can be

preceded by a label.)

Java Semantics

We say that the Java language has "reference semantics" and C/C++ have "copy

semantics." This means that Java objects are passed to methods by reference in

Java, while objects are passed by value in C/C++.

Java primitive types, however, are not treated in the same way as Java objects.

Primitive types are assigned, compared, and passed as arguments using copy

semantics, just as in C/C++. For example, i = j for two int variables i and j

performs a 32-bit integer copy.

Assignment of Objects

Assignment makes two variables refer to the same object. For example,

class Data {

 public int data = 0;

 public Data(int d) { data = d; }

}

I Data a = new Data(1); // a.data is 1

I Data b = new Data(2); // b.data is 2

II b = a; // b.data and a.data are 1

III a.data = 3; // b.data and a.data are 3

IV a = new Data(4); // b.data is 3, a.data is 4

To copy objects, define and use clone():

class Data implements Cloneable {

 public int data = 0;

 public Data(int d) { data = d; }

 public Object clone() {

 Data d = (Data) super.clone();

 d.data = data;

 return d;

 }

}

...

Data a = new Data(1); // a.data is 1

Data b = new Data(2); // b.data is 2

b = a.clone(); // b.data and a.data are 1

a.data = 3; // b.data is 1, a.data is 3

Note: The above class definition requires exception handling code. We, however,

have not yet discussed exception handling. For now, pretend that it is not

necessary.

Method Parameters and Return Values

Arguments and return values for primitive types are passed by value to and from

all Java methods because they are implied assignments, as in C/C++. However, all

Java objects are passed by reference. For example, the C/C++ code:

// C++ code

int foo(int j) { return j + 34;}

Button *bfoo(Button *b) {

 if ( b != NULL ) return b;

 else return new Button();

}

or, in C

/* C code */

int foo(int j) { return j + 34;}

Button *bfoo(Button *b) {

 if ( b != NULL ) return b;

 else return calloc(sizeof(Button));

}

would be written in the Java language:

// Java code

int foo(int j) { return j + 34;}

Button bfoo(Button b) {

 if ( b != null ) return b;

 else return new Button("OK");

}

Equality

Two Java primitive types are equal (using the == operator) when they have the

same value (e.g., "3 == 3"). However, two object variables are equal if and only if

they refer to the same instantiated object--a "shallow" comparison. For example,

void test() {

 Data a = new Data(1);

 Data b = new Data(2);

 Data c = new Data(1);

 // a == b is FALSE

 // a == c is FALSE (in C++, this'd be TRUE)

 Data d = a;

 Data e = a;

 // d == e is TRUE,

 // d,e are referring to same object

}

To perform a "deep" comparison, the convention is to define a method called

equals(). You would rewrite Data as:

class Data {

 public int data = 0;

 public Data(int d) { data = d; }

 boolean equals(Data d) {

 return data == d.data;

 }

}

...

Data a = new Data(1);

Data b = new Data(1);

// a.equals(b) is true!!!!

No Pointers!

The Java language does not have pointer types nor address arithmetic. Java

variables are either primitive types or references to objects. To illustrate the

difference between C/C++ and Java semantics, consider the following equivalent

code fragments.

// C++ code (C code would be similar)

Stack *s = new Stack; // point to a new Stack

s->push(...);

// dereference and access method push()

The equivalent Java code is:

// Java code

// internally, consider s to be a (Stack *)

Stack s = new Stack();

// dereference s automatically

s.push(...);

Garbage Collection

An automatic garbage collector deallocates memory for objects that are no longer

needed by your program, thereby relieving you from the tedious and error-prone

task of deallocating your own memory.

As a consequence of automatic garbage collection and lack of pointers, a Java

object is either null or valid--there is no way to refer to an invalid or stale object

(one that has been deallocated).

To illustrate the effect of a garbage collector, consider the following C++ function

that allocates 1000 objects on the heap via the new operator (a similar C function

would allocate memory using calloc/malloc):

// C++ code

void f() {

 T *t;

 for (int i = 1; i <= 1000; i++) {

 t = new T; // ack!!!!!

 }

}

Every time the loop body is executed, a new instance of class T is instantiated, and

t is pointed to it. But what happens to the instance that t used to point to? It's

still allocated, but nothing points to it and therefore it's inaccessible. Memory in

this state is referred to as "leaked" memory.

In the Java language, memory leaks are not an issue. The following Java method

causes no ill effects:

// Java code

void f() {

 T t;

 for (int i = 1; i <= 1000; i++) {

 t = new T();

 }

}

In Java, each time t is assigned a new reference, the old reference is now available

for garbage collection. Note that it isn't immediately freed; it remains allocated

until the garbage collector thread is next executed and notices that it can be freed.

Put simply, automatic garbage collection reduces programming effort,

programming errors, and program complexity.