Java Platform
One characteristic of Java is portability, which means that computer programs written in the Java language must run similarly on any supported hardware/operating-system platform. This is achieved by compiling the Java language code to an intermediate representation called
Java bytecode, instead of directly to platform-specific
machine code. Java bytecode instructions are analogous to machine code, but are intended to be
interpreted by a
virtual machine (VM) written specifically for the host hardware.
End-users commonly use a
Java Runtime Environment (JRE) installed on their own machine for standalone Java applications, or in a Web browser for Java
applets.
Standardized libraries provide a generic way to access host-specific features such as graphics,
threading, and
networking.
A major benefit of using bytecode is porting. However, the overhead of interpretation means that interpreted programs almost always run more slowly than programs compiled to native executables would. Just-in-Time compilers were introduced from an early stage that compile bytecodes to machine code during runtime. Over the years, this JVM built-in feature has been optimized to a point where the JVM's performance competes with natively compiled C code.
[citation needed]
Implementations
Sun Microsystems officially licenses the Java Standard Edition platform for
Linux,
[21] Mac OS X,
[22] and
Solaris. Although in the past Sun has licensed Java to Microsoft, the license has expired and has not been renewed.
[23] Through a network of third-party vendors and licensees,
[24] alternative Java environments are available for these and other platforms.
Sun's trademark license for usage of the Java brand insists that all implementations be "compatible". This resulted in a legal dispute with
Microsoft after Sun claimed that the Microsoft implementation did not support
RMI or
JNI and had added platform-specific features of their own. Sun sued in 1997, and in 2001 won a settlement of US$20 million, as well as a court order enforcing the terms of the license from Sun.
[25] As a result, Microsoft no longer ships Java with
Windows, and in recent versions of Windows,
Internet Explorer cannot support Java applets without a third-party plugin. Sun, and others, have made available free Java run-time systems for those and other versions of Windows.
Performance
Programs written in Java have a reputation for being slower and requiring more memory than those written in C.
[26] However, Java programs' execution speed improved significantly with the introduction of
Just-in-time compilation in 1997/1998 for
Java 1.1,
[27] the addition of language features supporting better code analysis (such as inner classes, StringBuffer class, optional assertions, etc.), and optimizations in the
Java Virtual Machine itself, such as
HotSpot becoming the default for Sun's JVM in 2000.
To boost even further the speed performances that can be achieved using the Java language, Systronix made
JStik,
[28] a
microcontrollerbased on the aJile Systems
[29] line of embedded
Java processors. In addition, the widely used ARM family of CPUs has hardware support for executing Java bytecode through its
Jazelle option.
Automatic memory management
Java uses an
automatic garbage collector to manage memory in the
object lifecycle. The programmer determines when objects are created, and the Java runtime is responsible for recovering the memory once objects are no longer in use. Once no references to an object remain, the
unreachable memory becomes eligible to be freed automatically by the garbage collector. Something similar to a memory leak may still occur if a programmer's code holds a reference to an object that is no longer needed, typically when objects that are no longer needed are stored in containers that are still in use. If methods for a nonexistent object are called, a "null pointer exception" is thrown.
[30][31]One of the ideas behind Java's automatic memory management model is that programmers can be spared the burden of having to perform manual memory management. In some languages, memory for the creation of objects is implicitly allocated on the
stack, or explicitly allocated and deallocated from the
heap. In the latter case the responsibility of managing memory resides with the programmer. If the program does not deallocate an object, a
memory leak occurs. If the program attempts to access or deallocate memory that has already been deallocated, the result is undefined and difficult to predict, and the program is likely to become unstable and/or crash. This can be partially remedied by the use of
smart pointers, but these add overhead and complexity. Note that garbage collection does not prevent "logical" memory leaks, i.e. those where the memory is still referenced but never used.
Garbage collection may happen at any time. Ideally, it will occur when a program is idle. It is guaranteed to be triggered if there is insufficient free memory on the heap to allocate a new object; this can cause a program to stall momentarily. Explicit memory management is not possible in Java.
Java does not support C/C++ style
pointer arithmetic, where object addresses and unsigned integers (usually long integers) can be used interchangeably. This allows the garbage collector to relocate referenced objects and ensures type safety and security.
As in
C++ and some other object-oriented languages, variables of Java's
primitive data types are not objects. Values of primitive types are either stored directly in fields (for objects) or on the
stack (for methods) rather than on the heap, as commonly true for objects (but see
Escape analysis). This was a conscious decision by Java's designers for performance reasons. Because of this, Java was not considered to be a pure object-oriented programming language. However, as of Java 5.0,
autoboxing enables programmers to proceed as if primitive types were instances of their wrapper class.
Syntax
Main article:
Java syntaxThe syntax of Java is largely derived from
C++. Unlike C++, which combines the syntax for structured, generic, and object-oriented programming, Java was built almost exclusively as an object-oriented language. All code is written inside a class, and everything is an object, with the exception of the intrinsic data types (ordinal and real numbers, boolean values, and characters), which are not classes for performance reasons.
Java uses similar commenting methods to C++. There are three different styles of comment: a single line style marked with two slashes (//), a multiple line style opened with a slash asterisk (/*) and closed with an asterisk slash (*/), and the
Javadoc commenting style opened with a slash and two asterisks (/**) and closed with an asterisk slash (*/). The Javadoc style of commenting allows the user to run the Javadoc executable to compile documentation for the program.
Example:
// This is an example of a single line comment using two slashes
/* This is an example of a multiple line comment using the slash and asterisk.
This type of comment can be used to hold a lot of information or deactivate
code but it is very important to remember to close the comment. */
/**
* This is an example of a Javadoc comment; Javadoc can compile documentation
* from this text.
*/
Examples
Hello world
// Outputs "Hello, world!" and then exits
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, world!");
}
}
Source files must be named after the public class they contain, appending the suffix
.java, for example,
HelloWorld.java. It must first be compiled into bytecode, using a
Java compiler, producing a file named
HelloWorld.class. Only then can it be executed, or 'launched'. The java source file may only contain one public class but can contain multiple classes with less than public access and any number of public
inner classes.
A
class that is not declared
public may be stored in any .java file. The compiler will generate a class file for each class defined in the source file. The name of the class file is the name of the class, with
.class appended. For class file generation,
anonymous classes are treated as if their name were the concatenation of the name of their enclosing class, a
$, and an integer.
The
keyword public denotes that a method can be called from code in other classes, or that a class may be used by classes outside the class hierarchy. The class hierarchy is related to the name of the directory in which the .java file is located.
The keyword
static in front of a method indicates a
static method, which is associated only with the class and not with any specific instance of that class. Only static methods can be invoked without a reference to an object. Static methods cannot access any method variables that are not static.
The keyword void indicates that the main method does not return any value to the caller. If a Java program is to exit with an error code, it must call System.exit() explicitly.
The method name "
main" is not a keyword in the Java language. It is simply the name of the method the Java launcher calls to pass control to the program. Java classes that run in managed environments such as applets and
Enterprise JavaBean do not use or need a
main()method. A java program may contain multiple classes that have
main methods, which means that the VM needs to be explicitly told which class to launch from.
The main method must accept an
array of
String objects. By convention, it is referenced as
args although any other legal identifier name can be used. Since Java 5, the main method can also use
variable arguments, in the form of
public static void main(String... args), allowing the main method to be invoked with an arbitrary number of
String arguments. The effect of this alternate declaration is semantically identical (the
args parameter is still an array of
String objects), but allows an alternative syntax for creating and passing the array.
The Java launcher launches Java by loading a given class (specified on the command line or as an attribute in a
JAR) and starting its
public static void main(String[]) method. Stand-alone programs must declare this method explicitly. The
String[] args parameter is an
array of
String objects containing any arguments passed to the class. The parameters to
main are often passed by means of a
command line.
Printing is part of a Java standard library: The
System class defines a public static field called
out. The
out object is an instance of the
PrintStream class and provides many methods for printing data to
standard out, including
println(String) which also appends a new line to the passed string.
The string "Hello, world!" is automatically converted to a String object by the compiler.
A more comprehensive example
// OddEven.java
import javax.swing.JOptionPane;
public class OddEven {
// "input" is the number that the user gives to the computer
private int input; // a whole number("int" means integer)
/*
* This is the constructor method. It gets called when an object of the OddEven type
* is being created.
*/
public OddEven() {
/*
* Code not shown for simplicity. In most Java programs constructors can initialize objects
* with default values, or create other objects that this object might use to perform its
* functions. In some Java programs, the constructor may simply be an empty function if nothing
* needs to be initialized prior to the functioning of the object. In this program's case, an
* empty constructor would suffice, even if it is empty. A constructor must exist, however if the
* user doesn't put one in then the compiler will create an empty one.
*/
}
// This is the main method. It gets called when this class is run through a Java interpreter.
public static void main(String[] args) {
/*
* This line of code creates a new instance of this class called "number" (also known as an
* Object) and initializes it by calling the constructor. The next line of code calls
* the "showDialog()" method, which brings up a prompt to ask you for a number
*/
OddEven number = new OddEven();
number.showDialog();
}
public void showDialog() {
/*
* "try" makes sure nothing goes wrong. If something does,
* the interpreter skips to "catch" to see what it should do.
*/
try {
/*
* The code below brings up a JOptionPane, which is a dialog box
* The String returned by the "showInputDialog()" method is converted into
* an integer, making the program treat it as a number instead of a word.
* After that, this method calls a second method, calculate() that will
* display either "Even" or "Odd."
*/
input = Integer.parseInt(JOptionPane.showInputDialog("Please Enter A Number"));
calculate();
} catch (NumberFormatException e) {
/*
* Getting in the catch block means that there was a problem with the format of
* the number. Probably some letters were typed in instead of a number.
*/
System.err.println("ERROR: Invalid input. Please type in a numerical value.");
}
}
/*
* When this gets called, it sends a message to the interpreter.
* The interpreter usually shows it on the command prompt (For Windows users)
* or the terminal (For Linux users).(Assuming it's open)
*/
private void calculate() {
if (input % 2 == 0) {
System.out.println("Even");
} else {
System.out.println("Odd");
}
}
}
- The import statement imports the
JOptionPane class from the javax.swing package.
- The
OddEven class declares a single private field of type int named input. Every instance of the OddEven class has its own copy of the input field. The private declaration means that no other class can access (read or write) the input field.
OddEven() is a public constructor. Constructors have the same name as the enclosing class they are declared in, and unlike a method, have no return type. A constructor is used to initialize an object that is a newly created instance of the class.- The
calculate() method is declared without the static keyword. This means that the method is invoked using a specific instance of the OddEven class. (The reference used to invoke the method is passed as an undeclared parameter of type OddEven named this.) The method tests the expression input % 2 == 0 using the if keyword to see if the remainder of dividing the input field belonging to the instance of the class by two is zero. If this expression is true, then it prints Even; if this expression is false it prints Odd. (The inputfield can be equivalently accessed as this.input, which explicitly uses the undeclared this parameter.)
OddEven number = new OddEven(); declares a local object reference variable in the main method named number. This variable can hold a reference to an object of type OddEven. The declaration initializes number by first creating an instance of the OddEvenclass, using the new keyword and the OddEven() constructor, and then assigning this instance to the variable.- The statement
number.showDialog(); calls the calculate method. The instance of OddEven object referenced by the number local variable is used to invoke the method and passed as the undeclared this parameter to the calculate method.
input = Integer.parseInt(JOptionPane.showInputDialog("Please Enter A Number")); is a statement that converts the type of String to the primitive data type int by using a utility function in the primitive wrapper class Integer.
Special classes
Applet
Main article:
Java appletJava applets are programs that are embedded in other applications, typically in a Web page displayed in a
Web browser.
// Hello.java
import javax.swing.JApplet;
import java.awt.Graphics;
public class Hello extends JApplet {
public void paintComponent(Graphics g) {
g.drawString("Hello, world!", 65, 95);
}
}
The
import statements direct the
Java compiler to include the
javax.swing.JApplet and
java.awt.Graphics classes in the compilation. The import statement allows these classes to be referenced in the
source code using the
simple class name (i.e.
JApplet) instead of the
fully qualified class name (i.e.
javax.swing.JApplet).
The
Hello class
extends (
subclasses) the
JApplet (Java Applet) class; the
JApplet class provides the framework for the host application to display and control the
lifecycle of the applet. The
JApplet class is a JComponent (Java Graphical Component) which provides the applet with the capability to display a
graphical user interface (GUI) and respond to user
events.
The
Hello class
overrides the
paintComponent(Graphics) method inherited from the
Container superclass to provide the code to display the applet. The
paintComponent() method is passed a
Graphics object that contains the graphic context used to display the applet. The
paintComponent() method calls the graphic context
drawString(String, int, int) method to display the
"Hello, world!" string at a
pixel offset of (
65, 95) from the upper-left corner in the applet's display.
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Hello World Applet</title>
</head>
<body>
<applet code="Hello" width="200" height="200">
</applet>
</body>
</html>
An applet is placed in an
HTML document using the
. The
applet tag has three attributes set:
code="Hello"specifies the name of the
JApplet class and
width="200" height="200" sets the pixel width and height of the applet. Applets may also be embedded in HTML using either the
object or
embed element,
[32] although support for these elements by Web browsers is inconsistent.
[33] However, the
applet tag is deprecated, so the
object tag is preferred where supported.
The host application, typically a Web browser, instantiates the
Hello applet and creates an
AppletContext for the applet. Once the applet has initialized itself, it is added to the AWT display hierarchy. The
paintComponent() method is called by the AWT
event dispatching thread whenever the display needs the applet to draw itself.
Servlet
Main article:
Java ServletJava Servlet technology provides Web developers with a simple, consistent mechanism for extending the functionality of a Web server and for accessing existing business systems. Servlets are
server-side Java EE components that generate responses (typically
HTML pages) to requests (typically
HTTP requests) from
clients. A servlet can almost be thought of as an applet that runs on the server side—without a face.
// Hello.java
import java.io.*;
import javax.servlet.*;
public class Hello extends GenericServlet {
public void service(ServletRequest request, ServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
final PrintWriter pw = response.getWriter();
pw.println("Hello, world!");
pw.close();
}
}
The
import statements direct the Java compiler to include all of the public classes and
interfaces from the
java.io and
javax.servlet packages in the compilation.
The
Hello class
extends the
GenericServlet class; the
GenericServlet class provides the interface for the
server to forward requests to the servlet and control the servlet's lifecycle.
The
setContentType(String) method in the response object is called to set the
MIME content type of the returned data to
"text/html". The
getWriter() method in the response returns a
PrintWriter object that is used to write the data that is sent to the client. The
println(String) method is called to write the
"Hello, world!" string to the response and then the
close() method is called to close the print writer, which causes the data that has been written to the stream to be returned to the client.
JavaServer Pages
JavaServer Pages (JSP) are
server-side Java EE components that generate responses, typically
HTML pages, to
HTTP requests from
clients. JSPs embed Java code in an HTML page by using the special
delimiters . A JSP is compiled to a Java
servlet, a Java application in its own right, the first time it is accessed. After that, the generated servlet creates the response.
Swing application
Main article:
Swing (Java)Swing is a graphical user interface
library for the Java SE platform. It is possible to specify a different look and feel through the
pluggable look and feel system of Swing. Clones of
Windows,
GTK+ and
Motif are supplied by Sun.
Apple also provides an
Aqua look and feel for
Mac OS X. Where prior implementations of these looks and feels may have been considered lacking, Swing in Java SE 6 addresses this problem by using more native
GUI widget drawing routines of the underlying platforms.
This example Swing application creates a single window with "Hello, world!" inside:
// Hello.java (Java SE 5)
import javax.swing.*;
public class Hello extends JFrame {
public Hello() {
super("hello");
setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
add(new JLabel("Hello, world!"));
pack();
}
public static void main(String[] args) {
new Hello().setVisible(true);
}
}
The first
import includes all of the public classes and interfaces from the
javax.swing package.
The
Hello() constructor initializes the frame by first calling the superclass constructor, passing the parameter
"hello", which is used as the window's title. It then calls the
setDefaultCloseOperation(int) method inherited from
JFrame to set the default operation when the close control on the title bar is selected to
WindowConstants.EXIT_ON_CLOSE — this causes the
JFrame to be disposed of when the frame is closed (as opposed to merely hidden), which allows the JVM to exit and the program to terminate. Next, a
JLabel is created for the string
"Hello, world!" and the
add(Component) method inherited from the
Container superclass is called to add the label to the frame. The
pack() method inherited from the
Window superclass is called to size the window and lay out its contents.
The
main() method is called by the JVM when the program starts. It
instantiates a new
Hello frame and causes it to be displayed by calling the
setVisible(boolean) method inherited from the
Component superclass with the boolean parameter
true. Once the frame is displayed, exiting the
main method does not cause the program to terminate because the AWT
event dispatching thread remains active until all of the Swing top-level windows have been disposed.
Generics
In 2004
generics were added to the Java language, as part of J2SE 5.0. Prior to the introduction of generics, each variable declaration had to be of a specific type. For container classes, for example, this is a problem because there is no easy way to create a container that accepts only specific types of objects. Either the container operates on all subtypes of a class or interface, usually
Object, or a different container class has to be created for each contained class. Generics allow compile-time type checking without having to create a large number of container classes, each containing almost identical code.
Critical analysis
Class libraries
Java Platform and Class libraries diagram
- Java libraries are the compiled bytecodes of source code developed by the JRE implementor to support application development in Java. Examples of these libraries are:
- The core libraries, which include:
- The integration libraries, which allow the application writer to communicate with external systems. These libraries include:
- User interface libraries, which include:
- The (heavyweight, or native) Abstract Window Toolkit (AWT), which provides GUI components, the means for laying out those components and the means for handling events from those components
- The (lightweight) Swing libraries, which are built on AWT but provide (non-native) implementations of the AWT widgetry
- APIs for audio capture, processing, and playback
- A platform dependent implementation of Java Virtual Machine (JVM) that is the means by which the byte codes of the Java libraries and third party applications are executed
- Plugins, which enable applets to be run in Web browsers
- Java Web Start, which allows Java applications to be efficiently distributed to end-users across the Internet
- Licensing and documentation.
Documentation
Javadoc is a comprehensive documentation system, created by
Sun Microsystems, used by many Java developers. It provides developers with an organized system for documenting their code. Whereas normal comments in Java and
C are set off with /* and */, the multi-line comment tags, Javadoc comments have an extra asterisk at the beginning, so that the tags are /** and */.
