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Exam Objectives Covered

• Describe the features of Java


• Describe the real-world applications of Java


• Describe the Java Development Kit (JDK) and the Java Runtime Environment (JRE)

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Java Is a Language

• Java is a modern, strongly-typed, OO programming language
• Invented by Sun Microsystems in 1995
• And it's come a long way since then
• Acquired by Oracle


• Built-in support for:
• Networking and database access
• GUI and internationalization
• Multithreading, security, error handling, and much more


• Syntax based on C/C++


• Very similar to Microsoft .NET C#

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Java Is a Platform

• Java Virtual Machine (JVM) executes the code


• Java Runtime Environment (JRE) includes the JVM and other runtime facilities outside the JVM


• Java Core API
• Thousands of built-in classes to help you
• The "built-in support" on the previous page is mostly done via these
• Getting to know them and how to use them is one of your challenges
• But once you know OO and how to read an API, you're there

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Java Is Actually Multiple Platforms

• Java Standard Edition (Java SE)
• "Foundation" platform - runtime and Core API are in here
• Generally for client-based applications
• Contains some APIs for server-side enterprise systems, too, e.g., JDBC for database access


• Java Enterprise Edition (Java EE)
• Geared toward server-side and multitier enterprise systems
• Adds APIs, e.g., servlet/JSP, EJB, transactions, etc.
• Depends on Java SE


• Java Micro Edition (Java ME)
• For smaller devices - PDAs, cell phones, etc.
• Subset of Java SE (smaller footprint)

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Features of Java - Condensed

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Java Is Standardized and Portable

• Standardized - Sun (with input from others) specifies the language and its Core API
• Sun was once the owner, but Java is now open source and specified via the Java Community Process (JCP)


• Portable - runs on any platform (without recompiling)
• Sun's motto: "Write Once Run Anywhere"


• But there's lots of different computing platforms out there...
• Unix (lots of different flavors), Windows, MacOS, MVS, VMS, etc.
• So how does Java accomplish this?


• Code runs on the JVM, not directly on the operating system
• The JVM, though, does run on the target operating system

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Java Provides Some Safety for Developers

• Java was designed to protect developers from common pitfalls
• Things in other languages deliberately omitted
• No multiple inheritance (can have only one superclass)
• No manipulation of object references to memory locations
• No direct access to memory addresses


• Java was designed to help you write reliable software
• Strongly typed (enforced by compiler)
• Case sensitive
• Compiler-checked exceptions
• Automatic array bounds checking
• Automatic memory management (garbage collection)
• Portable

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Java Development Cycle

1. Write source file with any text editor
• FirstExample.java
• Your write the class definition (class) in here
• NOTE: classes are named using CamelCase


2. Compile it with javac
• javac FirstExample.java
• This will give error(s) or FirstExample.class


3. Run it with java
• java FirstExample
• This launches the JVM and executes FirstExample.class

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Java Development Kit (JDK)

• JRE + command line tools for compiling and other tasks
• Free
• Often obtained from operating system vendor
• Remember, the VM itself is platform-specific


• There is also a JRE-only package
• No compiler or other tools, just the runtime
• If you have compiled Java code, all you need is a JRE to run it
• Vendors that provide Java-based applications usually bundle the JRE with their installer
• Why would an application vendor choose Java for its language?

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JDK Illustrated

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Java Source Files

• Filename must have .java extension
• HelloWorld.java


• Contains 1 or more class definitions
• You can put more than one class in a source file, but don't
• Unless you enjoy getting yourself and others confused


• Max of one public class (details later)
• Should only be one class per file anyway


• Classname should match filename, including case
• If public class, classname must match filename

class HelloWorld {

}

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main Method

• Entry point for every Java application


• Signature is important (args variable name is not critical)


• Memorize it


• The void means main doesn't return any value
• To whom? Who calls main, anyway?


• We'll talk later about public and static

class HelloWorld {

  public static void main(String[] args) {
    // code goes here
  }

}

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Diagnosing Problems

• Common problems:
• javac YourClass.java
• path
• java YourClass
• path or classpath (more later)


• Multiple JDKs installed on your machine and java -version doesn't give desired one
• Check PATH
• Delete any java.exe and javaw.exe files located other than in JDK installation directory, especially on Windows (see notes)

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Classpath

• Review: JDK tools (java, javac, etc.) are located by looking on your operating system path
• Which needs to contain <java>\bin


• At runtime, JVM looks for classes on the classpath


• Much like a system path, classpath is a list of directories or JAR files (more on these later)
• If you get errors at runtime that look like "cannot find class" or "class not found," this is a classpath issue


• Can be specified by CLASSPATH environment variable


• Can be provided to JDK tools with -classpath flag
• java -classpath C:\Student\Java FirstExample

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Classpath Rules

• If you have no classpath, your classpath is implicitly "dot" (.) (current directory)


• If you do have a classpath, you give up the free "dot" (.)
• So if you want current directory on classpath, put "dot" (.) back in


• If you have a CLASSPATH environment variable and use the -classpath flag to the JDK tools, -classpath overrides