Lesson 2: First Java Programs

2.1 Why Java? Java is one of the most popular programming languages in the world. Java is a modern object-oriented programming language. Java has benefited by learning from the less desirable features of early object-oriented programming languages.

2.1 Why Java? Java is ideally suited to develop distributed, network-based applications because it: –Enables the construction of virus-free, tamper-free systems (security) –Supports the development of programs that do not overwrite memory –Has little or no failure rate (robust) –Yields programs that can be run on different types of computers without change (portable)

2.1 Why Java? Java supports advanced programming concepts such as threads. –A thread is a process that can run concurrently with other processes. Java resembles C++, the world’s most popular industrial strength programming language. Java however, runs more slowly than most modern programming languages.

2.2 The Java Virtual Machine and Byte Code Java compilers translate Java into pseudomachine language called java byte code. To run java byte code on a particular computer, a Java virtual machine (JVM) must be installed.

2.2 The Java Virtual Machine and Byte Code A Java virtual machine is a program that acts like a computer. It is called an interpreter. Disadvantage: –Runs more slowly than an actual computer To combat slower processing, some JVMs translate code when first encountered. This is known as just-in-time compilation (JIT).

2.2 The Java Virtual Machine and Byte Code Advantages: –Portability. Any computer can run Java byte code. –Applets. Applets are small Java programs already translated into byte code. Applets run in a JVM incorporated in a web browser Applets can be decorative (like animated characters on a web page.) Applets can be practical (like continuous streams of stock market quotes.) –Security. It is possible to limit the capabilities of a Java program since it runs inside a virtual machine.

2.3 Choosing a User Interface Style There are two types of user interfaces available to use to create Java programs. –Graphical User Interface (GUI) –Terminal I/O interface Figure 2-1 illustrates both interfaces used to create the same program.

2.3 Choosing a User Interface Style

There are 3 reasons for beginning with terminal I/O: –It is easier to implement than a GUI –There are programming situations that require terminal I/O –Terminal-oriented programs are similar in structure to programs that process files of sequentially organized data. (What is learned here is easily transferred to that setting.)

2.4 Hello World Figure 2-2 displays the results of a small Java program, entitled “hello world”

2.4 Hello World A program is a sequence of instructions for a computer. The following is the bulk of instructions, or source code, for the “hello world” program.

2.4 Hello World Sending messages to objects always takes the following form:. ( ) The period between the object name and the message is called a method selector operator.

2.4 Hello World The original “hello world” program needs to be embedded in a larger framework defined by several additional lines of code, in order to be a valid program.

Program Framework public class { public static void main(String [ ] args) {... place statements here... * ( Each statement must end with a ; ) } * Notice the open and closed braces ( { } ).