Using the Java programming language compiler

1
Using the Java programming language compiler
2
Review of relevant material from previous
lectures

• From previous lectures

• A computer can only execute machine instruction
  codes (binary numbers)
• Writing algorithms in machine instruction codes
  is extremely tedious

3
Review of relevant material from previous
lectures (cont.)

• People have invented a number of English-like
  languages that is highly suitable for writing
  algorithms
• People have also written computer applications
  that translate algorithm written in a programming
  language into machine instructions

• These languages are called (high level)
  programming languages

• These applications are called compilers

4
How to write an algorithm that a computer can
execute

• The hard way
• You will hate your professor if he/she make you
  do this.... (and yet, the pioneers in Computer
  Science had done this... we must be grateful to
  them)

• Write the algorithm (yourself) in machine
  instruction (binary numbers)

5
How to write an algorithm that a computer can
execute (cont.)

• The easy way

1. Write the algorithm (yourself) in a programming
   language (using an editor like gedit)
2. Translate the program using a compiler into
   machine instructions (The translated program is
   stored in a computer file)
3. Let the computer execute the translated program
   in machine instructions (This translated program
   is an computer application because it can be run
   by a computer)

6
How to write an algorithm that a computer can
execute (cont.)

• Pictorially
• (An application is a file that contains machine
  instructions (which can be executed by a
  computer))

7
Using the Java compiler

• The name of the Java compiler (application) is

• javac

8
Using the Java compiler (cont.)

• A Java program ( a computer algorithm written in
  the Java programming language) is first written
  and stored in a file
• The name of the file must end with the
  characters
• The ending of a filename is called the file
  extension and it is commonly used to identify the
  kind of file.

• .java

9
Using the Java compiler (cont.)

• The following command will translate a Java
  program into machine code
• The Java compiler javac will store the
  translated machine codes into a file name
  ProgramFileName.class

• UNIX promptgtgt javac ProgramFileName.java

10
Using the Java compiler (cont.)

• Example

11
Using the Java compiler (cont.)

• Example Program (Demo above example)  
• The Hello.java Prog file http//mathcs.emory.edu/
  cheung/Courses/170/Syllabus/02/Progs/Hello.java
     
• What to do

• Right click of the link and save in some scratch
  directory
• Change current directory to that scratch
  directory
• Compile Hello.java with javac Hello.java

12
The Java machine language

• The translated machine code produced by the Java
  compiler javac is called

• Java bytecode

13
The Java machine language

• Java bytecodes are executed by a Java machine ---
  a computer that executes machine instructions in
  Java bytecodes
• However, no such Java machine has ever been
  built
• In other words

• There does not exist a physical computer
  (machine) that can execute Java byte codes

14
The Java virtual machine

• Emulators
• Emulators are also known as virtual machines
• (Because they do the same thing as a real
  machine, but is not a physical machine)

• Emulators are (special) computer programs that
  emulate the exact behavior of a machine or entity

15
The Java virtual machine (cont.)

• Java virtual machine
• (A Java virtual machine is an emulator)

• A Java virtual machine is a computer program
  (application) than can execute an algorithm
  written in Java bytecode

16
The Java virtual machine (cont.)

• The name of this computer application is

• java

17
Executing the translated Java program with a Java
virtual machine

• Suppose the program containing the Java bytecode
  is called
• This program can be executed by the Java virtual
  machine java by executing the following command

ProgramFileName.class
java ProgramFileName
18
Executing the translated Java program with a Java
virtual machine (cont.)

• Note

• You must omit the extension .class
• The Java virtual machine java will append the
  extension .class to its argument

19
Executing the translated Java program with a Java
virtual machine (cont.)

• Example

20
Executing the translated Java program with a Java
virtual machine (cont.)

• Example Program (Demo above code)  
• Hello.java Prog file http//mathcs.emory.edu/che
  ung/Courses/170/Syllabus/02/Progs/Hello.java
• What to do          

• Right click on the link ans save in some scratch
  directory
• Change current directory to that scratch
  directory
• Compile Hello.java with nsbp javac Hello.java
  
• Then execute the Java bytecode Hello.class with
    java Hello