Download Using the Java programming language compiler

Using the Java programming
language compiler
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
Review of relevant material from previous
lectures (cont.)
• People have invented a number of English-like languages that
is highly suitable for writing algorithms
• These languages are called (high level)
programming languages
• People have also written computer applications that translate
algorithm written in a programming language into machine
instructions
• These applications are called compilers
How to write an algorithm that a computer
can execute
• The hard way:
• Write the algorithm (yourself) in machine instruction
(binary numbers)
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)
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)
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))
Using the Java compiler
• The name of the Java compiler (application) is:
• javac
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:
•.java
The ending of a filename is called the file extension and it
is commonly used to identify the kind of file.
Using the Java compiler (cont.)
• The following command will translate a Java program into
machine code:
• UNIX prompt>> javac
ProgramFileName.java
The Java compiler javac will store the translated machine
codes into a file name ProgramFileName.class
Using the Java compiler (cont.)
• Example:
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
The Java machine language
• The translated machine code produced by the Java
compiler javac is called:
• Java bytecode
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
The Java virtual machine
• Emulators:
• Emulators are (special) computer programs that
emulate the exact behavior of a machine or entity
Emulators are also known as virtual machines
(Because they do the same thing as a real machine, but is
not a physical machine)
The Java virtual machine (cont.)
• Java virtual machine:
• A Java virtual machine is a computer program
(application) than can execute an algorithm
written in Java bytecode
(A Java virtual machine is an emulator)
The Java virtual machine (cont.)
• The name of this computer application is:
• java
Executing the translated Java program
with a Java virtual machine
• Suppose the program containing the Java bytecode is
called:
ProgramFileName.clas
s
This program can be executed by the Java virtual machine
java by executing the following command:
java
ProgramFileName
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
Executing the translated Java program
with a Java virtual machine (cont.)
• Example:
Executing the translated Java program
with a Java virtual machine (cont.)
• Example Program: (Demo above code)
– Hello.java Prog file:
http://mathcs.emory.edu/~cheung/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