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Using the Java programming language compiler

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• 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 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

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• 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)

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  • The easy way:

    Write the algorithm (yourself) in a programming language (using an editor like gedit) Translate the program using a compiler into machine instructions (The translated program is stored in a computer file) 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)

    Pictorially:

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

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• Using the Java compiler
  • The name of the Java compiler (application) is:

    javac

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  • 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.

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  • 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

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  • Example:

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  • Example Program: (Demo above example)                                                
    • The Hello.java Prog file: click here

    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

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

    Java bytecode

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  • 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

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• 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)

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  • 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)

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  • The name of this computer application is:

    java

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• Executing the translated Java program with a Java virtual machine
  • Suppose the program containing the Java bytecode is called:

    ProgramFileName.class

    This program can be excuted by the Java virtual machine java by executing the following command:

    java ProgramFileName

    Note:

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

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  • Example:

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  • Example Program: (Demo above code)                                                
    • Hello.java Prog file: click here

    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:   javac Hello.java Then execute the Java bytecode Hello.class with:   java Hello