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 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

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)

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: The ending of a filename is called the file extension and it is commonly used to identify the kind of file..java

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 prompt>> javac ProgramFileName.java

Using the Java compiler (cont.) Example:

Using the Java compiler (cont.) Example Program: (Demo above example) –The Hello.java Prog file: 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 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

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

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: This program can be executed by the Java virtual machine java by executing the following command: ProgramFileName.clas s 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: 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