Pratik Bahl
Running Java Programs
Introduction to Java
Simple Java Program
● As a text file, you can print it, display it on the monitor, copy it to another text file, or alter it with
a text editor.
● A file name is case sensitive
● A source program is a text file that contains a program (such as above) written in a programming
language. Since it contains ordinary text (stored as bytes) it cannot be directly executed (run) by
the computer system.
Bytecodes
● To run a Java program the source file is first translated into a file of bytecodes.
● A Java bytecode is a machine instruction for a Java processor. A file of bytecodes is a machine
language program for a Java processor.
● a Java processor is a silicon processor chip that directly executes a machine language called Java
bytecodes. This is like an Intel processor that directly executes Intel machine language.
● An entirely new file Hello.class containing bytecodes is created by the compiler. (If there already
is a file named Hello.class, it will be replaced by a new one. Usually, this is what you want.)
● In this picture, the source program Hello.java is examined by a program called javac running on
your computer. The javac program is a compiler (a translator) that translates the source program
into a bytecode file called Hello.class.
○ Important Idea: The bytecode file will contain exactly the same bytecodes no matter what
computer the javac compiler runs on.
● The architecture of the processor that executes Java bytecodes is well-documented and is available
to anyone.
Java Virtual Machine
● Since people normally use PCs and Macintoshes, they do not have hardware java processor chips.
However, the Java processor can be implemented as a program that reads the bytecodes and
performs the operations they specify.
● Some interpreters run source code written in a high level language like Basic
● Some interpreters run bytecodes (like the Java interpreter)
● One of the interpreters is called an emulator because it emulates hardware, but it's software. Eg//
A Java bytecode interpreter can be created for any computer system.
● Once you have a Java compiler and a Java interpreter you can run any Java program no matter
what type of computer you have.
● The actual processor of the computer does not run java code. The java interpreter program, which
is part of the computer software, runs the code. The actual processor is the intel or mac processor
chip.
● Important Idea: When the Java interpreter is running on a computer system, that system acts just
like a hardware Java bytecode processor. It is a Java Virtual Machine. (thats why the java
interpreter is like an emulator).
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● After a java interpreter is specifically written for a particular computer system, that computer
system can now become a Java virtual machine. It is almost like the computer has a hardware Java
processor chip.
● When a Java program is translated into bytecodes, the bytecodes are the same no matter what
computer system is used.
● A Java source program (such as Hello.java) can be written and compiled on one computer (say a
Windows computer) to produce bytecode (say Hello.class). Now that bytecode can run on any
computer that has a Java interpreter.
Portability
● Apple developed a new Java interpreter for their new system. That interpreter can run any java
bytecode program.
● Java programs are portable, meaning that the same bytecode program can run on any computer
system that has a java interpreter.
● A source program can be compiled into bytecodes on any computer that has a Java compiler.No
matter what computer you have, you can write the same Java programs.
● Many other languages, such as JavaScript, Python and PHP are portable like Java. Your web
browser contains a JavaScript interpreter and can run JavaScript source code embedded in a web
page, regardless of where the page is hosted. JavaScript is not Java. Its name is intended to draw
attention to its Java-like portability.
Running on Windows
● In order to access a file that is in a folder on the c drive, you need to find the folder on file
explorer and right click to open the address on command prompt.
● Another way to do that would be through the DIR command. The DIR command lists the files in
the DIRectory.
● Every character of a text file encoded as one byte or end-of-line characters.
Compile the File
● The command Javac Hello.java runs the Java compiler on the source file. Make sure that the
upper and lower case characters match.
● For MAC dir is actually ls and to find a certain folder you have to use cd.
Running the Program
● The command java Hello starts up the Java Virtual Machine with the bytecode file Hello.class.
● Aggravations: don't use the extension class with the file name. Also, the command is java without
a "c" at the end.
● You don't have to use javac on mac because it is one OS.
○ Pratik@Pratiks-MacBook-Pro ~ % java /Users/Pratik/Documents/ICS4U/Hello.java
Contents of the Bytecode File
● The command TYPE is used in command prompt to type a text file to the monitor
● The TYPE command accepts bytes that encode characters. It does this by sending those bytes to
the graphics board which puts each character on the monitor.
● The TYPE command does not work with a bytecode file.
● Usually a file that contains bytes that do not correspond to characters is called a binary file. It is
named this even though all files are composed of bytes, and all bytes contain binary patterns.
Pratik Bahl
Questions:
1. Can the processor of a computer system directly execute source programs written in Java?
a. No
2. What are the two ways that a source program (written in any programming language) could be run
on a computer system?
a. Translation (into machine instructions, which are then directly executed by the processor)
b. Interpretation (by an interpreter program running on the processor.
3. Could a processor chip be built that executes Java bytecodes directly, just as an Intel processor
chip executes its machine language directly?
a. Yes. Hardware Java processor chips have been created out of silicon and can execute
bytecode files directly.
4. Say that Apple has just come out with a new computer and wants this computer to run Java
programs. What must Apple do?
a. Apple must write a Java interpreter for their new system.
5. Can bytecodes be sent from computer to computer over the Internet?
a. Yes. One of the advantages of portability is that Java bytcodes can be copied unaltered to
many types of computers and those computers can run the program.
6. Will you see exactly the same things as above on your computer system?
a. No. Probably not.
b. On recent Windows, the differences should be small.
7. What is the mistake
a. The source file contained a syntax error (in this case a missing ") and could not be
compiled.
b. Notice that the compiler says there are "3 errors" even though there is really only one. This
is common. Sometimes when there is a syntax error the compiler gets confused and prints
messages that are misleading.
c. Find the first error, use the text editor to correct the program, save it (to the same filename)
and compile again.
8. Could Hello.class be sent to a printer?
a. No.
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b. The bytes in the file Hello.class are machine language for the Java Virtual Machine. These
bytes of machine language do not usually correspond to printable characters (and when
they do, it is only by accident.)
9. What happens if you try to run the Java Virtual Computer on the source code?