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COMP 1010 Home > Java In-depth

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How a traditional program is born.

Compiling

Writing lines of code does nothing by itself. We have to compile code before we can run it.

When you compile a program, a program called the compiler translates text written in one programming language into another programming language. Java is a programming language that is (relatively) comprehensible by humans; it takes a while to learn, but once you learn the fundamentals of Java, you can read another Java program and understand what it is doing. A computer, on the other hand, understands a different language that is not comprehensible by humans; each instruction is encoded as a sequence of 0's and 1's (e.g., 4+5 might be 1010 1000 1011 0100). The language that a computer understands is called machine code, or machine language. Machine code is explained in greater detail in Chapter 2's further reading.

In general, a compiler takes lines of code and outputs the equivalent machine code that a computer can run.

How a Java program is born.

The Java Compiler

One of the reasons Java is different than other languages is that it is designed to be platform independent.

We use many different types of computers every day: Windows PCs, Macs, cell phones, game consoles, etc. Each of these computers is different, and understands a different machine language. With most programming languages, you will need to compile your program differently for each type of computer. You might even need to change the code itself to run correctly on a certain type of computer!

Java is different. It is designed to be run on any type of computer. To do this, the Java compiler does not convert lines of code to machine code, it converts lines of code to another language called Java bytecode. You can think of Java bytecode as a happy medium between Java code and machine code. Instead of creating a file that can be run by a computer, the computer runs a Java virtual machine that understands Java bytecode. This moves the responsibility of creating programs that work on multiple platforms from a programmer to the creators of Java.

Another way of think of what the Java compiler does is by looking at file types. You are probably aware of most common file types, like .doc for Microsoft Word documents, .mp3 for MP3 encoded music files, and so on. The table below details the file types that are involved when programming in a traditional programming language and the Java programming language. In the example, we are using C++ as a representative traditional programming language, and we are using Windows file extensions.

Notice that when we run our Java program, we are actually running java.exe, the Java virtual machine.