The Architecture
Java’s architecture arises out of four distinct but interrelated technolo-
gies, each of which is defined by a separate specification from Sun Mi-
crosystems:
the Java programming language
~ the Java class file format
the Java Application Programming Interface
the Java Virtual Machine
When you write and run a Java program, you are tapping the power of
these four technologies. You express the program in source files written
in the Java programming language, compile the source to Java class files,
and run the class files on a Java Virtual Machine. When you write your
program, you access system resources (such as I/O, for example) by call-
ing methods in the classes that implement the Java Application Pro-
gramming Interface, or Java API. As your program runs, it fulfills your
program’s Java API calls by invoking methods in class files that imple-
ment the Java API. You can see the relationship among these four parts
in Figure 1-1.
Together, the
Java Virtual Machine and Java API form a “platform” for
which all Java programs are compiled. In addition to being called the Java
runtime system, the combination of the Java Virtual Machine and Java
API is called the Java Platform.
Java programs can run on many differ-
ent kinds of computers because the Java Platform can itself be imple-
mented in software. As you can see in Figure 1-2, a
Java program can run
anywhere the Java Platform is present.