Loading classes

We already talked about the fact that classes are complex data types. Now let's talk a little about the other side of classes — how classes are handled by the Java machine. Remember that in Java everything is an object, even a class. A class is an object. Does that surprise you? Then let's keep going.

Loading a class into memory

Actually, when a class is loaded into memory, three special "objects" are created:

Brief description of the illustration:

Yellow rectangle:

The code file is stored on disk as a file with the ".class" extension. It contains information about the class, its fields and methods, as well as the source code of methods compiled into bytecode.

Orange rectangle:

When the Java machine loads a class into memory, it compiles the bytecode into machine code specific to the computer's processor and operating system. Only the Java machine has access to this machine code. As Java programmers, we do not have access to it.

Green rectangle:

The Java machine creates an object that contains all the static variables and methods of the class. You access to this "object" using the class name.

For example, when you write java.lang.Math.PI, you are referring to the static PI variable located in the java.lang.Math class. This java.lang.Math object is our green rectangle. And that's where the static PI variable is stored.

Blue rectangle:

When the Java machine loads the code of a class into memory, it creates a special java.lang.Class object, which stores information about the loaded class: its name, method names, field names and types, etc.

The name "Class" can be a little confusing. It would make more sense to call it ClassInfo, since this class just stores some information about the loaded class.

You can get the Class object for any type using a command like this:

Class name = ClassName.class;

Examples:

Class a = String.class;

Class b = Object.class;

Class c = Integer.class;

Class d = int.class;

Class e = void.class;

You can also get a reference to a class description object from any object, since each object has the getClass() method, which it inherits from the Object class.

Examples:

Class a = "Hello".getClass();

Class b = new Integer().getClass();

Class c = Boolean.TRUE.getClass();