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Classes

Functions and variables are always declared inside a class. Even the application entry function that is called upon a program launch, called the main() function, is a function that is located inside a class.

JVM only supports the single-inheritance model. Classes always inherit from one class at the maximum. This is not a big loss. As we will see in the next chapter, a structure called an interface comes to the rescue. An interface is basically a list of function prototypes (only the definition of functions, without code) and constants. Classes that implement an interface are required by the compiler to have implementations for those functions. Classes can implement as many interfaces as they want, but they must provide implementations for each method of all the implemented interfaces.

Some languages covered in this book hide these facts completely from the developer. For example, unlike Java, some languages allow functions and variables to be written outside class declarations or even executable code outside function definitions. Other languages support inheritance of multiple classes. Internally, these languages do clever tricks to work around JVM limitations and design decisions.

JVM classes are usually grouped in packages. In the next chapter, we will see how classes are organized.