Lessons

Well, we've completed our second "flyby" of the key principles of OOP. We studied polymorphism and encapsulation in greater detail. We also learned about a new concept: abstract classes. That said, these topics aren't as simple as the other topics you are already familiar with. So feel free to use the following articles to bolster...

Static variables. When a class is loaded into memory a static object is created immediately. This object stores static class variables (static class fields). The static object exists even if no ordinary (non-static) objects of the class have been created. When we declare...

Static methods. In addition to static variables, classes can also have static methods. Regular methods are bound to objects (instances) of a class and can refer to ordinary (non-static) variables of the class (as well as static variables and methods). Static methods are bound to the class's static object...

Inner classes. You recently learned that there are static variables and static methods. It turns out that there are also static classes.  But we'll approach this topic from a distance. In Java, you are entirely allowed to declare classes within classes. And even classes within classes that are within classes...

Order of initialization when using constructors and static blocks

In this level, we have taken a few more steps forward in learning Java. We explored class loading and talked about static variables, methods, and classes. We figured out the most common reasons why the compiler yells at you, as well as the situations where you need to be especially careful to avoid...

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...

I think you won't be too surprised if I tell you that your computer has a limited amount of memory :) Even a hard drive — generally many times larger than RAM storage — can be packed to capacity with your favorite games, TV shows, and more...

List of wrapper types. You know that Java has 8 primitive types, which are types that are not classes. On the one hand, this is good — they are simple and take up little space. On the other hand, sometimes classes are just what we need. You'll find out exactly why that is in the next lesson. So what can be done?

Integer class. Integer is also good in that it is a class, which means that it can have fields and methods. And, of course, it has them. A lot of them — dozens of them. Here we'll consider the most basic ones. The Integer class has two fields that contain the maximum and minimum possible values of the int...

ArrayList class. Today we will explore the ArrayList class. This is the first of several classes known as collections. In Java, collections are such a broad and useful topic that an entire CodeGym quest is devoted to them. To fully understand how collections are structured and all their nuances, you need to first learn...

How ArrayList is structured. ArrayList is the Java class used most commonly for storing elements. So how does ArrayList work and why does everyone like it so much? The structure of ArrayList is simple and ingenious. Each ArrayList object contains two fields: An array of elements...