Java Android App Development Course, Types of Java Classes

In Android app development, Java is still one of the important languages. In this article, we will discuss the types of Java classes and how they are utilized in Android apps. Java is an object-oriented programming (OOP) language, and classes and objects are the basic concepts. A class is a blueprint for creating objects, and an object is an instance of a class.

1. Basic Concept of Java Class

A class in Java is a data structure that can contain variables and methods. The main components of a class are as follows:

• Attribute: Represents the state of the class. Defined through variables.
• Method: Defines the behavior that the class can perform.

Here is a basic example of defining a Java class:

public class Dog {
    // Attributes
    String name;
    int age;

    // Constructor
    public Dog(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // Method
    public void bark() {
        System.out.println(name + " is barking.");
    }
}

2. Types of Java Classes

In Java, various types of classes can be defined. The main types of classes are as follows:

2.1. Object Class

All classes in Java inherit from the Object class. Therefore, the Object class is the top-level parent class in Java. It contains basic methods that can be used in all classes (e.g., toString(), equals(), hashCode()).

2.2. User-defined Class

A user-defined class is a class defined by a developer to meet their needs. The Dog class mentioned above is an example.

2.3. Abstract Class

An abstract class is a class that contains one or more abstract methods. An abstract method is a method without an implementation, which must be implemented by the subclass that inherits from that class. An abstract class cannot be instantiated.

abstract class Animal {
    abstract void sound();

    void eat() {
        System.out.println("Animal is eating");
    }
}

class Cat extends Animal {
    void sound() {
        System.out.println("Meow");
    }
}

2.4. Interfaces

An interface contains only the definition of methods and requires classes that implement those methods. Interfaces provide multiple inheritance. That is, a class can implement multiple interfaces.

interface Flyable {
    void fly();
}

class Bird implements Flyable {
    public void fly() {
        System.out.println("The bird is flying.");
    }
}

2.5. Inner Class

An inner class is a class defined as a member of another class. An inner class has the ability to access members of the outer class.

class Outer {
    class Inner {
        void display() {
            System.out.println("This is an inner class.");
        }
    }
}

2.6. Static Class

A static class is a class defined as a static member of the outer class. Instances of the static class can be created without an instance of the outer class.

class Outer {
    static class StaticInner {
        void display() {
            System.out.println("This is a static inner class.");
        }
    }
}

3. Utilization of Classes in Android Apps

Classes are very important in Android app development. Classes are used to implement various UI elements and functionalities. For example, Activity and Fragment can be seen as classes that define the UI state, which are major components of Android.

3.1. Activity Class

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

The code above is an example of defining a basic Activity class. The onCreate() method is called when the Activity is created.

3.2. Fragment Class

public class SampleFragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_sample, container, false);
    }
}

Fragment is a class that allows for the reuse of UI. The onCreateView() method creates the UI for the Fragment.

3.3. View Class

All UI components (buttons, text views, etc.) are implemented by inheriting the View class. Each UI element is based on this class and provides various attributes and methods.

public class CustomButton extends Button {
    public CustomButton(Context context) {
        super(context);
    }

    public void changeColor() {
        setBackgroundColor(Color.RED);
    }
}

4. Conclusion

By understanding the types of Java classes, one can effectively utilize the concepts of object-oriented programming in Android app development. Utilizing various classes and the concept of objects allows for writing more efficient and reusable code. Additionally, the types of Java classes can guide the structure of code in a better direction by leveraging OOP characteristics (inheritance, polymorphism, etc.).

In this article, we explored various classes in Java and how to utilize them in Android app development. If you understand and utilize Java’s class concepts well, it will greatly help in developing more effective Android apps.