Wrapping of data and functions that operate on that data into a single unit is called Encapsulation. A medical capsule is a real world example of Encapsulation as it keeps the drug safe inside it. More on knowledgeboat.com

You'd have a spaghetti mess of code in any non-trivial project which would be difficult to maintain. More on reddit.com

Properties can be defined on interfaces and implemented in classes. More on reddit.com

Encapsulation is when you make some parts of your program unable to access other parts of your program. The purpose of encapsulation is to help you (or others) safely make changes to one part of your program without damaging other parts. Programs that use encapsulation are also easier to understand. In Java, one of the main ways to use encapsulation is to create classes with private member variables. Suppose you want to make a class that represents a circle. Let's pretend that for some reason it's really important to you that the area of the circle does not need to be computed using pi*r2 each time you want to find it. You might write the Circle class like this: public class Circle { public double radius; public double area; } And then when you want to create a circle you just call new Circle() and then set circle.radius to r and circle.area to pi*r2. One problem with this is that you are forcing anyone who uses the Circle class to understand the formula for finding the area of a circle. It would be better if only Circle needed to know how to find the area. So you might change your code to this: public class Circle { public double radius; public double area; public Circle(double radius) { this.radius = radius; this.area = computeArea(radius); } public double computeArea(double radius) { return Math.PI * radius * radius; } } Now you can create a circle without worrying how to find its area. But there is still a problem: what if you want to change the radius of the circle? When you change the radius, you also need to change the area. This is something that anyone who uses the Circle class needs to remember. It would be better if they didn't have to remember this. Here is how to fix this problem: public class Circle { private double radius; private double area; public Circle(double radius) { setRadius(radius); } public double getRadius() { return this.radius; } public void setRadius(double radius) { this.radius = radius; this.area = Math.PI * radius * radius; } public double getArea() { return this.area; } } Now whenever someone wants to change the radius of the circle they call setRadius, and the area is automatically updated to match the new radius. People using Circle don't need to understand how to find the area of a circle and don't need to worry about messing anything up if they set the radius. Finally, suppose you decide that you don't want to store the area of the circle inside Circle anymore, and you just want to compute it when someone asks for the area. Then you can change the class to this: public class Circle { private double radius; public Circle(double radius) { this.radius = radius; } public double getRadius() { return this.radius; } public void setRadius(double radius) { this.radius = radius; } public double getArea() { return Math.PI * radius * radius; } } Even though you've changed the way Circle works on the inside, anyone who looks at it from the outside sees the exact same thing. So if somebody has some code that uses Circle, you can make this change and their code will still work. I hope this helps! More on reddit.com