Instruction

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Properties

In the previous lesson, you were briefly introduced to properties as a member of Kotlin classes. They’re often used to describe the object’s attributes or hold state. Look again at the following example:

Constructor Properties

Although you’ve done this many times already, now is a good time to dig slightly deeper into the definition of a class.

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Mutability and Immutability

Take a look at the statement var name: String. This is how you define variables in functions and define class level variables or properties.

Access Modifiers

You’ve seen how to define properties in a class and how to control their mutability. Now, let’s talk about access modifiers. These are keywords that control the visibility of properties and methods in a class.

Default Values

If you can reasonably assume what the value of a property should be when the type is initialized, you can give that property a default value. It doesn’t make sense to create a default name or price for a food, but imagine there’s a new property type to indicate what kind of food it is. Create a food class that has a default value:

Custom Accessors

Many properties work fine with the default accessor implementation, in which dot notation returns the value directly, and an assignment statement sets the value. Properties can also be defined with custom getter and setter methods. If a custom setter is provided, the property must be declared a var.

Custom Getter

A good example of a custom accessor is a food label printed to display in the store.

Custom Setter

The property you wrote in the previous section is called a read-only property. It has a block of code to compute the value of the property: the custom getter.

Companion Object Properties

You’ve explored how properties within a class are unique to each instance. Imagine two bowls of soup, each with its own temperature and ingredients. Companion objects offer a different approach. They let you define properties that belong to the class itself, shared by all instances – like a universal recipe for the perfect bowl of soup!

Delegated Properties

So far, property initialization has been pretty basic: setting a value directly, using defaults, or calculating it with custom accessors. But what if you need more power? Delegated properties come to the rescue! These handy tools, introduced with the by keyword, let you offload property initialization or behavior to another object. Please see the next section of the lesson to learn more about by keyword with a real-life example.

Lateinit

Sometimes, a property might not have a value assigned immediately when a class instance is created. Maybe you’ll inject the value later, or it’s only needed under certain conditions. The lateinit keyword comes in handy here.

Extension Properties

Say you bought a shop, but the business isn’t doing well, and you decided to do a sale for all products. The discount is 30%, and you need to update the prices of all your food items. It would be nice not to do the calculation of a new price every time you see a food item.
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