Скачать или смотреть Using data class in Kotlin with Constructor Parameters having Defaults Dependent on Another Paramet

Learn how to define Kotlin data classes with constructor parameters that have default values depending on other parameters, without making all parameters properties.
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This video is based on the question https://stackoverflow.com/q/79475081/ asked by the user 'k314159' ( https://stackoverflow.com/u/13963086/ ) and on the answer https://stackoverflow.com/a/79475114/ provided by the user 'Sweeper' ( https://stackoverflow.com/u/5133585/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Problem Overview

In Kotlin, you often want to use a data class for its convenient features like autogenerated toString(), copy(), and equals() methods. However, a common challenge arises when you want a constructor parameter to have a default value that depends on another parameter, but you don't want that dependent parameter to be a property.

For example, you might want a small immutable object with:

a name property

a timeCreated property that defaults to the current instant

a clock parameter used only internally for testing time creation, not as a property

The initial attempt in a normal class works:

[[See Video to Reveal this Text or Code Snippet]]

But converting this directly to a data class fails because all primary constructor parameters in data class must be properties (val or var):

[[See Video to Reveal this Text or Code Snippet]]

Why This Happens

A data class requires all primary constructor parameters to be properties. This is to enable automatic generation of equals(), copy(), toString(), and hashCode(). Parameters used only internally, like clock here, cannot be part of the primary constructor if they aren’t properties.

The Modern, Clean Solution: Secondary Constructor

The best practice is to define a primary constructor with only the properties you actually want in your class, such as name and timeCreated, and then add a secondary constructor that accepts the additional parameter (clock) and handles defaults accordingly.

[[See Video to Reveal this Text or Code Snippet]]

Key points:

The primary constructor only has properties (name, timeCreated).

The secondary constructor adds the clock parameter with its default, but clock is not a property.

You can still call MyObject(name = "Fred") which uses the secondary constructor.

Additional Tips

Place the clock parameter after required parameters like name to avoid ambiguity.

When using the secondary constructor in more complex cases, always use named arguments if needed for clarity.

Summary

To have default constructor parameters dependent on other parameters in a Kotlin data class, avoid adding non-property parameters to the primary constructor. Instead, leverage a secondary constructor to handle these parameters without exposing them as class properties. This preserves the benefits of data class while maintaining clean and testable code.