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Learn how to effectively convert `String` dates to `Long` values for storage in an Android Room database using Kotlin with clear, step-by-step guidance.
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This video is based on the question https://stackoverflow.com/q/64608755/ asked by the user 'AndroidDev123' ( https://stackoverflow.com/u/12813514/ ) and on the answer https://stackoverflow.com/a/64610981/ provided by the user 'sergiy tikhonov' ( https://stackoverflow.com/u/12718414/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Converting String Dates to Long in Room Database: A Kotlin Guide

Storing dates in an Android Room database can sometimes be tricky, especially when you're dealing with different data types. One common issue developers face is converting String date formats into Long values suitable for storage in the Room database. In this guide, we’ll explore the problem and provide a clear solution to convert your string dates effectively.

The Problem

Imagine you have a date string formatted like this: 2020/10/30 12:48:34. Your goal is to convert this string to a Long value so that it can be stored efficiently in your Room database, which supports INTEGER types for dates in milliseconds since the epoch (January 1, 1970).

You may have already set up your type converter for converting Date to Long, but the challenge here is specifically converting a String to Long.

Why Type Converters Aren’t Applied to Strings

The Room persistence library in Android uses type converters for custom data types that it can't automatically map to SQLite types. However, for standard types like String, Int, or Double, Room handles these without the need for type converters. Here’s how Room typically processes the data types during the build:

Entity Detection: Room scans for classes annotated with @ Entity to determine the database schema.

Type Mapping: For built-in types like Int and String, Room directly maps these types to SQLite equivalents (e.g., INTEGER for Int, TEXT for String).

Custom Types: For custom data types, such as Date, Room checks for type converters. If missing, it raises an error; if present, it generates methods using the provided type converter.

Since String is already mapped to TEXT, your custom type converters for String to Long will be ignored by Room as it doesn't believe they are needed.

The Solution

Although Room doesn’t allow direct conversion of String to Long, you can work around this limitation by using a custom wrapper class. This approach involves creating a custom string wrapper and implementing a type converter for it.

Step-by-Step Guide: Creating a String Wrapper Class

Here’s how you can set up a solution:

Create a String Wrapper Class:

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

Update Your Type Converter:

Implement a type converter for converting between StringWrapper and Long.

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

Modify Your Entity Class:

Ensure that your Room entity uses StringWrapper instead of String.

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

Final Thoughts

By wrapping the String type into a custom class, you have instructed Room to treat it as a custom type and utilize your type converter. This method will effectively allow you to convert String dates to Long for storage in your Room database without any trouble.

Now, you can confidently handle date conversions in your Android application, keeping your data organized and optimized. If you have any further questions or need assistance, feel free to drop a comment below!