𝗛𝗲𝗿𝗲’𝘀 𝘄𝗵𝘆 𝘆𝗼𝘂 𝘀𝗵𝗼𝘂𝗹𝗱 𝗯𝗲 𝗰𝗮𝗿𝗲𝗳𝘂𝗹 𝘄𝗵𝗲𝗻 𝗿𝗲𝗹𝘆𝗶𝗻𝗴 𝗼𝗻 𝙝𝙖𝙨𝙝𝘾𝙤𝙙𝙚() 𝗮𝗻𝗱 𝙚𝙦𝙪𝙖𝙡𝙨() 𝗶𝗻 𝗞𝗼𝘁𝗹𝗶𝗻 𝗱𝗮𝘁𝗮 𝗰𝗹𝗮𝘀𝘀𝗲𝘀:

Data classes offer many benefits, the most prominent of which is that they help us avoid boilerplate code.

Data classes will provide a default implementation of equals(), toString(), hashCode(), copy(), etc.

While that’s really useful in terms of saving us time - we should be careful when relying on these default implementations. Most notably, we should be careful with the hashCode() method.

From the documentation:

𝙏𝙝𝙚 𝙘𝙤𝙢𝙥𝙞𝙡𝙚𝙧 𝙤𝙣𝙡𝙮 𝙪𝙨𝙚𝙨 𝙩𝙝𝙚 𝙥𝙧𝙤𝙥𝙚𝙧𝙩𝙞𝙚𝙨 𝙙𝙚𝙛𝙞𝙣𝙚𝙙 𝙞𝙣𝙨𝙞𝙙𝙚 𝙩𝙝𝙚 𝙥𝙧𝙞𝙢𝙖𝙧𝙮 𝙘𝙤𝙣𝙨𝙩𝙧𝙪𝙘𝙩𝙤𝙧 𝙛𝙤𝙧 𝙩𝙝𝙚 𝙖𝙪𝙩𝙤𝙢𝙖𝙩𝙞𝙘𝙖𝙡𝙡𝙮 𝙜𝙚𝙣𝙚𝙧𝙖𝙩𝙚𝙙 𝙛𝙪𝙣𝙘𝙩𝙞𝙤𝙣𝙨. 𝙏𝙤 𝙚𝙭𝙘𝙡𝙪𝙙𝙚 𝙖 𝙥𝙧𝙤𝙥𝙚𝙧𝙩𝙮 𝙛𝙧𝙤𝙢 𝙩𝙝𝙚 𝙜𝙚𝙣𝙚𝙧𝙖𝙩𝙚𝙙 𝙞𝙢𝙥𝙡𝙚𝙢𝙚𝙣𝙩𝙖𝙩𝙞𝙤𝙣𝙨, 𝙙𝙚𝙘𝙡𝙖𝙧𝙚 𝙞𝙩 𝙞𝙣𝙨𝙞𝙙𝙚 𝙩𝙝𝙚 𝙘𝙡𝙖𝙨𝙨 𝙗𝙤𝙙𝙮.

In our example below, I’m doing exactly that. I’m declaring 𝚝𝚒𝚖𝚎𝚂𝚎𝚗𝚝 inside the generated implementation.

So 𝙚𝙦𝙪𝙖𝙡𝙨() and 𝙝𝙖𝙨𝙝𝘾𝙤𝙙𝙚() will give us identical values for 𝚖𝚎𝚜𝚜𝚊𝚐𝚎𝟷 and 𝚖𝚎𝚜𝚜𝚊𝚐𝚎𝟸. This might lead us to believe these two objects are identical, but they’re not. We can easily verify that by printing 𝚝𝚒𝚖𝚎𝚂𝚎𝚗𝚝.

So there you have it. While default implementations in data classes save us a lot of headaches - they can also cause problems. Make sure you know what you’re doing when using them.

Thanks for reading!