Gson silent bug that Never Said a Word(Interview Prep)

Here's a bug that looks impossible until you understand Gson. You have this data class in your Pokedex app: data class PokemonStat ( @SerializedName ( "base_stat" ) val baseStat : Int , val stat : StatInfo ) A teammate cleans up the code and deletes what looks like a redundant line: data class PokemonStat ( val baseStat : Int , // @SerializedName removed val stat : StatInfo ) It compiles . No red errors. He runs the app — and every Pokemon's baseStat is 0 . Not the real 45 or 49. Just 0 . Everywhere. And Gson never threw an error, never logged a warning, never said a single word. If you can explain why this happens, you understand Gson better than most juniors. This is also a favorite interview question. Let's break it fully. What Gson actually does When the JSON comes back from the server, Gson goes key by key: Read a key from the JSON — say base_stat . Look for a Kotlin property with the exact same name . Found it? Pour the value in. Not found? Leave that property at its default value and move on. That's the entire matching game — exact name match, or nothing. Now look at the "cleaned up" class. The JSON key is base_stat . The Kotlin property is baseStat . Those are not the same string . Gson looks for a property called base_stat , doesn't find one, shrugs, and leaves baseStat at its default. The default for an Int is 0 . That's your bug. @SerializedName("base_stat") was never redundant. It was the sticky note telling Gson: "this property is called baseStat in Kotlin, but look for base_stat in the JSON." Delete the note, and Gson stops matching. Two ways to fix it: Rename the property to base_stat — works, but breaks Kotlin's camelCase convention. Put @SerializedName("base_stat") back — keeps the clean name and matches. This is the right one. But why 0 ? Why not a crash? This is the part that surprised me. A missing field feels like it should be an error. It isn't. Gson treats a missing key as allowed . It builds your object, fills the fields it found, and leaves