Changing Keys, Losing Values TL;DR: When you use mutable objects as keys in hashed collections, changing them breaks contracts. Problems 😔 Lost Values
Hard Debugging
The Least Surprise Principle Violation
Unexpected Behavior Solutions 😃 Use immutable objects as keys.
Override equals/hashCode well.
Use final fields (If your language allows it)
Rehash after mutation (This is an over-engineering solution) Context 💬 When you use mutable objects as keys in hashed collections, changing them key after after you add a related objec can make it unretrievable. This happens because the hash code changes and the collection can't find the object in the correct bucket. Sample Code 📖 Wrong ❌ class MutableKey {
    int id;

    MutableKey(int newId) {
        this.id = newId;
    }

    @Override
    public int hashCode() {
        return this.id;
    }

    @Override
    public boolean equals(Object objectToCompare) {
        if (this == objectToCompare) return true;
        MutableKey that = (MutableKey) objectToCompare;
        return id == that.id;
    }
}

MutableKey key = new MutableKey(42);
Map<MutableKey, String> map = new HashMap<>();
map.put(key, "Yes Album");

// The key mutates
key.id = 90125;

// Now you cannont retrieve the album
System.out.println(map.get(key)); 

// Output: null Right 👉 class ImmutableKey {
    private final int id;

    ImmutableKey(int newId) {
        this.id = newId;
    }

    @Override
    public int hashCode() {
        return this.id;
    }

    @Override
    public boolean equals(Object objectToCompare) {
        if (this == objectToCompare) return true;
        ImmutableKey that = (ImmutableKey) objectToCompare;
        return id == that.id;
    }
}

ImmutableKey key = new ImmutableKey(42);
Map<ImmutableKey, String> map = new HashMap<>();
map.put(key, "Yes Album");

System.out.println(map.get(key)); 
// Output: Yes Album Detection 🔍 Semi-Automatic You can detect this smell by checking if you use mutable objects as keys in hash-based collections. Automated tools like linters or IDE inspections can also flag mutable keys. Tags 🏷️ Mutability Level 🔋 Intermediate Why the Bijection Is Important 🗺️ The bijection between the real world and your program is important because it ensures that your objects accurately reflect the relationships they are supposed to represent. In the real world, keys are often immutable (e.g., IDs, names). When you model these keys as mutable objects, you break the one-to-one correspondence between the real world and your program in the MAPPER. When you break this bijection using mutable keys, you make the map's inconsistent leading to retrieval failures and unexpected behavior. AI Generation 🤖 AI generators might create this smell if they generate mutable objects as keys without considering the implications. This is seldom the case since AI generators suffer from primitive obsession. AI Detection 🥃 AI generators can detect this smell with instructions to analyze the use of mutable objects in hash-based collections and flag potential issues. You can instruct the AI to look for classes without final fields or methods that modify the object's state after creation. Try Them! 🛠 Remember: AI Assistants make lots of mistakes Without Proper Instructions

With Specific Instructions



ChatGPT

ChatGPT



Claude

Claude



Perplexity

Perplexity



Copilot

Copilot



Gemini

Gemini



DeepSeek

DeepSeek



Meta AI

Meta AI Conclusion 🏁 When you use mutable objects as keys, you risk breaking the contract between the key's state and hash code. Use immutable objects to avoid this issue. Relations 👩‍❤️‍💋‍👨 https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxxiv https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxv https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxiv https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxxvi https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xviii https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxvi https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xlii More Information 📕 https://en.wikipedia.org/wiki/Hash_function?embedable=true https://hackernoon.com/is-it-crystal-clear-for-everybody-that-a-date-should-not-mutate-wuoy3z03?embedable=true Disclaimer 📘 Code Smells are my opinion. Credits 🙏 Photo by Kathyryn Tripp on Unsplash The most important property of a program is whether it accomplishes the intention of its user.
C.A.R. Hoare https://hackernoon.com/400-thought-provoking-software-engineering-quotes?embedable=true This article is part of the CodeSmell Series. https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-i-xqz3evd?embedable=true Changing Keys, Losing Values Changing Keys, Losing Values TL;DR: When you use mutable objects as keys in hashed collections, changing them breaks contracts. TL;DR: When you use mutable objects as keys in hashed collections, changing them breaks contracts. Problems 😔 Lost Values Hard Debugging The Least Surprise Principle Violation Unexpected Behavior Lost Values Hard Debugging The Least Surprise Principle Violation Unexpected Behavior Solutions 😃 Use immutable objects as keys. Override equals/hashCode well. Use final fields (If your language allows it) Rehash after mutation (This is an over-engineering solution) Use immutable objects as keys. immutable Override equals/hashCode well. equals/hashCode Use final fields (If your language allows it) Rehash after mutation (This is an over-engineering solution) Context 💬 When you use mutable objects as keys in hashed collections , changing them key after after you add a related objec can make it unretrievable. hashed collections This happens because the hash code changes and the collection can't find the object in the correct bucket. Sample Code 📖 Wrong ❌ class MutableKey {
    int id;

    MutableKey(int newId) {
        this.id = newId;
    }

    @Override
    public int hashCode() {
        return this.id;
    }

    @Override
    public boolean equals(Object objectToCompare) {
        if (this == objectToCompare) return true;
        MutableKey that = (MutableKey) objectToCompare;
        return id == that.id;
    }
}

MutableKey key = new MutableKey(42);
Map<MutableKey, String> map = new HashMap<>();
map.put(key, "Yes Album");

// The key mutates
key.id = 90125;

// Now you cannont retrieve the album
System.out.println(map.get(key)); 

// Output: null class MutableKey {
    int id;

    MutableKey(int newId) {
        this.id = newId;
    }

    @Override
    public int hashCode() {
        return this.id;
    }

    @Override
    public boolean equals(Object objectToCompare) {
        if (this == objectToCompare) return true;
        MutableKey that = (MutableKey) objectToCompare;
        return id == that.id;
    }
}

MutableKey key = new MutableKey(42);
Map<MutableKey, String> map = new HashMap<>();
map.put(key, "Yes Album");

// The key mutates
key.id = 90125;

// Now you cannont retrieve the album
System.out.println(map.get(key)); 

// Output: null Right 👉 class ImmutableKey {
    private final int id;

    ImmutableKey(int newId) {
        this.id = newId;
    }

    @Override
    public int hashCode() {
        return this.id;
    }

    @Override
    public boolean equals(Object objectToCompare) {
        if (this == objectToCompare) return true;
        ImmutableKey that = (ImmutableKey) objectToCompare;
        return id == that.id;
    }
}

ImmutableKey key = new ImmutableKey(42);
Map<ImmutableKey, String> map = new HashMap<>();
map.put(key, "Yes Album");

System.out.println(map.get(key)); 
// Output: Yes Album class ImmutableKey {
    private final int id;

    ImmutableKey(int newId) {
        this.id = newId;
    }

    @Override
    public int hashCode() {
        return this.id;
    }

    @Override
    public boolean equals(Object objectToCompare) {
        if (this == objectToCompare) return true;
        ImmutableKey that = (ImmutableKey) objectToCompare;
        return id == that.id;
    }
}

ImmutableKey key = new ImmutableKey(42);
Map<ImmutableKey, String> map = new HashMap<>();
map.put(key, "Yes Album");

System.out.println(map.get(key)); 
// Output: Yes Album Detection 🔍 Semi-Automatic Semi-Automatic You can detect this smell by checking if you use mutable objects as keys in hash-based collections. Automated tools like linters or IDE inspections can also flag mutable keys. Tags 🏷️ Mutability Mutability Level 🔋 Intermediate Intermediate Why the Bijection Is Important 🗺️ The bijection between the real world and your program is important because it ensures that your objects accurately reflect the relationships they are supposed to represent. bijection In the real world, keys are often immutable (e.g., IDs, names). When you model these keys as mutable objects, you break the one-to-one correspondence between the real world and your program in the MAPPER . MAPPER When you break this bijection using mutable keys, you make the map's inconsistent leading to retrieval failures and unexpected behavior. AI Generation 🤖 AI generators might create this smell if they generate mutable objects as keys without considering the implications. This is seldom the case since AI generators suffer from primitive obsession . primitive obsession AI Detection 🥃 AI generators can detect this smell with instructions to analyze the use of mutable objects in hash-based collections and flag potential issues. You can instruct the AI to look for classes without final fields or methods that modify the object's state after creation. final Try Them! 🛠 Remember: AI Assistants make lots of mistakes Remember: AI Assistants make lots of mistakes Without Proper Instructions

With Specific Instructions



ChatGPT

ChatGPT



Claude

Claude



Perplexity

Perplexity



Copilot

Copilot



Gemini

Gemini



DeepSeek

DeepSeek



Meta AI

Meta AI Without Proper Instructions

With Specific Instructions ChatGPT

ChatGPT Claude

Claude Perplexity

Perplexity Copilot

Copilot Gemini

Gemini DeepSeek

DeepSeek Meta AI

Meta AI Without Proper Instructions With Specific Instructions Without Proper Instructions Without Proper Instructions With Specific Instructions With Specific Instructions ChatGPT ChatGPT ChatGPT ChatGPT ChatGPT ChatGPT ChatGPT ChatGPT Claude Claude Claude Claude Claude Claude Claude Claude Perplexity Perplexity Perplexity Perplexity Perplexity Perplexity Perplexity Perplexity Copilot Copilot Copilot Copilot Copilot Copilot Copilot Copilot Gemini Gemini Gemini Gemini Gemini Gemini Gemini Gemini DeepSeek DeepSeek DeepSeek DeepSeek DeepSeek DeepSeek DeepSeek DeepSeek Meta AI Meta AI Meta AI Meta AI Meta AI Meta AI Meta AI Meta AI Conclusion 🏁 When you use mutable objects as keys, you risk breaking the contract between the key's state and hash code. Use immutable objects to avoid this issue. Relations 👩‍❤️‍💋‍👨 https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxxiv https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxxiv https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxv https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxv https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxiv https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxiv https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxxvi https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxxvi https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xviii https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xviii https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxvi https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xxvi https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xlii https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-xlii More Information 📕 https://en.wikipedia.org/wiki/Hash_function?embedable=true https://en.wikipedia.org/wiki/Hash_function?embedable=true https://hackernoon.com/is-it-crystal-clear-for-everybody-that-a-date-should-not-mutate-wuoy3z03?embedable=true https://hackernoon.com/is-it-crystal-clear-for-everybody-that-a-date-should-not-mutate-wuoy3z03?embedable=true Disclaimer 📘 Code Smells are my opinion . opinion Credits 🙏 Photo by Kathyryn Tripp on Unsplash Kathyryn Tripp Unsplash The most important property of a program is whether it accomplishes the intention of its user. C.A.R. Hoare The most important property of a program is whether it accomplishes the intention of its user. C.A.R. Hoare https://hackernoon.com/400-thought-provoking-software-engineering-quotes?embedable=true https://hackernoon.com/400-thought-provoking-software-engineering-quotes?embedable=true This article is part of the CodeSmell Series. https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-i-xqz3evd?embedable=true https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-i-xqz3evd?embedable=true

The code in this story is for educational purposes. The readers are solely responsible for whatever they build with it.

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