在Java中，覆蓋equals()方法時必須遵循五個規(guī)則：自反性、對稱性、傳遞性、一致性及非空性。 1. 自反性要求對象與自身比較返回true；2. 對稱性確保x.equals(y)與y.equals(x)結(jié)果一致；3. 傳遞性要求若x.equals(y)和y.equals(z)為true，則x.equals(z)也應(yīng)為true；4. 一致性保證相同對像多次調(diào)用equals()結(jié)果不變；5. 非空性規(guī)定對象與null比較應(yīng)返回false。此外，覆蓋equals()時必須同時覆蓋hashCode()，以確保相等的對象具有相同的哈希碼，從而保證集合類如HashMap和HashSet的正確行為。實(shí)現(xiàn)時需使用相同字段計算equals()和hashCode()，並避免常見錯誤如未處理null、誤用==、破壞對稱性或忘記同步重寫hashCode()。正確實(shí)現(xiàn)步驟包括：檢查是否為同一對象、判斷參數(shù)合法性、強(qiáng)制類型轉(zhuǎn)換、逐個比較關(guān)鍵字段，並可藉助Objects.equals()等工具方法簡化邏輯。

When you're working with Java and comparing objects, the equals() method is what you use to check if two objects are logically "equal" — but there's a specific contract you must follow when overriding it. If you don't, your code might behave unpredictably, especially when used with collections like HashMap or HashSet.

What Does the Equals Contract Require?

The equals method in Java has a clear set of rules defined in its contract. These are not enforced by the compiler, so it's up to you to make sure they're followed:

• Reflexive : For any non-null reference x, x.equals(x) should return true.
• Symmetric : If x.equals(y) returns true, then y.equals(x) must also return true.
• Transitive : If x.equals(y) and y.equals(z) both return true, then x.equals(z) should return true as well.
• Consistent : Multiple calls to equals() on the same objects (as long as they aren't modified) should return the same result consistently.
• Non-null : For any non-null reference x, x.equals(null) should return false.

These rules ensure that object comparison behaves predictably across different parts of your application.

Why Override HashCode When You Override Equals?

If two objects are equal according to the equals() method, their hash codes must be the same too. That's why whenever you override equals() , you should also override hashCode() .

Why? Because classes like HashMap and HashSet rely on hash codes to organize and retrieve data efficiently. If two equal objects return different hash codes, those collections might fail to find or manage your objects correctly.

So here's the rule:

• Always override hashCode() when you override equals() .
• Use the same fields in both methods to calculate equality and hash value.

This helps maintain consistency between object identity and how they're stored and retrieved in collections.

Common Mistakes to Avoid

Even experienced developers sometimes slip up when implementing equals() . Here are some common pitfalls:

• Not handling nulls properly: Make sure to check for null before calling methods on an object inside equals() .
• Using == instead of .equals() for field comparisons: The == operator checks references for objects, not values.
• Breaking symmetry accidentally: This often happens when extending classes. If you add new fields in a subclass, make sure the equals() logic doesn't violate symmetry.
• Forgetting to override hashCode() after changing equals() .

One example is this: say you have a Person class with a name field. If you only compare names in equals() but forget to update hashCode() , two equal Person objects could return different hash codes — which will cause issues in maps and sets.

How to Implement Equals Correctly

Implementing equals() properly involves several steps. Here's a practical approach:

1. Check if the object being compared is this — if yes, return true.
2. Check if the input object is null or of the wrong type — return false if so.
3. Cast the object to the correct type.
4. Compare each significant field one by one:
   • Use Objects.equals() for object fields.
   • Use == for primitives.
5. Return true only if all fields match.

You can also use helper methods like Objects.hash() or Arrays.deepEquals() depending on your object structure.

Here's a quick example:

 @Override
public boolean equals(Object obj) {
    if (this == obj) return true;
    if (!(obj instanceof Person)) return false;

    Person other = (Person) obj;
    return Objects.equals(this.name, other.name);
}

And always remember to pair it with a matching hashCode() method.

基本上就這些.

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