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Implementations of `Object#hashCode` should not incorporate fields which the
implementation of `Object#equals` does not. This violates the contract of
`hashCode`: specifically, equal objects must have equal hashCodes.
```java
class Foo {
private final int a;
private final int b;
Foo(int a, int b) {
this.a = a;
this.b = b;
}
@Override
public boolean equals(@Nullable Object o) {
return o instanceof Foo && ((Foo) o).a == a;
}
@Override
public int hashCode() {
return a + 31 * b;
}
}
Foo first = new Foo(10, 20);
Foo second = new Foo(10, 40);
first.equals(second) // true
first.hashCode() == second.hashCode() // false
```
The fix for this class is either to include a comparison of `b` in the `#equals`
method, or remove `b` from `#hashCode`. The former is more likely to be correct.
|
