Discussion
Which line contains the error?
public class Sample73
{
static class Point
{
int x;
public boolean equals(Object o)
{
return o instanceof Point && ((Point) o).x == x;
}
}
public static void main(String[] args)
{
Point a = new Point(), b = new Point();
assert a.equals(b);
assert a.hashCode() == b.hashCode();
}
}
Answer: A
Point overrides equals but not hashCode, so two equal points inherit identity-based hash codes that almost certainly differ. The first assertion passes and the second fails, which is the contract violation showing itself. Overriding equals obliges you to override hashCode.
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