JuniorTheoryCommon

What does the program print?

JAVA

import java.util.HashSet;
import java.util.Set;

public class Main {
    public static void main(String[] args) {
        Set<P> set = new HashSet<>();
        set.add(new P());
        set.add(new P());
        System.out.println(set.size() + " " + new P().equals(new P()));
    }
}
class P { @Override public boolean equals(Object o) { return o instanceof P; } }

2 true

Expecting 1 true: a HashSet compares hash codes first and calls equals only on a match; without a hashCode override they differ, so both objects stay.
Expecting a compile error: Java does not force you to override hashCode with equals; breaking the contract compiles and fails quietly at run time.

What must hashCode return for two objects that are equal by equals?
Can two objects that are not equal have the same hash code?

Back to questions Open topic in roadmap →Answer it yourself — Quiz

Back to questions

JuniorTheoryCommon

What does the program print?

JAVA

import java.util.HashSet;
import java.util.Set;

public class Main {
    public static void main(String[] args) {
        Set<P> set = new HashSet<>();
        set.add(new P());
        set.add(new P());
        System.out.println(set.size() + " " + new P().equals(new P()));
    }
}
class P { @Override public boolean equals(Object o) { return o instanceof P; } }

2 true

Expecting 1 true: a HashSet compares hash codes first and calls equals only on a match; without a hashCode override they differ, so both objects stay.
Expecting a compile error: Java does not force you to override hashCode with equals; breaking the contract compiles and fails quietly at run time.

What must hashCode return for two objects that are equal by equals?
Can two objects that are not equal have the same hash code?

Back to questions Open topic in roadmap →Answer it yourself — Quiz