HashSet and modifiable objects example

HashSetUnitTest.kt

package com.test.hashset

import org.junit.Assert.assertFalse
import org.junit.Assert.assertTrue
import org.junit.Test

class HashSetUnitTest {

    class Some(var a: Int, var b: Int) {
        override fun equals(other: Any?): Boolean {

            if (this === other) return true
            if (other !is Some) return false

            if (a == other.a && b == other.b) return true
            return false
        }

        override fun hashCode(): Int {
            var result = 17
            result = result * 31 + a
            result = result * 31 + b
            return result
        }

        override fun toString(): String {
            return "Some(a=$a, b=$b)"
        }

    }

    @Test
    fun objectModificationCausesBrokenContains() {

        val hashSet = HashSet<Some>()
        val control = Some(1, 2)
        hashSet.add(control)
        control.a = 42

        val contains = hashSet.contains(Some(42, 2))

        assertFalse(contains)
    }
    
    @Test
    fun objectWithouModificationAndCorrectContains() {

        val hashSet = HashSet<Some>()
        val control = Some(1, 2)
        hashSet.add(control)

        val contains = hashSet.contains(Some(1, 2))

        assertTrue(contains)
    }
    
    @Test
    fun objectWithModificationAndSetResizeAndIncorrectContains() {

        val hashSet = HashSet<Some>()
        val control = Some(1, 2)
        hashSet.add(control)
        control.a = 42

        for (i in 3..100000) {
            hashSet.add(Some(1, i))
        }


        val contains = hashSet.contains(control)

        assertFalse(contains)
    }
}