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19.7 Exercises
(a) True or False: Resizes are triggered if adding a key-value pair causes the load factor to be greater than or equal to the specified maximum load factor.
(b) True or False: The
hashCode() function can have varied return types.In order for a hash code to be valid, objects that are equivalent to each other (i.e. .equals() returns true) must return equivalent hash codes. If an object does not explicitly override the
hashCode() method, it will inherit the hashCode() method defined in the Object class, which returns the object’s address in memory.Here are four potential implementations of Integer's
hashCode() function. Assume that intValue() returns the value represented by the Integer object. Categorize each
hashCode() implementation as either a valid or an invalid hash function. If it is valid, point out a flaw or disadvantage. If it is invalid, explain why. Question (a)
public int hashCode() {
return -1;
}
Question (b)
public int hashCode() {
return intValue() * intValue();
}
Valid. Similar to (a), this hash function returns the same hash code for Integers that are .equals(). However, Integers that share the same absolute values will collide (for example, x = 5 and x = -5 will both return the same hashcode). A better hash function would be to just return intValue() itself.
Question (c)
public int hashCode() {
Random rand = new Random();
return rand.nextInt();
}
Question (d)
public int hashCode() {
return super.hashCode();
}
Given the provided
hashCode() implementation, hash the items listed below with external chaining (the first item is already inserted for you). Assume the load factor is 1, and the initial underlying array has size of 4. Use geometric resizing with a resize factor of 2. You may draw more boxes to extend the array when you need to resize./** Returns 0 if word begins with ’a’, 1 if it begins with ’b’, etc. */
public int hashCode() {
return word.charAt(0) - 'a';
}
["apple", "cherry", "fig", "guava", "durian", "apricot", "banana"]
Last modified 8mo ago