Think Data Structures by Allen B. Downey

Author:Allen B. Downey
Language: eng
Format: epub
Publisher: O'Reilly Media
Published: 2017-07-25T04:00:00+00:00

Hashing and Mutation

Strings are immutable, and SillyString is also immutable because innerString is declared to be final. Once you create a SillyString, you can’t make innerString refer to a different String, and you can’t modify the String it refers to. Therefore, it will always have the same hash code.

But let’s see what happens with a mutable object. Here’s a definition for SillyArray, which is identical to SillyString, except that it uses an array of characters instead of a String:

public class SillyArray { private final char[] array; public SillyArray(char[] array) { this.array = array; } public String toString() { return Arrays.toString(array); } @Override public boolean equals(Object other) { return this.toString().equals(other.toString()); } @Override public int hashCode() { int total = 0; for (int i=0; i<array.length; i++) { total += array[i]; } System.out.println(total); return total; }

SillyArray also provides setChar, which makes it possible to modify the characters in the array:

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