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10.3: Hashing and mutation

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  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    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);
        public boolean equals(Object other) {
            return this.toString().equals(other.toString());
        public int hashCode() {
            int total = 0;
            for (int i=0; i<array.length; i++) {
                total += array[i];
            return total;

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

    public void setChar(int i, char c) {
        this.array[i] = c;

    Now suppose we create a SillyArray and add it to a map:

    SillyArray array1 = new SillyArray("Word1".toCharArray());
    map.put(array1, 1); 

    The hash code for this array is 461. Now if we modify the contents of the array and then try to look it up, like this:

    array1.setChar(0, ’C’);
    Integer value = map.get(array1);

    the hash code after the mutation is 441. With a different hash code, there’s a good chance we’ll go looking in the wrong sub-map. In that case, we won’t find the key, even though it is in the map. And that’s bad.

    In general, it is dangerous to use mutable objects as keys in data structures that use hashing, which includes MyBetterMap and HashMap. If you can guarantee that the keys won’t be modified while they are in the map, or that any changes won’t affect the hash code, it might be OK. But it is probably a good idea to avoid it.

    This page titled 10.3: Hashing and mutation is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Allen B. Downey (Green Tea Press) .

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