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3.5: Cards are Immutable

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

    The instance variables of Card are private, so they can’t be accessed from other classes. We can provide getters to allow other classes to read the rank and suit values:

    public int getRank() {
        return this.rank;
    public int getSuit() {
        return this.suit;

    Whether or not to provide setters is a design decision. If we did, cards would be mutable, so you could transform one card into another. That is probably not a feature we need, and in general mutable objects are more error-prone. So it might be better to make cards immutable. To do that, all we have to do is not provide any modifier methods (including setters).

    That’s easy enough, but it is not foolproof, because some fool might come along later and add a modifier. We can prevent that possibility by declaring the instance variables final:

    public class Card {
        private final int rank;
        private final int suit;

    You can still assign values to these variables inside a constructor. But if someone writes a method that tries to modify these variables, they’ll get a compiler error.

    This page titled 3.5: Cards are Immutable 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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