58 lines
1.5 KiB
Plaintext
58 lines
1.5 KiB
Plaintext
Singletons that aren't actually singletons become problems instead. To make sure a singleton isn't instantiable, make sure all constructors are ``++private++``. If the singleton doesn't have any constructors then add a ``++private++`` no-args constructor to override the default constructor.
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This rule raises an issue when a class that holds a ``++public static final++`` instance of itself has non-``++private++`` constructors or no constructor.
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== Noncompliant Code Example
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----
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public class Highlander implements Immortal { // Noncompliant; no constructor; default, public constructor generated
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public static final Highlander INSTANCE = new Highlander();
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public void eliminateRival(Immortal immortal) {
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// ...
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}
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}
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public class Kurgan implements Immortal {
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public static final Kurgan INSTANCE = new Kurgan();
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Kurgan() { // Noncompliant; should be private
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}
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public void eliminateRival(Immortal immortal) {
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// ...
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}
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}
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----
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== Compliant Solution
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----
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public class Highlander implements Immortal {
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public static final Highlander INSTANCE = new Highlander;
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private Highlander() {
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}
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public void eliminateRival(Immortal immortal) {
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// ...
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}
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}
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public class Kurgan implements Immortal {
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public static final Kurgan INSTANCE = new Kurgan;
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private Kurgan() {
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}
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public void eliminateRival(Immortal immortal) {
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// ...
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}
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}
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----
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== See
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* https://wiki.sei.cmu.edu/confluence/x/_zZGBQ[CERT MSC07-J.] - Prevent multiple instantiations of singleton objects
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