59 lines
1.4 KiB
Java
59 lines
1.4 KiB
Java
// patterns/SingletonPattern.java
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// (c)2017 MindView LLC: see Copyright.txt
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// We make no guarantees that this code is fit for any purpose.
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// Visit http://OnJava8.com for more book information.
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interface Resource {
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int getValue();
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void setValue(int x);
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}
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// Since this isn't inherited from a Cloneable
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// base class and cloneability isn't added,
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// making it final prevents cloneability from
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// being added through inheritance. This also
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// implements thread-safe lazy initialization:
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final class Singleton {
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private static class
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ResourceImpl implements Resource {
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private int i;
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private ResourceImpl(int i) {
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this.i = i;
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}
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public synchronized int getValue() {
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return i;
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}
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public synchronized void setValue(int x) {
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i = x;
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}
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}
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private static class ResourceHolder {
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private static Resource resource =
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new ResourceImpl(47);
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}
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public static Resource getResource() {
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return ResourceHolder.resource;
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}
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}
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public class SingletonPattern {
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public static void main(String[] args) {
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Resource r = Singleton.getResource();
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System.out.println(r.getValue());
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Resource s2 = Singleton.getResource();
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s2.setValue(9);
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System.out.println(r.getValue());
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try {
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// Can't do this: compile-time error.
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// Singleton s3 = (Singleton)s2.clone();
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} catch(Exception e) {
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throw new RuntimeException(e);
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}
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}
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}
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/* Output:
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47
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9
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*/
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