#590 Add new presentation for Singleton
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Ilkka Seppälä
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@ -15,7 +15,32 @@ tags:
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Ensure a class only has one instance, and provide a global point of
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access to it.
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## Explanation
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Real world example
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> There can only be one ivory tower where the wizards study their magic. The same enchanted ivory tower is always used by the wizards. Ivory tower here is singleton.
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In plain words
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> Ensures that only one object of a particular class is ever created.
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Wikipedia says
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> In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system.
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**Programmatic Example**
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Joshua Bloch, Effective Java 2nd Edition p.18
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> A single-element enum type is the best way to implement a singleton
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```
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public enum EnumIvoryTower {
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INSTANCE;
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}
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```
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Then in order to use
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```
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EnumIvoryTower enumIvoryTower1 = EnumIvoryTower.INSTANCE;
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EnumIvoryTower enumIvoryTower2 = EnumIvoryTower.INSTANCE;
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assertEquals(enumIvoryTower1, enumIvoryTower2); // true
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```
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## Applicability
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Use the Singleton pattern when
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@ -40,7 +65,7 @@ Use the Singleton pattern when
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* Violates Single Responsibility Principle (SRP) by controlling their own creation and lifecycle.
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* Encourages using a global shared instance which prevents an object and resources used by this object from being deallocated.
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* Creates tightly coupled code that is difficult to test.
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* Creates tightly coupled code. The clients of the Singleton become difficult to test.
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* Makes it almost impossible to subclass a Singleton.
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## Credits
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Binary file not shown.
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@ -1,104 +0,0 @@
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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.1.8" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true"
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realizations="true" associations="true" dependencies="false" nesting-relationships="true">
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<class id="1" language="java" name="com.iluwatar.singleton.InitializingOnDemandHolderIdiom.HelperHolder"
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project="singleton" file="/singleton/src/main/java/com/iluwatar/singleton/InitializingOnDemandHolderIdiom.java"
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binary="false" corner="BOTTOM_RIGHT">
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<position height="88" width="133" x="328" y="457"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="2" language="java" name="com.iluwatar.singleton.IvoryTower" project="singleton"
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file="/singleton/src/main/java/com/iluwatar/singleton/IvoryTower.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="106" width="164" x="114" y="275"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="3" language="java" name="com.iluwatar.singleton.InitializingOnDemandHolderIdiom" project="singleton"
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file="/singleton/src/main/java/com/iluwatar/singleton/InitializingOnDemandHolderIdiom.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="142" width="295" x="328" y="275"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<enumeration id="4" language="java" name="com.iluwatar.singleton.EnumIvoryTower" project="singleton"
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file="/singleton/src/main/java/com/iluwatar/singleton/EnumIvoryTower.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="124" width="190" x="663" y="275"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</enumeration>
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<class id="5" language="java" name="com.iluwatar.singleton.ThreadSafeLazyLoadedIvoryTower" project="singleton"
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file="/singleton/src/main/java/com/iluwatar/singleton/ThreadSafeLazyLoadedIvoryTower.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="106" width="299" x="893" y="275"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="6" language="java" name="com.iluwatar.singleton.ThreadSafeDoubleCheckLocking" project="singleton"
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file="/singleton/src/main/java/com/iluwatar/singleton/ThreadSafeDoubleCheckLocking.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="106" width="292" x="1242" y="275"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<association id="7">
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<end type="SOURCE" refId="2" navigable="false">
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<attribute id="8" name="instance"/>
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<multiplicity id="9" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="2" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<association id="10">
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<end type="SOURCE" refId="6" navigable="false">
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<attribute id="11" name="INSTANCE"/>
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<multiplicity id="12" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="6" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<association id="13">
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<end type="SOURCE" refId="1" navigable="false">
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<attribute id="14" name="INSTANCE"/>
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<multiplicity id="15" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="3" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<nesting id="16">
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<end type="SOURCE" refId="3"/>
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<end type="TARGET" refId="1"/>
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</nesting>
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<association id="17">
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<end type="SOURCE" refId="5" navigable="false">
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<attribute id="18" name="instance"/>
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<multiplicity id="19" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="5" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</classifier-display>
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<association-display labels="true" multiplicity="true"/>
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</class-diagram>
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@ -1,43 +0,0 @@
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@startuml
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package com.iluwatar.singleton {
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class App {
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- LOGGER : Logger {static}
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+ App()
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+ main(args : String[]) {static}
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}
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enum EnumIvoryTower {
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+ INSTANCE {static}
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+ toString() : String
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+ valueOf(name : String) : EnumIvoryTower {static}
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+ values() : EnumIvoryTower[] {static}
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}
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class InitializingOnDemandHolderIdiom {
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- InitializingOnDemandHolderIdiom()
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+ getInstance() : InitializingOnDemandHolderIdiom {static}
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}
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-class HelperHolder {
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- INSTANCE : InitializingOnDemandHolderIdiom {static}
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- HelperHolder()
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}
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class IvoryTower {
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- INSTANCE : IvoryTower {static}
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- IvoryTower()
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+ getInstance() : IvoryTower {static}
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}
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class ThreadSafeDoubleCheckLocking {
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- instance : ThreadSafeDoubleCheckLocking {static}
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- ThreadSafeDoubleCheckLocking()
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+ getInstance() : ThreadSafeDoubleCheckLocking {static}
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}
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class ThreadSafeLazyLoadedIvoryTower {
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- instance : ThreadSafeLazyLoadedIvoryTower {static}
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- ThreadSafeLazyLoadedIvoryTower()
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+ getInstance() : ThreadSafeLazyLoadedIvoryTower {static}
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}
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}
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IvoryTower --> "-INSTANCE" IvoryTower
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ThreadSafeDoubleCheckLocking --> "-instance" ThreadSafeDoubleCheckLocking
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ThreadSafeLazyLoadedIvoryTower --> "-instance" ThreadSafeLazyLoadedIvoryTower
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HelperHolder ..+ InitializingOnDemandHolderIdiom
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HelperHolder --> "-INSTANCE" InitializingOnDemandHolderIdiom
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@enduml
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