Added Lazy Loading idiom.

lazy-loading/etc/lazy-loading.ucls

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+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.1.8" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true" 
+  realizations="true" associations="true" dependencies="false" nesting-relationships="true">  
+  <class id="1" language="java" name="com.iluwatar.HolderNaive" project="lazy-loading" 
+    file="/lazy-loading/src/main/java/com/iluwatar/HolderNaive.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="106" width="126" x="206" y="297"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="2" language="java" name="com.iluwatar.Heavy" project="lazy-loading" 
+    file="/lazy-loading/src/main/java/com/iluwatar/Heavy.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="88" width="104" x="289" y="479"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="3" language="java" name="com.iluwatar.Java8Holder" project="lazy-loading" 
+    file="/lazy-loading/src/main/java/com/iluwatar/Java8Holder.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="142" width="204" x="567" y="297"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="4" language="java" name="com.iluwatar.HolderThreadSafe" project="lazy-loading" 
+    file="/lazy-loading/src/main/java/com/iluwatar/HolderThreadSafe.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="106" width="155" x="372" y="297"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <association id="5">    
+    <end type="SOURCE" refId="4" navigable="false">      
+      <attribute id="6" name="heavy"/>      
+      <multiplicity id="7" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="2" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <dependency id="8">    
+    <end type="SOURCE" refId="3"/>    
+    <end type="TARGET" refId="2"/>  
+  </dependency>  
+  <association id="9">    
+    <end type="SOURCE" refId="1" navigable="false">      
+      <attribute id="10" name="heavy"/>      
+      <multiplicity id="11" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="2" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+    sort-features="false" accessors="true" visibility="true">    
+    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
+    <operations public="true" package="true" protected="true" private="true" static="true"/>  
+  </classifier-display>  
+  <association-display labels="true" multiplicity="true"/>
+</class-diagram>

lazy-loading/pom.xml

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+<?xml version="1.0"?>
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.0-SNAPSHOT</version>
+  </parent>
+  <artifactId>lazy-loading</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

lazy-loading/src/main/java/com/iluwatar/App.java

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+package com.iluwatar;
+
+/**
+ * 
+ * Lazy loading idiom defers object creation until needed.
+ * 
+ * This example shows different implementations of the pattern
+ * with increasing sophistication.
+ * 
+ * Additional information and lazy loading flavours are described in
+ * http://martinfowler.com/eaaCatalog/lazyLoad.html
+ *
+ */
+public class App 
+{
+    public static void main( String[] args ) {
+    	
+    	// Simple lazy loader - not thread safe
+    	HolderNaive holderNaive = new HolderNaive();
+    	Heavy heavy = holderNaive.getHeavy();
+    	System.out.println("heavy=" + heavy);
+    	
+    	// Thread safe lazy loader, but with heavy synchronization on each access
+    	HolderThreadSafe holderThreadSafe = new HolderThreadSafe();
+    	Heavy another = holderThreadSafe.getHeavy();
+    	System.out.println("another=" + another);
+    	
+    	// The most efficient lazy loader utilizing Java 8 features
+    	Java8Holder java8Holder = new Java8Holder();
+    	Heavy next = java8Holder.getHeavy();
+    	System.out.println("next=" + next);
+    }
+}

lazy-loading/src/main/java/com/iluwatar/Heavy.java

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+package com.iluwatar;
+
+/**
+ * 
+ * Heavy objects are expensive to create.
+ *
+ */
+public class Heavy {
+
+	public Heavy() {
+		System.out.println("Creating Heavy ...");
+		try {
+			Thread.sleep(1000);
+		} catch (InterruptedException e) {
+			e.printStackTrace();
+		}
+		System.out.println("... Heavy created");
+	}
+}

lazy-loading/src/main/java/com/iluwatar/HolderNaive.java

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+package com.iluwatar;
+
+/**
+ * 
+ * Simple implementation of the lazy loading idiom.
+ * However, this is not thread safe.
+ *
+ */
+public class HolderNaive {
+	
+	private Heavy heavy;
+	
+	public HolderNaive() {
+		System.out.println("HolderNaive created");
+	}
+
+	public Heavy getHeavy() {
+		if (heavy == null) {
+			heavy = new Heavy();
+		}
+		return heavy;
+	}
+}

lazy-loading/src/main/java/com/iluwatar/HolderThreadSafe.java

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+package com.iluwatar;
+
+/**
+ * 
+ * Same as HolderNaive but with added synchronization.
+ * This implementation is thread safe, but each {@link #getHeavy()}
+ * call costs additional synchronization overhead.
+ *
+ */
+public class HolderThreadSafe {
+	
+	private Heavy heavy;
+	
+	public HolderThreadSafe() {
+		System.out.println("HolderThreadSafe created");
+	}
+
+	public synchronized Heavy getHeavy() {
+		if (heavy == null) {
+			heavy = new Heavy();
+		}
+		return heavy;
+	}
+}

lazy-loading/src/main/java/com/iluwatar/Java8Holder.java

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+package com.iluwatar;
+
+import java.util.function.Supplier;
+
+/**
+ * 
+ * This lazy loader is thread safe and more efficient than HolderThreadSafe.
+ * It utilizes Java 8 functional interface Supplier<T> as Heavy factory.
+ *
+ */
+public class Java8Holder {
+	
+	private Supplier<Heavy> heavy = () -> createAndCacheHeavy();
+	
+	public Java8Holder() {
+		System.out.println("Java8Holder created");
+	}
+
+	public Heavy getHeavy() {
+		return heavy.get();
+	}
+	
+	private synchronized Heavy createAndCacheHeavy() {
+		class HeavyFactory implements Supplier<Heavy> {
+			private final Heavy heavyInstance = new Heavy();
+			@Override
+			public Heavy get() { return heavyInstance; }
+		}
+		if (!HeavyFactory.class.isInstance(heavy)) {
+			heavy = new HeavyFactory();
+		}
+		return heavy.get();
+	}
+}

lazy-loading/src/test/java/com/iluwatar/AppTest.java

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+package com.iluwatar;
+
+import org.junit.Test;
+
+public class AppTest {
+
+	@Test
+	public void test() {
+		String[] args = {};
+		App.main(args);
+	}
+}