Added guava set+function=map tutorial and tests

guava/src/test/java/org/baeldung/guava/GuavaMapFromSet.java

@@ -0,0 +1,101 @@
+package org.baeldung.guava;
+
+import java.util.AbstractMap;
+import java.util.AbstractSet;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Set;
+import java.util.WeakHashMap;
+
+import com.google.common.base.Function;
+
+public class GuavaMapFromSet<K, V> extends AbstractMap<K, V> {
+
+	private class SingleEntry implements Entry<K, V> {
+		private  K key;
+
+		public SingleEntry( K key) {
+			this.key = key;
+		}
+
+		@Override
+		public K getKey() {
+			return this.key;
+		}
+
+		@Override
+		public V getValue() {
+			V value = GuavaMapFromSet.this.cache.get(this.key);
+			if (value == null) {
+				value = GuavaMapFromSet.this.function.apply(this.key);
+				GuavaMapFromSet.this.cache.put(this.key, value);
+			}
+			return value;
+		}
+
+		@Override
+		public V setValue( V value) {
+			throw new UnsupportedOperationException();
+		}
+	}
+
+	private class MyEntrySet extends AbstractSet<Entry<K, V>> {
+
+		public class EntryIterator implements Iterator<Entry<K, V>> {
+			private  Iterator<K> inner;
+
+			public EntryIterator() {
+				this.inner = MyEntrySet.this.keys.iterator();
+			}
+
+			@Override
+			public boolean hasNext() {
+				return this.inner.hasNext();
+			}
+
+			@Override
+			public Map.Entry<K, V> next() {
+				 K key = this.inner.next();
+				return new SingleEntry(key);
+			}
+
+			@Override
+			public void remove() {
+				throw new UnsupportedOperationException();
+			}
+		}
+
+		private  Set<K> keys;
+
+		public MyEntrySet( Set<K> keys) {
+			this.keys = keys;
+		}
+
+		@Override
+		public Iterator<Map.Entry<K, V>> iterator() {
+			return new EntryIterator();
+		}
+
+		@Override
+		public int size() {
+			return this.keys.size();
+		}
+
+	}
+
+	private  WeakHashMap<K, V> cache;
+	private  Set<Entry<K, V>> entries;
+	private  Function<? super K, ? extends V> function;
+
+	public GuavaMapFromSet( Set<K> keys, Function<? super K, ? extends V> function) {
+		this.function = function;
+		this.cache = new WeakHashMap<K, V>();
+		this.entries = new MyEntrySet(keys);
+	}
+
+	@Override
+	public Set<Map.Entry<K, V>> entrySet() {
+		return this.entries;
+	}
+
+}

guava/src/test/java/org/baeldung/guava/GuavaMapFromSetTests.java

@@ -0,0 +1,66 @@
+package org.baeldung.guava;
+
+import static org.junit.Assert.assertTrue;
+
+import java.util.Arrays;
+import java.util.Map;
+import java.util.Set;
+import java.util.TreeSet;
+
+import org.junit.Test;
+
+import com.google.common.base.Function;
+
+public class GuavaMapFromSetTests {
+
+	@Test
+	public void givenStringSet_whenMapsToElementLength_thenCorrect() {
+		Function<Integer, String> function = new Function<Integer, String>() {
+
+			@Override
+			public String apply(Integer from) {
+				return Integer.toBinaryString(from.intValue());
+			}
+		};
+		Set<Integer> set = (Set<Integer>) new TreeSet<Integer>(Arrays.asList(
+				32, 64, 128));
+		Map<Integer, String> map = new GuavaMapFromSet<Integer, String>(set,
+				function);
+		assertTrue(map.get(32).equals("100000")
+				&& map.get(64).equals("1000000")
+				&& map.get(128).equals("10000000"));
+	}
+
+	@Test
+	public void givenIntSet_whenMapsToElementBinaryValue_thenCorrect() {
+		Function<String, Integer> function = new Function<String, Integer>() {
+
+			@Override
+			public Integer apply(String from) {
+				return from.length();
+			}
+		};
+		Set<String> set = (Set<String>) new TreeSet<String>(Arrays.asList(
+				"four", "three", "twelve"));
+		Map<String, Integer> map = new GuavaMapFromSet<String, Integer>(set,
+				function);
+		assertTrue(map.get("four") == 4 && map.get("three") == 5
+				&& map.get("twelve") == 6);
+	}
+	@Test
+	public void givenSet_whenNewSetElementAddedAndMappedLive_thenCorrect() {
+		Function<String, Integer> function = new Function<String, Integer>() {
+
+			@Override
+			public Integer apply(String from) {
+				return from.length();
+			}
+		};
+		Set<String> set = (Set<String>) new TreeSet<String>(Arrays.asList(
+				"four", "three", "twelve"));
+		Map<String, Integer> map = new GuavaMapFromSet<String, Integer>(set,
+				function);
+		set.add("one");
+		assertTrue(map.get("one") == 3 && map.size()==4);
+	}
+}