BAEL-1464 Guava Memoizer (#3420)

* BAEL-1464 Guava Memoizer

* Update codes based on code review and discussion in JIRA

guava/src/main/java/org/baeldung/guava/memoizer/CostlySupplier.java

@@ -0,0 +1,17 @@
+package org.baeldung.guava.memoizer;
+
+import java.math.BigInteger;
+import java.util.Random;
+import java.util.concurrent.TimeUnit;
+
+public class CostlySupplier {
+
+    public static BigInteger generateBigNumber() {
+        try {
+            TimeUnit.SECONDS.sleep(2);
+        } catch (InterruptedException e) {
+        }
+        return new BigInteger("12345");
+    }
+
+}

guava/src/main/java/org/baeldung/guava/memoizer/Factorial.java

@@ -0,0 +1,22 @@
+package org.baeldung.guava.memoizer;
+
+import com.google.common.cache.CacheBuilder;
+import com.google.common.cache.CacheLoader;
+import com.google.common.cache.LoadingCache;
+
+import java.math.BigInteger;
+
+public class Factorial {
+
+    private static LoadingCache<Integer, BigInteger> memo = CacheBuilder.newBuilder()
+            .build(CacheLoader.from(Factorial::getFactorial));
+
+    public static BigInteger getFactorial(int n) {
+        if (n == 0) {
+            return BigInteger.ONE;
+        } else {
+            return BigInteger.valueOf(n).multiply(memo.getUnchecked(n - 1));
+        }
+    }
+
+}

guava/src/main/java/org/baeldung/guava/memoizer/FibonacciSequence.java

@@ -0,0 +1,26 @@
+package org.baeldung.guava.memoizer;
+
+import com.google.common.cache.CacheBuilder;
+import com.google.common.cache.CacheLoader;
+import com.google.common.cache.LoadingCache;
+
+import java.math.BigInteger;
+import java.util.concurrent.TimeUnit;
+
+public class FibonacciSequence {
+
+    private static LoadingCache<Integer, BigInteger> memo = CacheBuilder.newBuilder()
+            .maximumSize(100)
+            .build(CacheLoader.from(FibonacciSequence::getFibonacciNumber));
+
+    public static BigInteger getFibonacciNumber(int n) {
+        if (n == 0) {
+            return BigInteger.ZERO;
+        } else if (n == 1) {
+            return BigInteger.ONE;
+        } else {
+            return memo.getUnchecked(n - 1).add(memo.getUnchecked(n - 2));
+        }
+    }
+
+}