Add function composition example

Author: tboychuk

lambdas/src/main/java/com/bobocode/FunctionComposition.java

@@ -0,0 +1,44 @@
+package com.bobocode;
+
+import java.util.Objects;
+import java.util.function.Function;
+import java.util.function.IntUnaryOperator;
+import java.util.function.Predicate;
+
+/**
+ * One of functional programming techniques if a function composition. Having two different functions f(x) and g(x) in
+ * math you would write f(g(x)) to compose those function. The same you can do in Java using method
+ * {@link IntUnaryOperator#compose} or {@link IntUnaryOperator#andThen(IntUnaryOperator)}
+ * <p>
+ * In fact most predefined functional interfaces have similar capabilities.
+ * E.g. {@link java.util.function.Function#compose(Function)} or {@link Predicate#and(Predicate)}
+ */
+public class FunctionComposition {
+    public static void main(String[] args) {
+        printSquareOfDoubleUsingFunctionComposition();
+        printStringIsBlankUsingPredicateComposition();
+    }
+
+    private static void printSquareOfDoubleUsingFunctionComposition() {
+        IntUnaryOperator squareFunction = a -> a * a; // s(x) = x * x
+        IntUnaryOperator doublerFunction = a -> 2 * a; // d(x) = 2 * x
+
+        IntUnaryOperator squareOfDoubleFunction = squareFunction.compose(doublerFunction); // s(d(x))
+
+        System.out.println("square(double(3)) = " + squareOfDoubleFunction.applyAsInt(3));
+    }
+
+    private static void printStringIsBlankUsingPredicateComposition() {
+        Predicate<String> isEmptyPredicate = String::isEmpty;
+        Predicate<String> isNotNullPredicate = Objects::nonNull;
+
+        Predicate<String> isNotBlank = isNotNullPredicate.and(isEmptyPredicate.negate());
+
+        String str = "Hi";
+        System.out.println("String \"" + str + "\" is not blank? " + isNotBlank.test(str));
+
+
+    }
+
+
+}