add examples for creating Function

src/section23/FunctionalInterfaces.java

@@ -0,0 +1,46 @@
+package section23;
+
+import java.util.function.DoubleToIntFunction;
+import java.util.function.Function;
+import java.util.function.IntUnaryOperator;
+
+public class FunctionalInterfaces {
+
+    static void functionExamples() {
+        // All of the following parse a String into Integer
+        Function<String, Integer>
+                parse1 = s -> Integer.parseInt(s),          // lambda expression
+                parse2 = (String s) -> Integer.parseInt(s), // lambda expression with explicit typing
+                parse3 = Integer::parseInt,                 // method reference to Integer.parseInt(String s)
+                parse4 = Integer::new;                      // method reference to constructor of Integer
+
+        // All of the following get the length of String
+        Function<String, Integer>
+                len1 = s -> s.length(),                     // lambda expression
+                len2 = String::length,                      // method reference to String#length
+                len3 = new Function<String, Integer>() {    // explicit implementation of Function interface
+                    public Integer apply(String s) {
+                        return s.length();
+                    }
+                };
+
+        // Function objects can be invoked with .apply()
+        print(parse1.apply("123456"));
+        print(len3.apply("foobar42"));
+
+        // Functions may be chained with andThen and compose
+        Function<String, String> repeatString = s -> s + s;
+        print(parse2.compose(repeatString).apply("13579")); // invoke repeatString and then parse2
+
+        Function<Integer, Integer> squaredValue = n -> n * n;
+        print(len2.andThen(squaredValue).apply("java10"));  // invoke len2 and then squaredValue
+    }
+
+    public static void main(String[] argv) {
+        functionExamples();
+    }
+
+    private static void print(Object o) {
+        System.out.println(o);
+    }
+}