Merge pull request #3112 from smhanes15/java-en-lambdas

[Java/en] Added Lambdas section.

java.html.markdown

@@ -11,6 +11,7 @@ contributors:
     - ["Michael Dähnert", "https://github.com/JaXt0r"]
     - ["Rob Rose", "https://github.com/RobRoseKnows"]
     - ["Sean Nam", "https://github.com/seannam"]
+    - ["Shawn M. Hanes", "https://github.com/smhanes15"]
 filename: LearnJava.java
 ---
 
@@ -858,6 +859,108 @@ public class EnumTest {
 // The enum body can include methods and other fields.
 // You can see more at https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html
 
+// Getting Started with Lambda Expressions
+//
+// New to Java version 8 are lambda expressions. Lambdas are more commonly found
+// in functional programming languages, which means they are methods which can
+// be created without belonging to a class, passed around as if it were itself
+// an object, and executed on demand.
+//
+// Final note, lambdas must implement a functional interface. A functional
+// interface is one which has only a single abstract method declared. It can
+// have any number of default methods. Lambda expressions can be used as an
+// instance of that functional interface. Any interface meeting the requirements
+// is treated as a functional interface. You can read more about interfaces
+// above.
+//
+import java.util.Map;
+import java.util.HashMap;
+import java.util.function.*;
+import java.security.SecureRandom;
+
+public class Lambdas {
+    public static void main(String[] args) {
+        // Lambda declaration syntax:
+	// <zero or more parameters> -> <expression body or statement block>
+
+        // We will use this hashmap in our examples below.
+        Map<String, String> planets = new HashMap<>();
+            planets.put("Mercury", "87.969");
+            planets.put("Venus", "224.7");
+            planets.put("Earth", "365.2564");
+            planets.put("Mars", "687");
+            planets.put("Jupiter", "4,332.59");
+            planets.put("Saturn", "10,759");
+            planets.put("Uranus", "30,688.5");
+            planets.put("Neptune", "60,182");
+
+        // Lambda with zero parameters using the Supplier functional interface
+        // from java.util.function.Supplier. The actual lambda expression is
+        // what comes after numPlanets =.
+        Supplier<String> numPlanets = () -> Integer.toString(planets.size());
+        System.out.format("Number of Planets: %s\n\n", numPlanets.get());
+
+        // Lambda with one parameter and using the Consumer functional interface
+        // from java.util.function.Consumer. This is because planets is a Map,
+        // which implements both Collection and Iterable. The forEach used here,
+        // found in Iterable, applies the lambda expression to each member of
+        // the Collection. The default implementation of forEach behaves as if:
+        /*
+            for (T t : this)
+                action.accept(t);
+        */
+
+        // The actual lambda expression is the parameter passed to forEach.
+        planets.keySet().forEach((p) -> System.out.format("%s\n", p));
+
+        // If you are only passing a single argument, then the above can also be
+        // written as (note absent parentheses around p):
+        planets.keySet().forEach(p -> System.out.format("%s\n", p));
+
+        // Tracing the above, we see that planets is a HashMap, keySet() returns
+        // a Set of its keys, forEach applies each element as the lambda 
+        // expression of: (parameter p) -> System.out.format("%s\n", p). Each
+        // time, the element is said to be "consumed" and the statement(s)
+        // referred to in the lambda body is applied. Remember the lambda body
+        // is what comes after the ->.
+
+        // The above without use of lambdas would look more traditionally like:
+        for (String planet : planets.keySet()) {
+            System.out.format("%s\n", planet);
+        }
+
+        // This example differs from the above in that a different forEach
+        // implementation is used: the forEach found in the HashMap class
+        // implementing the Map interface. This forEach accepts a BiConsumer,
+        // which generically speaking is a fancy way of saying it handles
+        // the Set of each Key -> Value pairs. This default implementation
+        // behaves as if:
+        /*
+            for (Map.Entry<K, V> entry : map.entrySet())
+                action.accept(entry.getKey(), entry.getValue());
+        */
+
+        // The actual lambda expression is the parameter passed to forEach.
+        String orbits = "%s orbits the Sun in %s Earth days.\n";
+        planets.forEach((K, V) -> System.out.format(orbits, K, V));
+
+        // The above without use of lambdas would look more traditionally like:
+        for (String planet : planets.keySet()) {
+            System.out.format(orbits, planet, planets.get(planet));
+        }
+
+        // Or, if following more closely the specification provided by the
+        // default implementation:
+        for (Map.Entry<String, String> planet : planets.entrySet()) {
+            System.out.format(orbits, planet.getKey(), planet.getValue());
+        }
+
+        // These examples cover only the very basic use of lambdas. It might not
+        // seem like much or even very useful, but remember that a lambda can be
+        // created as an object that can later be passed as parameters to other
+        // methods.
+    }
+}
 ```
 
 ## Further Reading