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-rw-r--r--java.html.markdown103
1 files changed, 103 insertions, 0 deletions
diff --git a/java.html.markdown b/java.html.markdown
index ab2be4a2..18a3b21a 100644
--- a/java.html.markdown
+++ b/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