Merge pull request #11 from ralf-ueberfuhr-ars/feature/lambda

Add lambda sample

account-service-provider/src/main/java/de/sample/lambda/LambdaDemo.java

@@ -0,0 +1,122 @@
+package de.sample.lambda;
+
+import java.util.*;
+import java.util.function.*;
+
+public class LambdaDemo {
+
+  static double sum(double d1, double d2) {
+    return d1 + d2;
+  }
+
+  static double sum2() {
+    double result = 0.0;
+    while (result <= 10) {
+      result = result + Math.random();
+    }
+    return result;
+  }
+
+  @FunctionalInterface
+  interface NumberGenerator {
+    double nextDouble();
+  }
+
+  static double sum3(NumberGenerator numberGenerator) {
+    double result = 0.0;
+    while (result <= 10) {
+      result = result + numberGenerator.nextDouble();
+    }
+    return result;
+  }
+
+  public static void main(String[] args) {
+
+    // Addieren zweier Zahlen
+    System.out.println(sum(3.4, 2.5));
+    // Addieren zweier Zufallszahlen
+    System.out.println(sum(Math.random(), Math.random()));
+    System.out.println(sum(Math.random(), Math.random()));
+    // Addiere Zufallszahlen solange, bis die Summe>10
+    System.out.println(sum2());
+    System.out.println(sum2());
+    // Die Entscheidung, dass es Zufallszahlen sind, MUSS in main()-Methode fallen
+    // Der Algorithmus soll in der sum-Methode bleiben.
+    System.out.println(sum3(new NumberGenerator() {
+      @Override
+      public double nextDouble() {
+        return Math.random();
+      }
+    }));
+    System.out.println(sum3(new NumberGenerator() {
+      @Override
+      public double nextDouble() {
+        return 0.4;
+      }
+    }));
+    // Kurzschreibweise
+    // Sonderfall: Functional Interface
+    // Entfernen der Redundanzen für den Compiler
+    System.out.println(sum3(/*new NumberGenerator() {
+      @Override
+      public double nextDouble*/() -> {
+        return Math.random();
+      }
+      /*}*/));
+    System.out.println(sum3(() -> {
+        return Math.random();
+      }
+    ));
+    // Sonderfall: nur 1 Anweisung in nextDouble()
+    System.out.println(sum3(() -> Math.random()));
+    // Sonderfall: Parameter matchen: nextDouble() -> random()
+    System.out.println(sum3(Math::random));
+    // :: = Method Reference Operator
+
+    // Gibt es bereits Functional Interfaces?
+    Supplier<Double> s1 = Math::random;
+    System.out.println(s1.get());
+    DoubleSupplier ds = Math::random;
+    Supplier<Date> dateSupplier = Date::new;
+    Date d = dateSupplier.get();
+    System.out.println(ds.getAsDouble());
+    Consumer<String> c1 = System.out::println;
+    c1.accept("Hallo Welt");
+    Function<Integer, String> f1 = Integer::toHexString;
+    System.out.println(f1.apply(1467));
+    Predicate<String> p1 = s -> s.toLowerCase().startsWith("hallo");
+    System.out.println(p1.test("gelbekatze"));
+
+    // Anwendungsfälle: Streams, Optional
+    Collection<String> namen = List.of("Simon", "Oraz", "Silvia", "Hannes", "Klaus");
+    // Anforderung: gesucht sind alle kurzen Namen (<6) in Großbuchstaben
+    {
+      Collection<String> kurznamen = new LinkedList<>();
+      for(String name: namen) {
+        if(name.length()<6) {
+          kurznamen.add(name.toUpperCase());
+        }
+      }
+      System.out.println(kurznamen);
+    }
+    {
+      System.out.println(
+        namen.stream()
+          .filter(s -> s.length()<6)
+          .map(String::toUpperCase)
+          .toList()
+      );
+    }
+    Optional<String> ersterName = namen.stream()
+      .filter(s -> s.length() < 6)
+      .map(String::toUpperCase)
+      .findFirst();
+    Optional<Integer> i = ersterName.map(String::length);
+    Integer result = i.orElseThrow(IllegalStateException::new);
+
+    // https://www.baeldung.com/java-8-streams-introduction
+    // https://www.baeldung.com/java-optional
+
+  }
+
+}