add examples in java 8 (#12)

* code for chapters 3-9 in Java

* code for chapters 1 in Java

* code for chapter 2 in Java

* missing CheckVoter for chapter 5 in Java

* add missing sample output for SetCovering as a comment

05_hash_tables/java/01_price_of_groceries/src/PriceOfGroceries.java

@@ -0,0 +1,17 @@
+import java.util.HashMap;
+import java.util.Map;
+
+public class PriceOfGroceries {
+
+    public static void main(String[] args) {
+        Map<String, Double> book = new HashMap<>();
+
+        // an apple costs 67 cents
+        book.put("apple", 0.67);
+        // milk costs $1.49
+        book.put("milk", 1.49);
+        book.put("avocado", 1.49);
+
+        System.out.println(book); // {apple=0.67, avocado=1.49, milk=1.49}
+    }
+}

05_hash_tables/java/02_check_voter/src/CheckVoter.java

@@ -0,0 +1,21 @@
+import java.util.HashMap;
+import java.util.Map;
+
+public class CheckVoter {
+    private static Map<String, Boolean> voted = new HashMap<>();
+
+    private static void checkVoter(String name) {
+        if (voted.containsKey(name)) {
+            System.out.println("kick them out!");
+        } else {
+            voted.put(name, true);
+            System.out.println("let them vote!");
+        }
+    }
+
+    public static void main(String[] args) {
+        checkVoter("tom"); // let them vote!
+        checkVoter("mike"); // let them vote!
+        checkVoter("mike"); // kick them out!
+    }
+}