String[] arrKeys = numbers.keySet().toArray(new String[0]); Arrays.sort(arrKeys);

Week 3: Objects, files and exceptions/21. Phone book 3: Listing numbers - Extract HASHMAP KEYS, then SORT

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+q21
+Write the method
+
+void listBook(HashMap<String,String> numbers
+
+which prints the users in the book and their numbers below them on the screen. 
+The names should be printed in alphabetical order.
+
+
+An example of how to call the method:
+public static void main(String[] args){
+    HashMap<String,String> numbers = new HashMap<>();
+    numbers.put("Tina", "12345");
+    numbers.put("Keith", "54321");
+    numbers.put("Jack", "55555");
+
+    listBook(numbers);
+}
+
+Program outputs:
+Name: Jack, number: 55555
+Name: Keith, number: 54321
+Name: Tina, number: 12345
+
+
+
+
+
+
+
+import java.util.Random;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Collections;
+import java.util.ArrayList;
+import java.util.Scanner;
+
+
+
+public class Test{
+    public static void main(String[] args){
+        final Random r = new Random();
+        
+        // <hide>
+        
+        HashMap<String, String> numbers = new HashMap<>();
+        numbers.put("Pete", "" + r.nextInt(1000000));
+        numbers.put("Lena", "" + r.nextInt(1000000));
+        numbers.put("Kim", "" + r.nextInt(1000000));
+        numbers.put("Laura", "" + r.nextInt(1000000));
+        numbers.put("Lisa", "" + r.nextInt(1000000));
+        numbers.put("Zorro", "" + r.nextInt(1000000));
+        numbers.put("Tarzan", "" + r.nextInt(1000000));
+
+        listBook(numbers);
+    }
+    
+
+
+    //q21
+    public static void listBook(HashMap<String,String> numbers) {
+        // https://stackoverflow.com/questions/9047090/how-to-sort-hashmap-keys
+        // https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html#keySet--
+        // sort the HashMap by key
+        // String[] arrKeys = numbers.keySet().toArray();              //incorrect
+
+        // new String[0] is an empty array of type String -> ie we want String[] array, even if empty array
+        String[] arrKeys = numbers.keySet().toArray(new String[0]);    //correct
+        // sort inplace
+        Arrays.sort(arrKeys);
+
+
+
+        // correct option 2 - extract keySet as ArrayList, then use Collection to sort
+        // Convert keySet to a List and sort it
+        // ArrayList<String> arrKeys = new ArrayList<>(numbers.keySet());
+        // Collections.sort(arrKeys);
+
+
+        
+
+        // print the sorted HashMap
+        for (String key : arrKeys) {
+            System.out.println("Name: " + key + ", number: " + numbers.get(key));
+        }
+    }
+
+
+
+
+}
+
+
+
+
+
+Name: Kim, number: 701584
+Name: Laura, number: 417806
+Name: Lena, number: 627033
+Name: Lisa, number: 506742
+Name: Pete, number: 292799
+Name: Tarzan, number: 297377
+Name: Zorro, number: 538240
+
+
+
+