HashMap<String, Integer>; put get containsKey keySet(= k-v pair)

Week 3: Objects, files and exceptions/11a Hashmap

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+In addition to lists (and tables), a very useful data structure is a HASHMAP. 
+It closely resembles Python's scatter table (aka DICTIONARY), 
+the idea being that each element has a KEY and a VALUE - the value is determined by the key.
+
+The elements are in NO particular ORDER - there is no question of first, second or last element.
+Instead, we have a set of elements. 
+You can see this when you print the scatterplot - the elements can be PRINTED in ANY ORDER.
+
+
+
+Let's first look at an example of a hashmap initialization and usage:
+
+import java.util.HashMap;
+
+public class Example {
+    public static void main(String[] args){
+        HashMap<String, Integer> heights = new HashMap<>();
+        heights.put("Jack", 172);
+        heights.put("Jane", 169);
+        heights.put("Kim", 181);
+        heights.put("Karl", 158);
+
+        System.out.println(heights);
+    }
+}
+
+
+Program outputs:
+{Kim=181, Jack=172, Jane=169, Karl=158}
+ 
+
+
+
+==========================================================
+
+
+
+In the example, you will notice that 2 generic type specifications are given when defining a hashmap: KEY TYPE and VALUE TYPE. 
+
+A new element can be inserted using the PUT method. 
+The method takes as parameters the key and the value. 
+If the key is NOT FOUND in the table, a new element is INSERTed. 
+If the key already EXISTS, the value of the element is REPLACED.
+
+
+Similarly, the GET method can be used to return a value from a hashmap: the method takes the key as a parameter.
+
+
+HashMap<String, Integer> heights = new HashMap<>();
+heights.put("Jane", 169);
+heights.put("Karl", 158);
+
+System.out.println(heights.get("Karl"));
+System.out.println(heights.get("Jane"));
+
+
+// Karl grows
+heights.put("Karl", 161); //UPDATE
+System.out.println(heights.get("Karl"));
+
+
+
+Program outputs:
+158
+169
+161
+ 
+
+
+
+==========================================================
+
+
+
+
+If the get method does not find a value for the given key in the hashmap, it returns null, which is an empty value.
+
+
+
+
+The containsKey method can be used to test whether the given key can be found in the hashmap:
+
+HashMap<Integer, Double> squares = new HashMap<>();
+squares.put(9, 3.0);
+squares.put(4, 2.0);
+squares.put(16, 4.0);
+
+
+
+System.out.println(squares.get(9));
+System.out.println(squares.get(4));
+// this is not found
+System.out.println(squares.get(10)); //null
+
+System.out.println(squares.containsKey(16)); //true
+System.out.println(squares.containsKey(15)); //false
+
+
+Program outputs:
+3.0
+2.0
+null
+true
+false
+
+
+
+
+
+==========================================================
+
+
+
+Although a hashmap is primarily intended for situations where you know the key and can retrieve a value from it, 
+it is sometimes useful to iterate through all the elements of the table. 
+
+This is easily done with the keySet method:
+
+
+HashMap<Integer, Double> squares = new HashMap<>();
+squares.put(9, 3.0);
+squares.put(4, 2.0);
+squares.put(16, 4.0);
+squares.put(25, 5.0);
+
+for (int key : squares.keySet()) {
+    System.out.println(key + ": " + squares.get(key));
+}
+
+Program outputs:
+16: 4.0
+4: 2.0
+9: 3.0
+25: 5.0
+
+
+
+
+
+
+