What is a Dictionary in Java?

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section 25 Collections Frameworks/Map Interface/Dictionary In Java/src/Dictionary In Java.txt

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+What is a Dictionary in Java?
+
+In Java, there is no direct dictionary data type like in Python.
+However, a similar structure can be created using the `HashMap` class from the `java.util` package.
+
+- A `HashMap` stores key-value pairs.
+- **Key**: The unique identifier.
+- **Value**: The data associated with the key.
+
+Example:
+
+HashMap<String, Integer> studentAges = new HashMap<>();
+studentAges.put("Alice", 20);   // Key: "Alice", Value: 20
+studentAges.put("Bob", 22);     // Key: "Bob", Value: 22
+studentAges.put("Charlie", 19); // Key: "Charlie", Value: 19
+
+
+How to Iterate Over a Dictionary in Java
+----------------------------------------
+
+There are three main ways to iterate over a `HashMap`:
+
+
+1. Iterate Over Keys
+--------------------
+You can retrieve all the keys using the `keySet()` method and loop through them.
+
+import java.util.HashMap;
+public class Main {
+    public static void main(String[] args) {
+        HashMap<String, Integer> studentAges = new HashMap<>();
+        studentAges.put("Alice", 20);
+        studentAges.put("Bob", 22);
+        studentAges.put("Charlie", 19);
+
+        System.out.println("Keys:");
+        for (String key : studentAges.keySet()) {
+            System.out.println(key);
+        }
+    }
+}
+
+Output:
+Keys:
+Alice
+Bob
+Charlie
+
+
+2. Iterate Over Values
+----------------------
+You can retrieve all the values using the `values()` method and loop through them.
+
+import java.util.HashMap;
+public class Main {
+    public static void main(String[] args) {
+        HashMap<String, Integer> studentAges = new HashMap<>();
+        studentAges.put("Alice", 20);
+        studentAges.put("Bob", 22);
+        studentAges.put("Charlie", 19);
+
+        System.out.println("Values:");
+        for (Integer value : studentAges.values()) {
+            System.out.println(value);
+        }
+    }
+}
+
+Output:
+Values:
+20
+22
+19
+
+
+3. Iterate Over Key-Value Pairs
+-------------------------------
+You can retrieve both keys and values together using the `entrySet()` method.
+
+import java.util.HashMap;
+import java.util.Map;
+public class Main {
+    public static void main(String[] args) {
+        HashMap<String, Integer> studentAges = new HashMap<>();
+        studentAges.put("Alice", 20);
+        studentAges.put("Bob", 22);
+        studentAges.put("Charlie", 19);
+
+        System.out.println("Key-Value Pairs:");
+        for (Map.Entry<String, Integer> entry : studentAges.entrySet()) {
+            System.out.println(entry.getKey() + ": " + entry.getValue());
+        }
+    }
+}
+
+Output:
+Key-Value Pairs:
+Alice: 20
+Bob: 22
+Charlie: 19
+
+
+Step-by-Step Explanation of the Third Example
+---------------------------------------------
+- `entrySet()` → Returns a set of all key-value pairs in the HashMap.
+- `Map.Entry` → Represents a single key-value pair.
+- `for (Map.Entry<String, Integer> entry : studentAges.entrySet())` → Iterates through each pair.
+
+Inside the loop:
+- `entry.getKey()` → Access the key.
+- `entry.getValue()` → Access the value.
+
+
+Key Methods for Iterating a HashMap
+-----------------------------------
+Method       Purpose
+-----------  -------------------------------------------------
+keySet()     Returns all the keys in the map.
+values()     Returns all the values in the map.
+entrySet()   Returns all key-value pairs in the map.