What is Immutable class in java

How to create Immutable class in Java?

• once the object is created we cannot modify the content.
• Example: Wrapper classes (Integer,Boolean,Byte,Short,String)

Properties

1. class --> final --> child classes can't be created
2. data members --> private, and final
3. parameterized contructor will initialize all the fields
4. no setters --> since we don;t want anyone to change the value
5. only getters

[[What is the use of immutable class]].

// Java Program to Create An Immutable Class

// Importing required classes
import java.util.HashMap;
import java.util.Map;

// Class 1
// An immutable class
final class Student {

    // Member attributes of final class
    private final String name;
    private final int regNo;
    private final Map<String, String> metadata;

    // Constructor of immutable class
    // Parameterized constructor
    public Student(String name, int regNo,
                Map<String, String> metadata)
    {

        // This keyword refers to current instance itself
        this.name = name;
        this.regNo = regNo;

        // Creating Map object with reference to HashMap
        // Declaring object of string type
        Map<String, String> tempMap = new HashMap<>();

        // Iterating using for-each loop
        for (Map.Entry<String, String> entry :
            metadata.entrySet()) {
            tempMap.put(entry.getKey(), entry.getValue());
        }

        this.metadata = tempMap;
    }

    // Method 1
    public String getName() { return name; }

    // Method 2
    public int getRegNo() { return regNo; }

    // Note that there should not be any setters

    // Method 3
    // User -defined type
    // To get meta data
    public Map<String, String> getMetadata()
    {

        // Creating Map with HashMap reference
        Map<String, String> tempMap = new HashMap<>();

        for (Map.Entry<String, String> entry :
            this.metadata.entrySet()) {
            tempMap.put(entry.getKey(), entry.getValue());
        }
        return tempMap;
    }
}

// Class 2
// Main class
class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating Map object with reference to HashMap
        Map<String, String> map = new HashMap<>();

        // Adding elements to Map object
        // using put() method
        map.put("1", "first");
        map.put("2", "second");

        Student s = new Student("ABC", 101, map);

        // Calling the above methods 1,2,3 of class1
        // inside main() method in class2 and
        // executing the print statement over them
        System.out.println(s.getName());
        System.out.println(s.getRegNo());
        System.out.println(s.getMetadata());

        // Uncommenting below line causes error
        // s.regNo = 102;

        map.put("3", "third");
        // Remains unchanged due to deep copy in constructor
        System.out.println(s.getMetadata());
        s.getMetadata().put("4", "fourth");
        // Remains unchanged due to deep copy in getter
        System.out.println(s.getMetadata());
    }
}