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Immutable class means once an object is created, we cannot change it's content i.e.. State doesn't change.

• It is a thread safe
• can be cached.

In Java all the wrapper classes(like Boolean, Byte, Short etc) and String class is immutable.

Immutable class can be created by using below

• class must be declared as final(so that child classes can't be created).
• data members in the class must be declared as final(so that we can't change the value of it after object creation).
• Initialize all the data members using public parameterized constructor.
• getter methods for all variables in it.
• no setters(to not have an option to change the value of instance variable).
• make all fields private so that direct access is not allowed.
• return copy of mutable instance variable rather than returning original object.

Let's see sample code which is having only immutable members.

public final class Employee{
    private final String name;
    private final int empId;
    public Employee(String name, int empId) {
        this.name=name;
        this.empId=empId;
    }
    public String getName() {
            return name;
    }
    public int getEmpId() {
            return empId;
    }
}

Let's see sample code which is having both immutable and mutable data members.

public final class CustomImmutable {
    private final String sName;
    private final StringBuffer sbName;
    public CustomImmutable(String sName, StringBuffer sbName) {
        this.sName = sName;
        this.sbName = sbName;
    }
    public String getSName() {
        return sName;
    }
    public StringBuffer getsbName() { // return copy of mutable instance variable rather than returning original object.
        return new StringBuffer(sbName.toString());
    }
}

Let's see sample code

// Immutable class is nothing but content or state will not be changed
public final class EmployeeImmutableClass {

    private final String empName;
    private final long empID;
    private final long empSalary;
    private final Address address;

    public EmployeeImmutableClass(String empName, long empID, long empSalary,Address address) {
    this.empName = empName;
    this.empID = empID;
    this.empSalary = empSalary;
    this.address=address;
    }

    public String getEmpName() {
    return empName;
    }

    public long getEmpID() {
    return empID;
    }

    public long getEmpSalary() {
    return empSalary;
    }

    public Address getAddress() throws CloneNotSupportedException {
    return (Address) address.clone();
    }

    public static void main(String[] args) throws CloneNotSupportedException {
    EmployeeImmutableClass eic= new EmployeeImmutableClass("Praveen",149903L,1000000L,new Address(10001L,"Rangareddy","Hyderabad"));
    Address address=eic.getAddress();
    System.out.println(address); // Address [id=10001, desc=Rangareddy, location=Hyderabad]
    address.setLocation("Tirupati");
    address.setDesc("Railway Colony");
    System.out.println(eic.getAddress()); // Address [id=10001, desc=Rangareddy, location=Hyderabad]
    }

}

// This is an mutable class
public class Address  implements Cloneable{

    Long id;
    String desc;
    String location;
    public Address(Long id, String desc, String location) {
    super();
    this.id = id;
    this.desc = desc;
    this.location = location;
    }
    public void setId(Long id) {
    this.id = id;
    }
    public void setDesc(String desc) {
    this.desc = desc;
    }
    public void setLocation(String location) {
    this.location = location;
    }
    public Long getId() {
    return id;
    }
    public String getDesc() {
    return desc;
    }
    public String getLocation() {
    return location;
    }
    @Override
    public String toString() {
    return "Address [id=" + id + ", desc=" + desc + ", location=" + location + "]";
    }

    public Object clone()throws CloneNotSupportedException{
    return super.clone();
        }

}