How to make normal java class as immutable

public final class ImmutableClass

{

 

    /**

    * Integer class is immutable as it does not provide any setter to change its content

    * */

    private final Integer immutableField1;

    /**

    * String class is immutable as it also does not provide setter to change its content

    * */

    private final String immutableField2;

    /**

    * Date class is mutable as it provide setters to change various date/time parts

    * */

    private final Date mutableField;

 

    //Default private constructor will ensure no unplanned construction of class

    private ImmutableClass(Integer fld1, String fld2, Date date)

    {

        this.immutableField1 = fld1;

        this.immutableField2 = fld2;

        this.mutableField = new Date(date.getTime());

    }

 

    //Factory method to store object creation logic in single place

    public static ImmutableClass createNewInstance(Integer fld1, String fld2, Date date)

    {

        return new ImmutableClass(fld1, fld2, date);

    }

 }