Creating Immutable Class

CreatingImmutableClass

How to create immutable class
=========================


**Immutable Classes** are those who's object once created it can't be change (or value can't change). So to create **Immutable Class** in java there are some simple that are listed below. 


> **Rules for immutable class:**

> - No **setter** should provide for the field.
> - All field should be **final** and **private**.
> - Make the class **final** as an immutable class should not be inherited or an another for this could be to make the constructor **private** and construct instances in factory methods.
> - If there is any field which is a **references to mutable objects** make sure that it should not be modified : 
		> > - Don't provide methods that modify the mutable objects.
		>> - Don't share references to the mutable objects. Always create a copy of the object and provide that for any operation.
		
#### <i class="icon-file"></i> Sample of Immutable Class
```java
import java.util.Date;

public final class ImmutableSample
{
 
    private final int intField; // Is value type 
    private final String stringField;// Is Immutable
    private final Date dateField;// Is mutable
 
    private ImmutableSample(int intField, String stringField, Date dateField){
        this.intField = intField;
        this.stringField = stringField;
        this.dateField = new Date(dateField.getTime());
    }
 
   
    public static ImmutableSample getInstance(int intField, String stringField, Date dateField){
        return new ImmutableSample(fld1, fld2, date);
    }
 
    //Provide no setter methods
 
    /**
    * int is value type variable, So the value in intField going change every 
    * */
    public int getIntField() {
        return intField;
    }
 
    /**
    * String class is also immutable so we can return the instance variable as it is
    * */
    public String getStringField() {
        return stringField;
    }
 
    /**
    * As Date field is mutable so we create a copy of dateField. So that modification will not effect dateField.
    * */
    public Date getDateField() {
        return new Date(dateField.getTime());
    }
 
    @Override
    public String toString() {
        return intField +" - "+ stringField +" - "+ dateField;
    }
}
```

#Some advantage of Immutable Object#

> - They are **thread-safe** so don't have and synchronization issues.
> - They can be best use in Map's key and Set's element.
> - It easier to parallelize your program as there are no conflicts among immutable objects.
> - They have their class invariant established once upon construction, and it never needs to be checked again