Monday, 23 January 2017

What is an Immutable Class ? How to create Immutable class ? What are the advantage of Immutability ? Why is it important in multi-threading context ? Can you write a Custom immutable class ? What are semantics for writing an Immutable Class in java ?

In this article we will discuss  about following  topic
è What is an Immutable Class ?
è How to create Immutable class ?
è Write a Custom immutable class ?
è What are the advantage of  Immutability ?
è Why is it important in multi-threading context ?
è What are semantics for writing an Immutable Class in java ?

What is an Immutable Class ?
As per Oracle docs , An Immutable class is a class if its state of object  cannot change after it is constructed. It means any modification of  Immutable object will be another immutable object.  Since the state of the immutable objects can not be changed once they are created they are automatically synchronized/thread-safe.All wrapper classes in java.lang are immutable  String, Integer, Boolean, Character, Byte, Short, Long, Float, Double, BigDecimal, BigInteger.

Read this article :

Why is String immutable?


Example of  Immutable class:
·         java.lang.String
·         The wrapper classes for the primitive types: java.lang.Integer, java.lang.Byte, java.lang.Character, java.lang.Short, java.lang.Boolean, java.lang.Long, java.lang.Double, java.lang.Float
·         java.lang.StackTraceElement (used in building exception stacktraces)
·         Most enum classes are immutable, but this in fact depends on the concrete case. (Don't implement mutable enums, this will screw you up somewhen.) I think that at least all enum classes in the standard API are in fact immutable.
·         java.math.BigInteger and java.math.BigDecimal (at least objects of those classes themselves, subclasses could introduce mutability, though this is not a good idea)
·         java.io.File. Note that this represents an object external to the VM (a file on the local system), which may or may not exist, and has some methods modifying and querying the state of this external object. But the File object itself stays immutable. (All other classes in java.io are mutable.)
·         java.awt.Font - representing a font for drawing text on the screen (there may be some mutable subclasses, but this would certainly not be useful)
·         java.awt.BasicStroke - a helper object for drawing lines on graphic contexts
·         java.awt.Color - (at least objects of this class, some subclasses may be mutable or depending on some external factors (like system colors)), and most other implementations of java.awt.Paint .
How to create Immutable class ?
The steps to create a Immutable class  in Java  -
1)    Mark the class final .
2)    Mark the fields private and final.
3)    Set the values of properties of the class final and Private , i.e do not  use any setter methods,.
4)    Do not change the state of  the objects in any methods of  the class. If the instance fields includes references to mutable objects, don’t allow those objects to be changed.
5)    Don’t provide methods that modified the mutable objects.
6)    Don't share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.
Write a Custom immutable class ?

Example of  Immutable class in Java:

package Java91.blogspot.in;

public final class TestImmutableClass {
       private final Integer id;
       private final String name;
       private final String password;

       public TestImmutableClass(Integer id, String name, String password) {
              super();
              this.id = id;
              this.name = name;
              this.password = password;
       }
      
       public Integer getId() {
              return id;
       }

       public String getName() {
              return name;
       }

       public String getPassword() {
              return password;
       }
}
What are the advantage of  Immutability ?
1.     Immutable objects are thread-safe so you will not have any synchronization issues.
2.     Immutable objects are good Map keys and Set elements, since these typically do not change once created.
3.     Immutability makes it easier to write, use and reason about the code (class invariant is established once and then unchanged)
4.     Immutability makes it easier to parallelize your program as there are no conflicts among objects.
5.     The internal state of your program will be consistent even if you have exceptions.
6.     References to immutable objects can be cached as they are not going to change.
In Java the good programming practice is one should try to use immutable objects as far as possible. Immutability can have a performance cost, since when an object cannot be mutated we need to copy it if we want to write to it.
By Joshua Bloch
Classes should be immutable unless there's a very good reason to make them mutable....If a class cannot be made immutable, limit its mutability as much as possible.
Why is it important in multi-threading context ?
As we all know if an object is  Immutable , it is thread safe. But some scenario It is not thread safe . I am going to explain why Immutable class not fully thread safe .
Let look into the  String class code:
public class String {
       private final char value[];
       /** Cache the hash code for the string */
       private int hash; // Default to 0

       public String(char[] value) {
              this.value = Arrays.copyOf(value, value.length);
       }

       public int hashCode() {
              int h = hash;
              if (h == 0 && value.length > 0) {
                     char val[] = value;
                     for (int i = 0; i < value.length; i++) {
                           h = 31 * h + val[i];
                     }
                     hash = h;
              }
              return h;
       }
}
String is considered immutable. Looking at its implementation, we can deduce one thing: an immutable string can change its internal state (in this case, the hashcode which is lazy loaded) as long as it is not externally visible.Now I am going to rewrite the hashcode method in a non-thread safe way:
public int hashCode() {
    if (hash == 0 && value.length > 0) {
        char val[] = value;
        for (int i = 0; i < value.length; i++) {
            hash = 31 * hash + val[i];
        }
    }
    return hash;
}
As you can see, I have removed the local variable h and I have instead affected the variable hash directly. This implementation is NOT thread safe! If several threads call hashcode at the same time, the returned value could be different for each thread. The question is, 'is this class immutable?' Since two different threads can see a different hashcode, in an external point of view we have a change of state and so it is not immutable.We can so conclude that String is immutable because it is thread safe and not the opposite. So... What's the point of saying "Use an immutable object, it is thread-safe! But take care, you have to make your immutable object thread-safe!"?
What are semantics for writing an Immutable Class in java ?
1.     Immutable objects are thread-safe so you will not have any synchronization issues.
2.     Immutable objects are good Map keys and Set elements, since these typically do not change once created.
3.     Immutability makes it easier to write, use and reason about the code (class invariant is established once and then unchanged)
4.     Immutability makes it easier to parallelize your program as there are no conflicts among objects.
5.     The internal state of your program will be consistent even if you have exceptions.
6.     References to immutable objects can be cached as they are not going to change.

















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