Are all final class immutable?

Question

Are all final classes in Java immutable. String and Integer both are final classes and both are immutable i beleive.

Answer 1

There is no keyword for immutability, it's more like a design pattern.

EDIT:
This means, there is no keyword, that makes a class immutable. To make a class immutable, you have to protect the internals by make them final or private.

The confusing thing is this: The final keyword has different meanings when used on a class then it has when used on a field/variable. The former means "this class can not be extended". The Second means "this variable (or reference) can not be changed".

Answer 2

No - a final class means you cannot inherit from it. It has nothing to do with mutability. The following class is final yet mutable:

public final class FinalMutable {
  int value;
  public void setValue(int v) { value=v; }
  public int getValue() { return value; }
}

Answer 3

Further to the other responses, if you look at the code for java.lang.String you'll see it contains a field: hash, which is mutable and is in fact computed and stored when hashCode() is called for the first time.

However, the class is still immutable: The hash field cannot be accessed directly or modified outside of the class.

Also, you may notice a common approach within the JDK is the implementation of immutable wrappers that can be used to expose an object's internal state without allowing it to be modified; e.g.

private final List<String> values;

public List<? get String> getValues() {
  return Collections.unmodifiableList(values);
}

Answer 4

A final immutable classes cannot be mutated. Here below it shows a non-final immutable class being mutated:

// a class not intended to be mutated 
public class GoodClass{

   private String name;

   public GoodClass() {
       this.name = "Good Class Neme";
   }

   public String getName() {
       return name;
   }
}

public class BadClass extends GoodClass {

   private String name;

   public String getName() {
       return name;
   }

   // mutating state
   public void setName(String name) {
       this.name = name;
   }
}

Answer 5

Final keywords prevents the other classes to inherit it. Final keyword doesn't make it immutable but there are such condition as making the instance class members private as well final and by using getters and not using setters

Answer 6

The private field can't be accessed by sub class's overridden methods...So there is no way the sub class methods change the private field of super class...Then what is the use of making the immutable class as final?