Writing an immutable class is generally easy but there can be some tricky situations. Follow the following guidelines:
1. A class is declared final (i.e. final classes cannot be extended).
public final class MyImmutable { ? }
2. All its fields are final (final fields cannot be mutated once assigned).
private final int[] myArray; //do not declare as Æ private final int[] myArray = null;
3. Do not provide any methods that can change the state of the immutable object in any way ? not just setXXX
methods, but any methods which can change the state.
4. The ?this? reference is not allowed to escape during construction from the immutable class and the immutable
class should have exclusive access to fields that contain references to mutable objects like arrays, collections
and mutable classes like Date etc by:
Declaring the mutable references as private.
Not returning or exposing the mutable references to the caller (this can be done by defensive copying)
Wrong way to write a constructor:
public final class MyImmutable {
private final int[] myArray;
public MyImmutable(int[] anArray) {
this.myArray = anArray; // wrong
}
public String toString() {
StringBuffer sb = new StringBuffer("Numbers are: ");
for (int i = 0; i < myArray.length; i++) {
sb.append(myArray[i] + " ");
}
return sb.toString();
}
}
// the caller could change the array after calling the
constructor.
int[] array = {1,2};
MyImmutable myImmutableRef = new MyImmutable(array) ;
System.out.println("Before constructing " + myImmutableRef);
array[1] = 5; // change (i.e. mutate) the element
System.out.println("After constructing " + myImmutableRef);
Out put:
Before constructing Numbers are: 1 2
Right way is to copy the array before assigning in the constructor.
public final class MyImmutable {
private final int[] myArray;
public MyImmutable(int[] anArray) {
this.myArray = anArray.clone(); // defensive copy
}
public String toString() {
StringBuffer sb = new StringBuffer("Numbers are: ");
for (int i = 0; i < myArray.length; i++) {
sb.append(myArray[i] + " ");
}
return sb.toString();
}
}
// the caller cannot change the array after calling the constructor.
int[] array = {1,2};
MyImmutable myImmutableRef = new MyImmutable(array) ;
System.out.println("Before constructing " + myImmutableRef);
array[1] = 5; // change (i.e. mutate) the element
System.out.println("After constructing " + myImmutableRef);
Out put:
Before constructing Numbers are: 1 2
As you can see in the output that the ?MyImmutable? object
has been mutated. This is because the object reference gets
After constructing Numbers are: 1 2
As you can see in the output that the ?MyImmutable? object has not
been mutated.
As you can see in the output that the ?MyImmutable? object has not
been mutated.
Wrong way to write an accessor. A caller could get the array
reference and then change the contents:
public int[] getArray() {
return myArray;
}
Right way to write an accessor by cloning.
public int[] getAray() {
return (int[]) myArray.clone();
}
Concept : Beware of using the clone() method on a collection like a Map, List, Set etc because they are not only difficult
to implement but also the default behavior of an object?s clone() method automatically
yields a shallow copy. You have to deep copy the mutable objects referenced by your immutable class.
section for deep vs. shallow cloning and for why you will be modifying the original object if you do not
deep copy.
