How will you write an immutable class?

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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 -> 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 an immutable class Right way to write an immutable class
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
After constructing Numbers are: 1 5
As you can see in the output that the “MyImmutable” object has been mutated.		 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
After constructing Numbers are: 1 2
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();
}

 

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