Why are Java wrapper classes immutable?

Question

I know the usual reasons that apply to general immutable classes, viz

1. can not change as a side effect
2. easy to reason about their state
3. inhe

Answer 1

The wrapper classes are immutable because it just makes no sense to be mutable.

Consider following code:

int n = 5;
n = 6;
Integer N = new Integer(n);

At first, it looks straightforward if you can change the value of N, just like you can change the value of n.

But actually N is not a wrapper to n, but a wrapper to 6! Look at the following line again:

Integer N = new Integer(n);

You are actually passing the value of n, which is 6, to N. And since Java is pass-by-value, you cannot pass n into N, to make N a wrapper to n.

So, if we did add a set method to the wrapper:

Integer N = new Integer(n);
N.setValue(7);
print(N); // ok, now it is 7
print(n); // oops, still 6!

The value of n will not be changed and that will be confusing!

Conclusion:

1. wrapper classes are wrappers of values, not wrappers of the variables.

2. it will be confusing if you did add a set method.

3. if you know it is a wrapper of a value, you will no longer ask for a set method. For example, you will not do "6.setValue(7)".

4. it's impossible to make a wrapper to a variable in Java.