StringBuffer array fails where String works

Question

I am trying to understand the underlying difference between initializing Strings and StringBuffer The following code works fine. (al is an ArrayList<String>)

String[] sa = new String[al.size()];
System.arraycopy(al.toArray(), 0, sa, 0, al.size());

However when i use StringBuffer array it wont work.

StringBuffer[] sa = new StringBuffer[al.size()];
System.arraycopy(al.toArray(), 0, sa, 0, al.size());

It gives me following exception

java.lang.ArrayStoreException
at java.lang.System.arraycopy(Native Method)
at practice.ArrayListDemo.main(ArrayListDemo.java:34)

Can someone help me understand the logic/reason? It is not clear how string being immutable makes a difference.

Immutability has nothing to do with it. You're trying to mix array types. — chrylis -cautiouslyoptimistic-
– chrylis -cautiouslyoptimistic-, Commented Sep 20, 2014 at 23:22

Peter Pei Guo · Accepted Answer · 2014-09-20 23:18:57Z

3

How did you define al? Is it with the right type - StringBuffer? When I did the following there was no problem:

    List<StringBuffer> al = new ArrayList<StringBuffer>();
    StringBuffer[] sa = new StringBuffer[al.size()];
    System.arraycopy(al.toArray(), 0, sa, 0, al.size());

answered Sep 20, 2014 at 23:18

Peter Pei Guo

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1 Comment

Madhav Bissa Over a year ago

no, i made a mistake. my arraylist was initialised with String.

Sotirios Delimanolis · Accepted Answer · 2014-09-20 23:22:05Z

2

Since you've declared al as an ArrayList<String>, we can safely assume that you've stored String objects in it.

But here

StringBuffer[] sa = new StringBuffer[al.size()];
System.arraycopy(al.toArray(), 0, sa, 0, al.size());

you're trying to copy String elements in a StringBuffer array. That doesn't work since String is not a subtype of StringBuffer.

answered Sep 20, 2014 at 23:22

Sotirios Delimanolis

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2 Comments

Madhav Bissa Over a year ago

yes, thanks a lot. i just understood the exception is thrown by the arraycopy method for type mismatch.

Azar Over a year ago

+1 This is the only answer that provides the actual reasoning behind the solution.

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