JDK 8 code review request for 7140820 Add covariant overrides to Collections clone methods (original) (raw)

Tom Hawtin tom.hawtin at oracle.com
Mon Jan 30 13:28:07 UTC 2012

On 30/01/2012 13:16, Ulf Zibis wrote:

Isn't cloning faster than normal instantiation? I can imagine, that behind the scenes cloning mainly only needs to duplicate the binary footprint of an object.

I don't see a good reason why it should be (equally, I've not tried benchmarking).

For the immediate fields of an object, (partial) bitwise copying "by hand" should be of comparable performance to a bitwise clone. For copying the referenced objects, there is no benefit for the clone.

Implementation of HashMap.clone is:

     HashMap<K,V> result = null;
     try {
         result = (HashMap<K,V>)super.clone();
     } catch (CloneNotSupportedException e) {
         // assert false;
     }
     result.table = new Entry[table.length];
     result.entrySet = null;
     result.modCount = 0;
     result.size = 0;
     result.init();
     result.putAllForCreate(this);

     return result;

The "copy constructor" is slightly different in that it uses the default load factor (and minimum size) instead of copying.

     this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                   DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
     putAllForCreate(m);

Tom

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