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javaperformancelistarraylistnew-operator

Java clearing a list by creating a new instance

I was looking through Java code, and I came across this code:

list = new ArrayList();
uselist(list);

if (condition)
   list = new ArrayList();

What's the use of this, as opposed to simply using the clear() method of ArrayLists.

is using the new command to clear a list is ok and is it faster than clearing a list ?

i am using java version 1.6

Solution

  • Do note that clearing and re-instantiating a list is not the same thing!

    Consider this example:

    a = new ArrayList();
a.add("Hello")
b = a;
a = new ArrayList();   // a is now empty while b contains hello!

    Versus

    a = new ArrayList();
a.add("Hello")
b = a;
a.clear();            // Both a and b are now empty.

    If the side-effects (shared references) are not an issue, then it is just two ways of clearing a list. It should probably not be a performance issue unless this is called millions of times.