如何清除Java中的ArrayList？

Question

The following iterate over an ArrayList :

for (String[] line : dataList) {
//adding items in the list here
    item.add(line[0]);

  if(dataList.isEmpty()){ //attempting to clear dataList when it has iterated through to the last element but not working as expected
    dataList.clear()
   }
}

How can I clear the elements in the ArrayList after I have iterater over its elements?

Answer 1

Remove the if(dataList.isEmpty()) block and clear the list after the loop. Iterating does not remove items.

The desired code is:

for (String[] line : dataList) {
    item.add(line[0]);
}
dataList.clear();

Answer 2

It should be something like,

for (String[] line : dataList) {
    item.add(line[0]);
}

dataList.clear();

Answer 3

As others point out:

for (String[] line : dataList) {
    item.add(line[0]);
}
dataList.clear();

Will clear the list. If it's already empty, that's fine. clear() will do nothing and return normally.

I've popped an answer in because you might wonder if you need to clear it at all. If dataList goes out of scope at the end of the method then the garbage collector will clear up for you.

If you're 'reusing' datalist in subsequent code then the best answer is - don't. Declare a local variable at the start of the loop. It's certainly cleaner and often faster to just keep creating new objects rather than recycling.

NB: There could be circumstances where clearing the list is appropriate. If it's a class field that will continue beyond the method. However if it is the use as a temporary store seems inappropriate.