I have some difficulties in my java code with regards to vectors. Please help

Question

In my code, After using setsize(12), why is the vec.capacity(); (on code line 18) giving the output of 20(please check the code output and tell me why is the new size 20) ?

And why trimtosize func does not remove the null elements?

https://code.sololearn.com/cpc9sofwase9/?ref=app

Answer 1

You start with an empty vector:
capacity = 10
size = 0

Then you add 5 elements:
capacity = 10
size = 5

Then you change the size to 12:
12 > 10 -> capacity increases
capazity = 20
size = 12

Then you trim capacity to size:
capacity = 12
size = 12

Do you now understand it?

Answer 2

Size and capacity are not the same.

Vector<Integer> vec = new Vector<>(30);

now print vec.size() and vec.capazity()
The vector is empty so size = 0. But capacity is 30.
Means, you have zero elements in the vector but could store 30 if you want.

setSize() changes the size.
trimToSize() trims the capazity of this vector to be the current size.

For example you create this Vector with a capacity of 30. But later there are only 5 elements in it, so you could trim the vector (capacity = 5).

Answer 3

But my question here is how is vec.capacity() on the 18th line of the code showing output of 20 when initial capacity is 10?

Answer 4

And in my code, if you see the output there are only 5 elements and the rest are null, so why doesn't trimtosize() trim the vector to 5 elements?

Answer 5

Denise Roßberg

Can you explain

Then you change the size to 12:
12 > 10 -> capacity increases
capazity = 20
size = 12

This👆
(why capacity becomes 20 and isn't 12+10=22) ?

Answer 6

Merlyn J

Because the capacity doubles.

The capacity at the beginning is 10. 12 elements does not fit in the vector, so the capacity gets 20. If you change size to 21 the capacity gets 40.

Another example: You start with a capacity of 12.
Now you change the size to 13.
The new capacity gets 24.
If you change the size to 25 the capacity gets 48 and so on...

Answer 7

https://www.javatpoint.com/java-vector may be helpful in this regard.

Answer 8

Merlyn J

About the 5 elements.
After changing the size to 12, your array is filled with null. Those are also elements. That's the reason why the size gets 12.