Prob with ensureCapacity( ) in the context of vectors

User_AYF65

Hi,

I have made program:

import java.awt.*;

import javax.swing.*;

import java.util.Vector;

class testVector{

public static void main(String[] args) {

   Vector <Integer> v = new Vector<Integer>( ); //v1= [ ], size =0, capacity =10

   for (int j=1; j<= 5; j++)

      v.add(new Integer(j));

   // print them all back again

for (int i = 0; i < v.size(); i++)

JOptionPane.showMessageDialog(null,v.get(i));

// same thing with for-each

for (int a : v)

JOptionPane.showMessageDialog(null, a);

Vector <Integer> v2 = new Vector<Integer>(3,4);//capacity =3, increment=4

for(int j=4; j<=8;++j)

  v2.add(new Integer(j));

// print them all back again

for (int i = 0; i < v2.size(); i++)

JOptionPane.showMessageDialog(null,v2.get(i));

// same thing with for-each

for (int a : v2)

JOptionPane.showMessageDialog(null, a);

JOptionPane.showMessageDialog(null,"Cap="+v2.capacity());

v2.ensureCapacity(9);

JOptionPane.showMessageDialog(null,"Cap="+v2.capacity());

}

}

Its printinting capacity 11. I cant understand this.

for(int j=4; j<=8;++j)

  v2.add(new Integer(j));

I understand that after this loop capacity becomes 7 because of the increment argument in the constructor. But when we execute ensureCapacity(9) why it changes the capacity. We are not doing any insertion in the vector.

However, if i write:

v2.ensureCapacity(50)

capacity changes to 50.

Somebody please guide me.

Zulfi.

Unknown

  Vector <Integer> v2 = new Vector<Integer>(3,4);//capacity =3, increment=4

Its printinting capacity 11. I cant understand this.
for(int j=4; j<=8;++j)
  v2.add(new Integer(j));

I understand that after this loop capacity becomes 7 because of the increment argument in the constructor. But when we execute ensureCapacity(9) why it changes the capacity. We are not doing any insertion in the vector.

However, if i write:
v2.ensureCapacity(50)

capacity changes to 50.

The MINIMUM the capacity will increase is 4 because you used 'increment=4' in the constructor,

Since the capacity was 7 it becomes 11; 7 + 4 = 11.

When you use 'ensureCapacity(50)' that value is greater than the default increment so it is used. Java does NOT try to get to 50 by extending 4 at a time; that would be ridiculous.

See the Javadocs for Vector

https://docs.oracle.com/javase/7/docs/api/java/util/Vector.html

Each vector tries to optimize storage management by maintaining a capacity and a capacityIncrement. The capacity is always at least as large as the vector size; it is usually larger because as components are added to the vector, the vector's storage increases in chunks the size of capacityIncrement. An application can increase the capacity of a vector before inserting a large number of components; this reduces the amount of incremental reallocation. 

If you add items one at a time then Java will extend the capacity 4 elements at a time if it gets full.

But if you specify 'ensureCapacity' Java will extend the capacity by 4 or by the value you provide, whichever is larger.

User_AYF65

Hi,

Thanks for reply but i cant get answer to my question.

I have reduced the program to:

import java.awt.*;

import javax.swing.*;

import java.util.Vector;

class testVector2{

public static void main(String[] args) {

  

Vector <Integer> v2 = new Vector<Integer>(3,4);//capacity =3, increment=4

for(int j=4; j<=8;++j)

  v2.add(new Integer(j));

JOptionPane.showMessageDialog(null,"Cap="+v2.capacity());//prints capacity 7: right, no problem

v2.ensureCapacity(9);// prob with this statement

JOptionPane.showMessageDialog(null,"Cap="+v2.capacity());// prints capacity 11: Why not 9?

}

}

I have problem with the statement:

v2.ensureCapacity(9);

Before execution of this statement, capacity was 7. I understand this. But after execution of this statement capacity becomes 11. i cant understand this. Why Its not executing the statement:

v2.ensureCapacity(9);

However if i change this statement to:

v2.ensureCapacity(50);

it would make the capacity 50.  Why is

v2.ensureCapacity(9);

not working?? Somebody please guide me.

Zulfi.

Unknown

Thanks for reply but i cant get answer to my question.

Yes you can, and did.

I gave you the answer in my first reply. Read the ENTIRE reply again. Pay attention to this part:

The MINIMUM the capacity will increase is 4 because you used 'increment=4' in the constructor,

Since the capacity was 7 it becomes 11; 7 + 4 = 11.

When you use 'ensureCapacity(50)' that value is greater than the default increment so it is used. Java does NOT try to get to 50 by extending 4 at a time; that would be ridiculous.

Tell us specifically which of those three statements you don't understand.

Vector <Integer> v2 = new Vector<Integer>(3,4);//capacity =3, increment=4

That says make the MINIMUM INCREMENT = 4. That is whenever you need to increment add AT LEAST 4 to the capacity.

v2.ensureCapacity(9);// prob with this statement
JOptionPane.showMessageDialog(null,"Cap="+v2.capacity());// prints capacity 11: Why not 9?

Why not 9? Because the old capacity was 7 and the MINIMUM INCREMENT is 4. Focus on the word MINIMUM.

Vector <Integer> v2 = new Vector<Integer>(3,4);//capacity =3, increment=4

What is 7 + 4? Please post your answer.

Before execution of this statement, capacity was 7. I understand this. But after execution of this statement capacity becomes 11. i cant understand this.

You can't understand that 7 + 4 = 11? Not sure what to tell you. Let me try again:

Why not 9? Because the old capacity was 7 and the MINIMUM INCREMENT is 4. Focus on the word MINIMUM.

Did you see the word MINIMUM that time. If not read it a few more times.

Why Its not executing the statement:

v2.ensureCapacity(9);

IT DID EXECUTE THAT STATEMENT.

The capacity is now 11. That ENSURES ('ensureCapacity') that you can store at least 9 without extending any more.

11 is greater than 9. So if it can hold 11 it can hold 9.

The term 'ensureCapacity' means: make sure you can hold AT LEAST the number I tell you.

You told it to MAKE SURE it can hold 9. The MINIMUM INCREMENT is 4. The current capacity is 7.

Add 4 to 7 and you get 11. It is that simple.

The term 'ensureCapacity' does NOT mean set the capacity to the value I give you give it. That would be 'setCapacity'; there IS NO SUCH METHOD.

1. Your wife has 7 eggs in the refrigerator.

2. She needs 9 eggs to make three-egg omelets for you, her and your ugly rug rat.

3. The INCREMENT value for eggs is 12 - one dozen

4. How many eggs MUST YOU buy at the market if you want to buy eggs? (HINT: what is the value for INCREMENT?)

5. How many total eggs will you have after you return home from the market? (HINT: add 9 and 12)

It is REALLY SIMPLE.

There IS NO method to SET the capacity to any value you feel like.

User_AYF65

Hi,

I tried to focus on MINIMUM but i think word MAXIMUM is more appropriate here.

Now i would tell you what i understand.

At:

v2.ensureCapacity(9);

capacity is 7, so at this statement it would check whether 7+4> 9. If yes then capacity  would be set to 11

At:

v2.ensureCapacity(50);

capacity is 7, so at this statement it would check whether 7+4> 50 which is not true then capacity  would be set to 50.

Zulfi.