How to reverse the all element of ArrayList using reverse method?

Question

I have to make a method that can reverse all elements of ArrayList ... For exmaple, if my list have <12 1 34 56 43> elements. then it should reverse the whole list. <43 56 34 1 12>. I've tried this method, but it doesn't work. Here is the method

public void reverse() { 
int size=0;     
for (int i = 0, j = size - 1; i < size && j >= 0; i++, j--)     
{         
int temp = j;         
j = i;         
i = temp;         
}          
}

when I call this method list.revese(). it doesn't reverse the list. Can anyone please help me out!!!

Your method is not accessing any elements of the underlying list, only manipulating the indices. You need to use the get and set methods in order to actually modify the list. — Mick Mnemonic
– Mick Mnemonic, Commented Oct 4, 2015 at 23:59

Yibin Lin · Accepted Answer · 2015-10-05 00:03:08Z

3

Your code deals with the index but not the actual element of the index. Also note that if j < i, you are doing the reversing twice, thus there will be no effect.

public void reverse(List<Integer> list) { 
  int size=list.size();     
  for (int i = 0, j = size - 1; i < size && j >= 0 && i <= j; /** i must be smaller. */ i++, j--)     
  {         
    int temp = list.get(j);         
    list.set(j, list.get(i));         
    list.set(i,temp);         
  }          
}

edited Oct 5, 2015 at 0:03

answered Oct 5, 2015 at 0:00

Yibin Lin

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5 Comments

joe Over a year ago

I tried this using .get but actually I've to put this method in LList.java. which contains all method like append, remove, length, etc. but it doesn't contain the .get method so i got error "can not find the symbol"!!!! is there any other way to do this without using .get() method??

Yibin Lin Over a year ago

What is the LList class? Is it a customized class that you can modify the source code? In that case you need to get the element in that list (e.g. in an array).

joe Over a year ago

yes it is a customized class that can modify the source code.>

Yibin Lin Over a year ago

In that case if it is an array, you can replace the list.get(i) with something like arr[i].

Yibin Lin Over a year ago

hmm but <1,2,3,4,5> is not valid java code? Sorry I don't understand

Casey ScriptFu Pharr · Accepted Answer · 2015-10-04 23:55:41Z

2

Use the ArrayList object, and then after building your array you can just run the following to reverse the contents:

Collections.reverse(your ArrayList Here);

answered Oct 4, 2015 at 23:55

Casey ScriptFu Pharr

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1 Comment

Casey ScriptFu Pharr Over a year ago

Oh, then I will make the correct snippet for you, give me one second.

user3928919 · Accepted Answer · 2015-10-04 23:59:37Z

0

Initialize 'size' variable to the length of the ArrayList. You are initializing it to 0.

answered Oct 4, 2015 at 23:59

user3928919

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