CODEWITHSUNDEEP
javaarraylist
Hager Khaled
Hager Khaled

Reputation: 84

clear arraylist in java

i try to make a 2D arrayList when i accept data and store it into a single arraylist then added it to the 2d arraylist so i clear the single array list in every loop and save the new row the problem in clearing data it clear all bast row and just save the newest one enter code here

public class main6 {

    public static void main(String[] args) throws IOException {
        Scanner scan = new Scanner(System.in);
        InputStreamReader in = new InputStreamReader(System.in);
        BufferedReader b = new BufferedReader(in);
        int arrCol= scan.nextInt();     
        ArrayList<ArrayList<String>> arr = new ArrayList<ArrayList<String>>();
        ArrayList<String> x = new ArrayList<String>();
        
    for(int i = 0 ; i < arrCol ; i++) { 
        
        
        x.addAll(Arrays.asList(b.readLine().split("\\s"))); 
        arr.add(i,x);   
        x.clear();
    } 
         System.out.println(Arrays.deepToString(arr.toArray()));
    }

}

Upvotes: 0

Views: 89

Answers (3)

WJS
WJS

Reputation: 40057

A couple pf things.

  • Best practice is to assign to the interface type in lieu of the implementation. And you're declaring your "2D" ArrayList incorrectly. So
    List<List<String>> arr = new ArrayList<>();
  • No need to use an auxiliary List. Just do it as follows:
    for(int i = 0 ; i < arrCol ; i++) {         
        arr.add(Arrays.asList(b.readLine().split("\\s")));
    }
  • If you still want to create an explicit ArrayList<> implementation, then pass the output of Arrays.asList as an argument. No need to use add or addAll.
    for(int i = 0 ; i < arrCol ; i++) {         
        arr.add(new ArrayList<>(Arrays.asList(b.readLine().split("\\s"))));
    }
  • No need to convert to an array and do deep string conversion. Lists don't need it.
    System.out.println(arr);

Note: One exception to to the first bullet. Sometimes, a List implementation has more functionality in terms of methods that are not in the List interface. In that case, if you want that functionality, then you need to assign to the implementation type rather than the interface type. With List interfaces, I cannot recall having to do that.

Upvotes: 1

Aneesh
Aneesh

Reputation: 148

Update the for loop to instantiate the array list:

for(int i = 0 ; i < arrCol ; i++) { 
    
    ArrayList<String> x = new ArrayList<>();
    x.addAll(Arrays.asList(b.readLine().split("\\s"))); 
    arr.add(i,x);   
   
}

This way, the array list will be recreated for each loop and there is no need to clear it.

You can then remove the instantiation of the arraylist in the method body before the for-loop:

ArrayList<String> x = new ArrayList<String>();

Upvotes: 0

Ecto
Ecto

Reputation: 1147

Create the list in the for loop, so you have new instance for every addition:

for(int i = 0 ; i < arrCol ; i++) { 
    ArrayList<String> x = new ArrayList<String>();
    ...

... also, don't clear it afterwards...

Upvotes: 0

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