Emptying arraylist in loop

Question

How can I empty the attributeList every time it has "added" the value to largeAttributeList?. I've tried .clear() but then the largeAttributeList loses all the values.

ArrayList<String> attributeList = new ArrayList<String>();
ArrayList<ArrayList<String>> largeAttributeList = new 
ArrayList<ArrayList<String>>();

for (int i = 0; i < attribute.getLength(); i++) {
        String current = attribute.item(i).getTextContent();
        if(current.equals("Identifier")){
            largeAttributeList.add(attributeList);
        }
        else{
            attributeList.add(current);
        }
    }

Examples of your input vs expected output?

sameera sy
– sameera sy

2017-05-31 10:47:06 +00:00
Commented May 31, 2017 at 10:47 — sameera sy
– sameera sy, Commented May 31, 2017 at 10:47

Youcef LAIDANI · Accepted Answer · 2017-05-31 11:01:42Z

7

You can inisialize your array inside your loop :

....
ArrayList<String> attributeList;
for (int i = 0; i < attribute.getLength(); i++) {
    String current = attribute.item(i).getTextContent();
    if (current.equals("Identifier")) {
        largeAttributeList.add(attributeList);
        attributeList = new ArrayList<>();//<<<-------------
    } else {
        attributeList.add(current);
    }

}

edited May 31, 2017 at 11:01

answered May 31, 2017 at 10:47

Youcef LAIDANI

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1 Comment

Nico Over a year ago

This would initialize it at every round of the for. While he only wants to empty it when added so I would move your suggestion into the if(current.equals("Identifier")){. That way it would always be a new one once he added it, wouldn't it?

Yuri H · Accepted Answer · 2017-05-31 11:20:32Z

2

You need to make a copy of a list before its clearing:

    largeAttributeList.add(new ArrayList<>(attributeList));

Update: YCF_L solution is obviously better than my one cause there is no necessity to obtain overhead and give additional work for GC.

edited May 31, 2017 at 11:20

answered May 31, 2017 at 10:46

Yuri H

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Comments

Adriaan Koster · Accepted Answer · 2017-05-31 11:56:13Z

2

When you do:

largeAttributeList.add(attributeList);

You are not making a copy of attributeList, but adding its reference to largeAttributeList. I think the best solution would be to re-initialize attributeList in the loop:

List<List<String>> identifierAttributes = new ArrayList<List<String>>();
List<String> attributes = new ArrayList<String>();
for (int i = 0; i < attribute.getLength(); i++) {        
    String current = attribute.item(i).getTextContent();
    if(current.equals("Identifier")){
        identifierAttributes.add(attributes);
        attributes = new ArrayList<String>();
    }
    else {
        attributes.add(current);
    }
}

edited May 31, 2017 at 11:56

answered May 31, 2017 at 10:50

Adriaan Koster

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Comments

davidxxx · Accepted Answer · 2017-05-31 10:49:19Z

1

Create a new ArrayList object for attributeList when you add attributeList in largeAttributeList :

largeAttributeList.add(new ArrayList<String>(attributeList));

In this way when you execute attributeList.clear() you clear only attributeList and not the list object added in largeAttributeList.

answered May 31, 2017 at 10:49

davidxxx

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