Android and iOs tips

Question

Sorry If you are thinking that it is a repeated post. But the situation is different here. I have read several posts for it but not getting the exact thing I am looking for. I know How to develop a complex logic for this but please take a look this is for android and we can't expect too much processing on the device due to to limited resources available.

I have an ArrayList of an bean class objects consisting five fields as

Java Bean -- MyShares

fileName

filePath

fileSize

isShared

Creator

I have another ArrayList of String which contains only filepaths. Now what I want is to remove all the common elements between the two arraylist means the file paths in seconds arraylist and file path in first arraylist objects are similar then I have to remove from both of the arraylist But I Don't want a new arraylist which contains the uncommon elemenst But I want to get my both arraylist only without their common elements.

Please help me.

Accepted Answer · 2013-04-11 13:14:42Z

You could use a Map from String to your object type (I used Obj in order to make a SSCCE).

Assume we are given a list objects and a list strings.

Steps:

Put all objects in a map with their str variable as key

Get all those str variables using map.keySet()

Get all strings that are in objects but not in strings by keys.removeAll(strings)

Get all strings that are in strings but not in objects by strings.removeAll(keys)

Get the objects that correspond to the remaining keys

Note that you need to be careful in steps 3 and 4, because you need to back up one of the collections.


import java.util.*;

public class Test { 
    public static void main(String[] args) throws Exception {
        new Test();
    }

    public Test() {
        List objects = new ArrayList<>();
        objects.add(new Obj("a"));
        objects.add(new Obj("b"));
        objects.add(new Obj("c"));

        List strings = new ArrayList<>();
        strings.add("a");
        strings.add("d");
        strings.add("e");

        remove(objects, strings);

        System.out.println(objects);
        System.out.println(strings);
    }

    public void remove(List objects, List strings) {
        Map map = new HashMap<>();
        for (Obj object : objects) {
            map.put(object.str, object);
        }

        Set keys = map.keySet();
        List oldStrings = new ArrayList<>(strings);

        strings.removeAll(keys);
        keys.removeAll(oldStrings);

        objects.clear();
        for (String key: keys) {
            objects.add(map.get(key));
        }       
    }

    public class Obj {  
        public String str;
        public Obj(String str) {
            this.str = str;
        }
        @Override
        public String toString() {
            return str;
        }       
    }
}

Prints:


[b, c]
[d, e]

Answer 2 · 2013-04-11 13:11:34Z

Rough Java code:


HashSet commonKeys = new HashSet();
for (Share share : shares) {
    commonKeys.add(share.filePath);
}
commonKeys.retainAll(filePaths);
for (Iterator it = shares.iterator(); it.hasNext(); ) {
    Share share = it.next();
    if (commonKeys.contains(share.filePath)) {
        it.remove();
    }
}
filePaths.removeAll(commonKeys);

This won't be O(N) because remove on an ArrayList is expensive. To get O(N) behavior you either need to create new ArrayList instances, or add the elements you don't want removed to temporary lists, and then clear() and add them back into the original lists.

Answer 3 · 2013-04-11 13:29:57Z

I will go with some clues for you

Suppose you have two lists one for bean objects namely myBeans and another for filePaths namely filePaths


List beansToRemove = new ArrayList();
List filePathsToRemove = new ArrayList();

for(Bean myBean : myBeans) {
    for(FilePath filePath : filePaths) {
        if(myBean.getfilePath.equals(filePath.getFilePath())) {
            beansToRemove.add(myBean);
            filePathsToRemove.add(filePath);
        }
    }
}

//Now remove filePaths and beans if any

for(Bean myBean : beansToRemove) {
    myBeans.remove(myBean);
}

for(FilePath filePath : filePathsToRemove) {
    filePaths.remove(filePath);
}

it is just a flow to make you clear for what to do; you can further customize it according to your needs.

Er. Nikhil Agrawal · Answer 4 · 2013-04-11 13:07:41Z

You can use an outer loop to scan over the Bean objects, and an inner loop to scan over the file paths.

pseudo code:


for (Bean i in beans) {
    for (String p in paths) {
        if (i.path.equals(p)) {
            beansToRemove.add(i);
            pathsToRemove.add(p);
        }
    }
}
beans.removeAll(beansToRemove);
paths.removeAll(pathsToRemove);

I'm not sure if my extra arraylists to track the removed arraylists go against your question or not since the original arrays remain. If you presort both arrays on the path and keep track of the position in each area (not exhaustive search) you can improve it from n2 to nlgn

Android and iOs tips

Monday, May 20, 2013

[android help] Removing common elements between two different Arraylist

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