How to Compare element of Arraylist<Integer>

Question

I wrote below code to get duplicate elements from Arraylist. My aerospikePIDs list doesn't have any duplicate value but still when I am executing below code it is reading if condition.

ArrayList<Integer> aerospikePIDs = new ArrayList<Integer>();    

ArrayList<Integer> duplicates = new ArrayList<Integer>();
    boolean flag;
    for(int j=0;j<aerospikePIDs.size();j++) {
        for(int k=1;k<aerospikePIDs.size();k++) {
            if(aerospikePIDs.get(j)==aerospikePIDs.get(k)) {
            duplicates.add(aerospikePIDs.get(k));
            flag=true;
            }
            if(flag=true)
System.out.println("duplicate elements for term " +searchTerm+duplicates);
    }   
                }

Answer 1

Your inner loop should start from j + 1 (not from 1 ), otherwise when j = 1 (second iteration of j), for k = 1 (first iteration of k for j value equals to 1).

aerospikePIDs.get(j)==aerospikePIDs.get(k) 

returns true .

So the code should be:

ArrayList<Integer> aerospikePIDs = new ArrayList<Integer>();    

ArrayList<Integer> duplicates = new ArrayList<Integer>();

for (int j = 0; j < aerospikePIDs.size(); j++) {
    for (int k = j + 1; k < aerospikePIDs.size(); k++) {
        if (aerospikePIDs.get(j)==aerospikePIDs.get(k)) {
            duplicates.add(aerospikePIDs.get(k));
            System.out.println("duplicate elements for term " +searchTerm+duplicates);
        }
    }   
}

Note: the flag is not necessary, because if you addeda duplicate you can print it directly in the if, without defining new unnecessary variables and code.

Answer 2

Use higher level abstractions:

1. Push all list elements into a Map<Integer, Integer> - key is the entry in your PIDs list, value is a counter. The corresponding loop simply checks "key present? yes - increase counter; else, add key with counter 1".
2. In the end, you can iterate that map, and each entry that has a counter > 1 ... has duplicates in your list; and you even get the number of duplicates for free .

And questions/answers that show you nice ways to do such things ... are posted here on almost daily basis. You can start here for example; and you only need to adapt from "String" key to "Integer" key.

Really: when working with collections, your first step is always: find the most highlevel way of getting things done - instead of sitting down and writing such error-prone low-level code as you just did.

Answer 3

You are iterating using the same arraylist. You are checking every data in inner for loop, for sure it will display duplicates.