用于检查重复项的String和HashSet之间的最佳（性能+内存）是什么

Question

I want to do a simple implementation to do some operations based on a distinct Codes (aCode) among the bigCodeList contains duplicates. Below I have mentioned two approaches what i want to know is what is the more effective one among them in performance vice + memory consumption wise?

Approach 1 :

    String tempStr = "";

    for(String aCode : bigCodeList){
        if(tempStr.indexOf(aCode) == -1) {
            // deal With the aCode related work
            tempStr += aCode+"-"
        }
    }

Approach 2 :

        HashSet<String> tempHSet = new HashSet<String>();

        for(String aCode : bigCodeList){

            if(tempHSet.add(aCode)){

                // deal With the aCode related work

            }

        }

Note : aCode is a Three Letter code like LON

Answer 1

Approach 2 is by far better. You should not even consider approach 1.

First of all, approach 1 has linear time in searching. That means that when tempStr becomes twice as long, the time to search it becomes twice as long (on average, of course, if you always find the first element, it stays short).

Next: you're copying the entire tempStr each time your appending to it (because String objects are immutable and that's the only way to create a new one from an existing one). So the adding option takes ages as well.

Third (not a performance concern): Mixing data ( aCode ) and meta-data (the separator - ) like this leads to all kinds of undesired effects. You might be sure that now aCode can never contain a dash, but what if that changes in two weeks?

Fourth: HashSet is built for pretty much exactly this use case ! That's what it does best: hold a set of distinct objects, check if it already exists and add a new one.

Answer 2

I think, that the first one approach is worse: indexOf operation has O(n) , while for HashSet complexity could be O(1) for unique String keys look-up.

Furthermore, in the first approach you are using string concatenation operation, which implies new String object creation each time, which gives additional performance draw.

Answer 3

java.util.Set不允许重复，但它在拒绝重复方面相当“安静”。

Answer 4

Performance and memory wise Hashset is best than String to use in ur coding.

Appending values into string variable will take time