string concatenation in loop [duplicate]

Question

Good evening all,

Probably a simple fix but I just cannot find it. I made this to print out every second chr from a string:

    Scanner sc = new Scanner(System.in);
    System.out.println("Enter a string:");

    String input = sc.next();
    String total = "";

    for (int i = 0; i < input.length(); i += 2){
        total += input.charAt(i);
    }
    System.out.println(total);

It works like a charm, but the '+=' is highlighted and gives me the tip: string concatenation in loop. Am I using a wrong method to accomplish what I want?

Yes, string concat is a costly operation (in terms of performance) due to immutability of string. You should be using StringBuilder. — Vasan
– Vasan, Commented Feb 19, 2018 at 18:30
normally these types of hints are accompanied by alternatives or even refactoring options, are you sure you cannot simply do that? Or are you asking out of curiousity? — luk2302
– luk2302, Commented Feb 19, 2018 at 18:31

lexicore · Accepted Answer · 2018-02-19 18:32:48Z

3

You can use a StringBuilder instead:

StringBuilder total = new StringBuilder();

for (int i = 0; i < input.length(); i += 2){
    total.append(input.charAt(i));
}
System.out.println(total.toString());

5 Comments

Andy Turner Over a year ago

new StringBuilder((input.length()+1)/2), if you want to be more efficient (it preallocates the internal array, so it doesn't have to be resized).

ADS Over a year ago

could you please add explanation why it's more efficient? I suppose then answer be more helpful

Andy Turner Over a year ago

Or, just print the chars directly to System.out.

lexicore Over a year ago

@ADS The provided link explains this in full.

MrEmper Over a year ago

I see, thanks a lot!

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