string concatenation in loop

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](https://docs.oracle.com/javase/7/docs/api/java/lang/StringBuilder.html). — Vasan, Feb 19 '18 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, Feb 19 '18 at 18:31

lexicore · Accepted Answer · 2018-02-19T18:32:48.207

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());

See also:

http://www.pellegrino.link/2015/08/22/string-concatenation-with-java-8.html

edited Feb 19 '18 at 18:32

answered Feb 19 '18 at 18:30

lexicore

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`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). – Andy Turner Feb 19 '18 at 18:32
1

could you please add explanation *why* it's more efficient? I suppose then answer be more helpful – ADS Feb 19 '18 at 18:33
Or, just print the chars directly to `System.out`. – Andy Turner Feb 19 '18 at 18:33
1

@ADS The provided link explains this in full. – lexicore Feb 19 '18 at 18:34
I see, thanks a lot! – MrEmper Feb 19 '18 at 18:34

string concatenation in loop

1 Answers1