import java.util.*;

public class ConcatInLoop {

    private String logStatement = "";

    public void bad() {
        List<String> values = Arrays.asList("tic", "tac", "toe");
        for (String val : values) {
            logStatement = logStatement + val + ", "; // bad
        }
    }

    public void good() {
        List<String> values = Arrays.asList("tic", "tac", "toe");
        StringBuilder sb = new StringBuilder(logStatement);
        for (String val : values) {
            sb.append(val).append(", ");
        }
        logStatement = sb.toString();
    }
}

import java.util.ArrayList;
import java.util.List;

public class Foo {
    private List<String> fileExtensions = new ArrayList();
    public void good(List<String> fileExtensions) {
        for (String fileExtension : fileExtensions) {
                if (fileExtension.charAt(0) != '.') {
                    fileExtension = "." + fileExtension; // StringBuilder would just add noise here
                }
                this.fileExtensions.add(fileExtension);
            }
    }
}