java中的字符串连接

Question

I am trying to concat String array values in a String, The below code does not work

private void newString() {

    String str = "Split me"; 
    String[] tokens = str.split("[ ]+");

    String newStr = new String();

    for(int i=0; i<tokens.length; i++){
        newStr.concat(tokens[i]);
    }

    System.out.println("NEW STRING IS : " + newStr);
}


public static void main(String[] args){
    Main m = new Main();
    m.newString();
}

Answer 1

When you call newStr.concat(tokens[i]); this doesn't modify newStr , it returns a new String that is the result of the concatenation. You aren't doing anything with the result.

You need to do something like: newStr = newStr.concat(tokens[i]);

String is an immutable type in java, so it's methods work like this, returning a new String object. Consider using a (mutable) StringBuilder instead. StringBuilder maintains an internal buffer that can grow, and so avoids the unnecessary String copies that come with using concatenation and immutable Strings .

Answer 2

String is immutable. Naturally, String.concat returns a brand new String .

... a new String object is created, representing a character sequence that is the concatenation of the character sequence represented by this String object and the character sequence represented by the argument string.

The way you're trying to do it, the code should look like...

String glued = "";

for (final String token : tokens) {
  glued = glued.concat(token);
}

Now, the problem I have with this approach is that you're doing a lot of copies of the String data. Each concat copies all of the data in glued as well as the data in token . Now, think about the implications of that... I'm not a fan of premature optimization, but I believe the logical way to achieve this is to instead use a StringBuilder as follows...

final StringBuffer buf = new StringBuffer();
for (final String token : tokens) {
  buf.append(token);
}
final String glued = buf.toString();

Answer 3

I try to fix it with StringBuilder,here is the new codes.

private void newString() {
    String str = "Split me"; 
    String[] tokens = str.split("[ ]+");
    StringBuilder newStr = new StringBuilder();

    for(int i=0; i<tokens.length; i++){
        newStr.append(tokens[i]);
    }
    System.out.println("NEW STRING IS : " + newStr);
}