When this code runs, it gets the content of a webpage.
I wanted to concatenate that entire string rather than printing it to the console but when I uncomment the two lines in the code below, System.out.println(inputLine);
prints nothing (but it worked with the line below commented) and the value fileText = null
,
where does this error come from?
import java.net.*;
import java.io.*;
public class URLReader {
public static void main(String[] args) throws Exception {
URL oracle = new URL("http://www.oracle.com");
BufferedReader in = new BufferedReader(
new InputStreamReader(oracle.openStream()));
String fileText = "";
String inputLine;
while ((inputLine = in.readLine()) != null)
//fileText.concat(inputLine);
System.out.println(inputLine);
in.close();
//System.out.println(fileText);
}
}
String
is immutable and concat() will return a new String
(check the linked doc), which you're not collecting.
You should make use of a StringBuilder to build a string efficiently, and then call toString()
on that once you're complete to get he resultant String.
e.g.
StringBuilder sb = new StringBuilder();
while (....) {
sb.append("more string data");
}
String str = sb.toString();
You can append Strings
e.g.
str = str + "more string data";
but it's not very efficient, due to the implementation of String
. A StringBuilder
is built in order to perform concatenation efficiently. You can tune a StringBuilder
via its initial capacity if you have an idea of the size of String
you're building.
You may see some sources refer to a StringBuffer
. That's very similar, except it's older and synchronises its methods by default. In a non-threaded environment that's wasteful and the general advice is to prefer StringBuilder
.