Why Does Java's Concat() Method Not Do Anything

Why does Java's concat() method not do anything?

a String is immutable, meaning you cannot change a String in Java. concat() returns a new, concatenated, string.

String s = "TEST";
String s2 = s.trim();
String s3 = s.concat("ING");

System.out.println("S = "+s);
System.out.println("S2 = "+s2);
System.out.println("S3 = "+s3);

Why does 'concat' not change String?

s1.concat(" Y"); doesn't alter s1 (it can't, since Strings are immutable).

It returns a new String :

String s2 = s1.concat(" Y");
System.out.println("s2 refers to "+s2);

Why doesn't the String method concat work in a for loop?

The concat method doesn't mutate the current String, but in fact returns a new String containing the result.

Use it like this:

output = output.concat(str);

String.concat documentation.

String.concat() doesn't do concatenation

concat does not alter the invoking strings, but returns a new one.

You may assign each resulting String to your result like this.-

messages = messages.concat(String.valueOf(random));
messages = messages.concat(" ");
messages = messages.concat(String.valueOf(ch));
messages = messages.concat(" ");

Or just use the overloaded operator +

messages = String.valueOf(random) + " " + String.valueOf(ch) + " ";

Why doesn't my Java string concatenation work?

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.

Clarification on String concatenation in java

Because you're not assigning the concatenated String to another value (as concat() returns the resulting String), so it is lost.

If you want to get the result, you need to do:

test = test.concat("test");

The second println() works, because the resulting concatenated string is returned, captured and used immediately by System.out.println(), but is then lost outside of the println() as it is not assigned (so if you check test after, it still just contains "test").

Here's the doc: http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#concat%28java.lang.String%29

String concatenation not working, or is it a bug

Ok, we see this question at least couple of times a day.

Strings are immutable, so all operations on String results in new String.

conc= conc.concat("hello "); you need to reassign result to string again

When to use String#concat() method in Java?

String#concat and + exist to provide a minimalistic set of operations on the type String.

They are not efficient if used multiple times.

But they have their own right as type operations "xxx" + "yyy" you do not want to specify using a StringBuilder. (Furthermore there it is a compile time concatenation.)

StringBuffer is a mistake IMHO. It is slower that the newer StringBuilder as it is synchronized, but one would rarely add something rom two threads (unordered).

String::concat may be a method reference useful for stream reduction or such.

---

Related Topics