String.concat() doesn't do concatenation

Question

I am trying to concatenate a String with an int and char including spaces, but not getting any concatenation. What is the reason?

private String messages;
Random r = new Random();
int random = r.nextInt(1000);

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

You need to reassign the return value of `concat()`. `String` is immutable. — Sotirios Delimanolis, Sep 09 '13 at 20:35

score 19 · Accepted Answer · answered Sep 09 '13 at 20:36

19

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) + " ";

answered Sep 09 '13 at 20:36

ssantos

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how can I concat any specified string at the beginning of the string? – Sep 09 '13 at 20:40
@Martin `messages = "newstring" + messages` – Cruncher Sep 09 '13 at 20:41
1

@Martin Call it the other way around. `"yourString".concat(message);` if you want to use a method call. – Sotirios Delimanolis Sep 09 '13 at 20:41
Using the `+` operator again, something like `resultingString = specifiedString + originalString` – ssantos Sep 09 '13 at 20:41
Note: the '+' operator usage will actually use a StringBuilder – Jay Apr 28 '16 at 14:53
String.valueOf is really not needed, something like: "" + random + " " + ch + " " – Jay Apr 28 '16 at 14:54

score 11 · Answer 2 · edited May 23 '17 at 12:18

11

Strings are immutable. You can either append the result using String concatenation as shown in other answers or you can use StringBuilder

StringBuilder messages = new StringBuilder();
messages.append(String.valueOf(random));
messages.append(" ");
messages.append(String.valueOf(ch));
messages.append(" ");

Have a look at How do I concatenate two strings in Java?

Edit: (to insert at beginning of String)

messages.insert(0, "newstring");

edited May 23 '17 at 12:18

Community

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answered Sep 09 '13 at 20:37

Reimeus

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is there any way by which i can append any specified string at the beginning of the string? – Sep 09 '13 at 20:41
yes, `StringBuilder` has an [insert](http://docs.oracle.com/javase/8/docs/api/java/lang/StringBuilder.html#insert-int-java.lang.CharSequence-) method with an offset position – Reimeus Aug 31 '17 at 17:06

score 6 · Answer 3 · answered Sep 09 '13 at 20:45

6

A better way is :

Random r = new Random();
    int random = r.nextInt(1000);

StringBuilder str = new StringBuilder(String.valueOf(random));

str.append("s")
str.append(String.valueOf(random))
str.append(" ");
str.append(String.valueOf(ch));

answered Sep 09 '13 at 20:45

Stathis Andronikos

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thats better yes, since you wont need to create all those string Objects throw concat() method – Ahmed Adel Ismail Sep 09 '13 at 21:08
This should be the accepted answer. StringBuilder doesn't alloc a new string for every addition. Any time you're concat'ing more than 2 strings it's the way to go – jmaculate Jun 13 '14 at 13:56

score 2 · Answer 4 · answered Sep 09 '13 at 20:35

2

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

answered Sep 09 '13 at 20:35

Cruncher

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Would require to initialize `messages` to "" in its declaration – ssantos Sep 09 '13 at 20:40
1

Yes, this assumes that messages is initialized. The question was about concatenating. – Cruncher Sep 09 '13 at 20:40

score 0 · Answer 5 · answered Sep 09 '13 at 20:39

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

String should be RE-Assigned, when you edit a String object, you create a new String Object in the String constant pool, but you didnt assign this new String value to any reference variable, so you will need to re-assign your modifications to the SAME REFERENCE VARIABLE that created them

String.concat() doesn't do concatenation

5 Answers5