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(" ");

Solution 1

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) + " ";

Solution 2

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");

Solution 3

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));

Solution 4

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

View more solutions

Comments

[Admin]

how can I concat any specified string at the beginning of the string?

[ssantos]

Would require to initialize messages to "" in its declaration

[Cruncher]

Yes, this assumes that messages is initialized. The question was about concatenating.

[Cruncher]

@Martin messages = "newstring" + messages

[Sotirios Delimanolis]

@Martin Call it the other way around. "yourString".concat(message); if you want to use a method call.

[ssantos]

Using the + operator again, something like resultingString = specifiedString + originalString

[Admin]

is there any way by which i can append any specified string at the beginning of the string?

[Ahmed Adel Ismail]

thats better yes, since you wont need to create all those string Objects throw concat() method

[jmaculate]

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

[Jay]

Note: the '+' operator usage will actually use a StringBuilder

[Jay]

String.valueOf is really not needed, something like: "" + random + " " + ch + " "

[Reimeus]

yes, StringBuilder has an insert method with an offset position