The following fragment concatenates three strings:

    String age = "9";
    String s = "He is " + age + " years old.";

This displays the string “He is 9 years old.”

One practical use of string concatenation is found when you are creating very long strings. Instead of letting long strings wrap around within your source code, you can break them into smaller pieces, using the + to concatenate them. Here is an example:

// Using concatenation to prevent long lines.

class ConCat {
public static void main(String args[]) {
String longStr = "This could have been " +
"a very long line that would have " +
"wrapped around. But string concatenation " +
"prevents this.";

String Concatenation with Other Data Types 

You can concatenate strings with other types of data. For example, consider this slightly a different version of the earlier example:

   int age = 9;
   String s = "He is " + age + " years old.";

In this case, age is an int rather than another String, but the output produced is the same as before. This is because the int value in age is automatically converted into its string representation within a String object. This string is then concatenated as before.

Be careful when you mix other types of operations with string concatenation expressions, however, you might get surprising results. Consider the following:

   String s = "four: " + 2 + 2;

This fragment displays four: 22 rather than the four: 4 

To complete the integer addition first, you must use parentheses, like this:

String s = "four: " + (2 + 2);

Now s contains the string “four: 4”.