How to add a prefix and suffix to a particular word occurrences in the String?

I am trying to add prefix and suffix to a particular occurrence of the word in the string in java. Can anyone help me and tell me where am i going wrong? Below is my code.

public static void main(String[] args) {
        String str = "Hello world. welcome world java.";
        String arr[] = str.split("[. ]");
        if(str.contains("world")) {
            System.out.println("PREFIX_"+str+"_SUFFIX");
        }
    }

output expected :

Hello PREFIX_world_SUFFIX. welcome PREFIX_world_SUFFIX java

output getting:

PREFIX_Hello world. welcome world java_SUFFIX

String replaced = str.replaceAll("world", "PREFIX_world_SUFFIX");
System.out.println(replaced);

Your code is wrong since you're not changing the str variable while calling the split() function.
Also, from what I can gather, you also want to add prefix and suffix to those words containing "world".
Like if your string is something like this: Hello worldJava! welcome to java world, you'd want to display something like this: Hello PREFIX_worldJava_SUFFIX! welcome to java PREFIX_world_SUFFIX. (Note, the previous answers wouldn't be able to do this kind of substitution).

    String str = "Hello world. welcome world java.";
    String[] wordArr = str.split("[. ]");
    Set<String> words = new HashSet<>(Arrays.asList(wordArr));
    for (String w: words) {
        if(w.toLowerCase().contains("world")){
            str = str.replace(w, "PREFIX_"+ w +"_SUFFIX");
        }
    }
    System.out.println(str);

Note here that I am using java Set to parse unique words from the input string and then replacing them in the original string with the added prefix/suffix.

Just do this simply :

public class Example {

     public static void main(String[] args){
        String str = "Hello world. welcome world java.";
        System.out.println(str.replace("world", "PREFIX_world_SUFFIX"));
     }
}

Output :

Hello PREFIX_world_SUFFIX. welcome PREFIX_world_SUFFIX java.

You are doing it wrong on the print side. You can do this.

String stringToCheck = "world";

if(str.contains(stringToCheck)) {
 str = str.replaceAll(stringToCheck , "PREFIX_"+stringToCheck+"_SUFFIX");
 System.out.println(str);
}