String.replace() is replacing one of two strings only

I have this line of text:

> number of troops the united states has stationed in south viet nam as compared with the number of troops it has stationed in west germany .

And what i'm trying to do is check the line of text for any words that may have a meaning when combined.

For example: replace the viet nam String with the correct word vietnam.

I wrote this function:

private String correctLine(ArrayList<String> newWords, String line) {
    line = line.toLowerCase();
    String[] words = line.split(" ");
    for (int i = 0; i < newWords.size(); i++) {
        for (int j = 0; j < words.length; j++) {
            if(newWords.get(i).toLowerCase().equals(words[j].concat((j + 1 == words.length)?"":words[j+1]))){
                line = line.replace(words[j].toLowerCase(), newWords.get(i).toLowerCase());
                line = line.replace(words[j+1].toLowerCase(), "");
            }
        }
    }
    return line;
}

What this function do is it takes a List of words newWords that will be replaced in the text, and the target line of text line.

The problem:

When the if condition is met:

vietnam is equal to words[j] (viet) & words[j+1] (nam)

I replace the line twice:

• replace viet with vietnam.
• replace nam with .

But what happens is only one of them is being replaced, while the other is ignored.

1. If i comment the first replace the result is:

   number of troops the united states has stationed in south viet as compared with the number of troops it has stationed in west germany .

2. If i comment the second replace the result is:

   number of troops the united states has stationed in south vietnam nam as compared with the number of troops it has stationed in west germany .

Any idea on why this is happening?

luk2302 is correct, although you will end up with two consecutive spaces with that approach which has to be addressed as well. Might I suggest another approach which is simpler and more flexible?

    Map<String, String> replace = new HashMap<>();
    replace.put("viet nam", "vietnam");
    
    String line = "number of troops the united states has stationed in south viet nam as compared with the number of troops it has stationed in west germany .";
    
    for (String key : replace.keySet()) {
        line = line.replaceAll(key, replace.get(key));
    }

You should think very hard about the two statements about what you replace, if you replace viet with vietnam and then replace nam with nothing then you obviously will replace the nam of the just replaced vietnam again. Resulting in viet nam -> vietnam nam -> viet nam.

You might get away by simply switching the two statements around:

line = line.replace(words[j+1].toLowerCase(), "");
line = line.replace(words[j].toLowerCase(), newWords.get(i).toLowerCase());

You don't actually need the two replace.

Below code is tested and working fine.

public class Main {
    
	public static void main(String args[]) { 
        
        String line = "in south viet nam as compared with the number of troops it has stationed in west germany";
        ArrayList<String> newWords = new ArrayList<>();
        newWords.add("vietnam");
        newWords.add("India");
        
        line = line.toLowerCase();
        String[] words = line.split(" ");
        for (int i = 0; i < newWords.size(); i++) {
            for (int j = 0; j < words.length; j++) {
                if(newWords.get(i).toLowerCase().equals(words[j].concat((j + 1 == words.length)?"":words[j+1]))){
                    line = line.replace((words[j] + " " + words[j+1]), newWords.get(i).toLowerCase());
                }
            }
        }
        System.out.println(line);
	}
}

Output :

in south vietnam as compared with the number of troops it has stationed in west germany