Question

I try to replace some string in StringBuilder using the replace method, but unfortunately it running like the insert method.

Here is some code:

public class StringBuilderReplace {

    public static void main(String[] args) {
        StringBuilder builder = new StringBuilder();

        builder.append("Line 1\n");
        builder.append("Line 2\n");
        builder.append("Line 3\n");

        builder.replace(builder.indexOf("Line 2"), builder.indexOf("Line 2"), "Temporary Line\n");

        System.out.println(builder.toString());
    }
}

The result for this code:

Line 1
Temporary Line
Line 2
Line 3

What I want is:

Line 1
Temporary Line
Line 3

How to do this to get the result I want?

Update based on AljoshaBre answer

It works if I change the code like this one:

builder.replace(builder.indexOf("Line 2"), builder.indexOf("Line 3"), "Temporary Line\n");

But new problem occur, what if the next string (for this example Line 3) I don’t know the content?

Answer 1

It’s because you’re getting index of “Line 2” as starting index, which is the beginning of that line, and you do the same for the last index.

I think you should do the following:

public class StringBuilderReplace {
        public static void main(String[] args) {
            StringBuilder builder = new StringBuilder();

            builder.append("Line 1\n");
            builder.append("Line 2\n");
            builder.append("Line 3\n");

            String lineToReplace = "Line 1\n";
            int startIndex = builder.indexOf(lineToReplace);
            int lastIndex = startIndex + lineToReplace.length();

            builder.replace(startIndex, lastIndex, "Temporary Line\n");
            System.out.println(builder.toString());
        }
}