Java - How to insert characters in a String at a specific index?

I was translating a DotNet project to Java. In code, CSharp insert String function was used and I wrote my own Java function to insert characters/string at a specific index of string. Following is the code

    public static String insert(String originalString,int offset, String injectString) {
        if(offset>= originalString.length()) {
            return originalString;
        }
        String firstPart = originalString.substring( 0, offset);
        String secondPart = originalString.substring( offset);
     
        return firstPart.concat( injectString ).concat( secondPart );
    } 

Later on, I found the StringBuffer's built-in insert function for the same purpose

        StringBuffer buffer = new StringBuffer();
        buffer.append( "Hello Java" );
        buffer.insert( 6, "to " );

        System.out.println( buffer ); // OUTPUT  Hello to Java