I changed the following so it will use the same mecanism to find the header file (some header files are named .hpp. I also adde .C as a source extension
String[] sourceExtensions = new String[]{"C", "cpp", "c", "cxx" };
String[] headerExtensions = new String[]{"h", "hpp"};

String defaultSourceExtension = "C";
String defaultHeaderExtension = "h";

String getSourceFile(String baseName, String[] extensions)
{
  int numExt = extensions.length;
  String tryFile = null;
  for (int i=numExt-1; i>=0; --i)
  {
    String ext = extensions[i];
    String tryFile = baseName + "." + ext;
    File f = new File(tryFile);
    if (f.canRead()) return f.getPath();
  }
  return baseName + "." + defaultSourceExtension;
}

boolean isFileType(String extension, String[] extensions)
{
  for (int i=0; i<extensions.length; ++i) {
    if (extension.equals(extensions[i])) return true;
  }
  return false;
}

void toggleHeaderSource()
{

  String currentFile = buffer.getPath();
  int pos = currentFile.lastIndexOf('.');
  String extension = currentFile.substring(pos+1);
  String baseName = currentFile.substring(0, pos);
  if (isFileType(extension, sourceExtensions))
  {
    String sourceFile = getSourceFile(baseName,headerExtensions);
    jEdit.openFile(view, baseName + ".h");
  }
  else if (isFileType(extension, headerExtensions) ) {

    String sourceFile = getSourceFile(baseName,sourceExtensions);
    jEdit.openFile(view, sourceFile);
  }
}

toggleHeaderSource();