java String in following code leak memery? how to fix it

Here is the function:

String gen() {
      String raw_content = "";
      String line;
      for (int i=0; i<10000; i++) {
        line = randString();
        raw_content += line + "\n";
      }
      return  raw_content;
}

When I call gen() for 100 times in main(), my program will stuck. I suspect this is related to memory leak caused by Java String. So will the no-longer used memory be freed by JVM automatically? How to fix this?

Thanks!