FileHashMap <K, V> ：重复使用相同的哈希图

Question

   FileHashMap<int, String> myMap = new FileHashMap<int, String>("/myFile");       
   myMap.put(1, "First");
   myMap.put(2, "Second");
   myMap.save();
   myMap.close();

According to my understanding this code will save the Hashmap files on to disk... But when I am creating the object of FileHashMap again, like this:

    FileHashMap<int, String> myMap = new FileHashMap<int, String>("/myFile");       
    if(myMap.containsKey(1){
    System.out.println("Yes");
    }
    else{
    System.out.println("No");
    }

This is giving the output as "No".. I want to reuse the file, reuse the Hashmap... How can I do this.. Can anybody explain??

Answer 1

Here's your problem: the API is confusing. It says that "[a] FileHashMap is persistent by default," but the zero- and one-arg constructors (the one you're using which specifies the file name is the one-arg) create a new transient FileHashMap — that is, the file is deleted after the FileHashMap is closed. The two-arg constructor takes the filename and an int options flag, but there's no specific option for "persistent", so you just have to make sure the flag is not "transient" (to be precise, flags & FileHashMap.TRANSIENT == 0 ).

In order to get it to save and open the file, pass 0 as the flag to the constructor (unless you want other options — read the docs) as such:

new FileHashMap<Integer, String>(fileName, 0);

Note that the generic parameter needs to be Integer rather than int , as comments have stated. Also note that you probably want your file path to be something other than "/myFile" as that will most likely give you a permission denied error, unless you're running as root, which you probably shouldn't be.