How do I fix this error since I'm the newbie in developing mobile apps?

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Java

Hi guys, this is my java file. I hope there is someone who can help me run out of this problem.What i'm trying to do is to integrate DropboxAPI into my apps and i'm keep getting the below problem:

==========================Gradle Error================================================

Error:(39, 41) error: cannot find symbol method metadata(DropboxAPI,int,<null>,boolean,<null>)
Error:(40, 56) error: cannot find symbol variable contents
Error:(41, 32) error: cannot find symbol method fileName()
Error:(53, 34) error: cannot find symbol method obtainMessage()
Error:(57, 16) error: cannot find symbol method sendMessage(Message)

======================================================================================


--------------------------------------my java file------------------------------------
package com.example.user.myapps;

import android.os.AsyncTask;
import android.os.Bundle;
import android.os.DropBoxManager;
import android.os.Message;
import android.os.DropBoxManager.Entry;


import com.dropbox.client2.DropboxAPI;
import com.dropbox.client2.exception.DropboxException;

import java.security.KeyStore;
import java.util.ArrayList;
import java.util.logging.Handler;

/**
* Created by FaiqIkhwan on 4/13/2016.
*/
public class ListFiles extends AsyncTask<void,void,arraylist> {

private String dropboxApi;
private DropboxAPI path;
private Handler handler;

public ListFiles(Handler handler, DropboxAPI path, String dropboxApi) {
this.handler = handler;
this.path = path;
this.dropboxApi = dropboxApi;
}

@Override
protected ArrayList doInBackground(Void... params) {

ArrayList files = new ArrayList();

try {

Entry directory = dropboxApi.metadata(path, 1000, null, true, null);
for (DropBoxManager.Entry entry : directory.contents){
files.add(entry.fileName());
}

}catch (DropboxException e){


}
return files;
}

@Override
protected void onPostExecute(ArrayList result) {
Message message = handler.obtainMessage();
Bundle bundle = new Bundle();
bundle.putStringArrayList("data",result);
message.setData(bundle);
handler.sendMessage(message);
}
}
--------------------------------------------------------------------------------------

What I have tried:

i had googled the problem and nothing is done.