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Store multiple values in a file - best format?

I want to store multiple values (String, Int and Date) in a file via Java in Android Studio.
I don't have that much experience in that area, so I tried to google a bit, but I didn't get the solution, which I've been looking for. So, maybe you can recommend me something?
What I've tried so far:
Android offers a SharedPreferences feature, which allows a user to save a primitive value for a key. But I have multiple values for a key, so that won't work for me.
Another option is saving data on an external storage medium as file. As far as good. But I want to keep the filesize at minimum and load the file as fast as possible. That's the place, where I can't get ahead. If I directly save all values as simple text, I would need to parse the .txt file per hand to load the data which will take time for multiple entries.
Is there a possibility to save multiple entries with multiple values for a particular key in an efficient way?

No need to reinvent a bicycle. Most probably the best option for your case is using the databases. Look into Sqlite or Realm.

You don’t divulge enough details about your data structure or volume, so it is difficult to give a specific solution.
Generally speaking, you have these three choices.
Serialize a collection
I have multiple values for a key
You could use a Map with a List or Set as its value. This has been discussed countless times on Stack Overflow.
Then use Serialization to write and read to storage.
Text file
Write a text file.
Use Tab-delimited or CSV format if appropriate. I suggest using the Apache Commons CSV library for that.
Database
If you have much data, or concurrency issues with multiple threads, use a database such as the H2 Database Engine.

how to implement RowLoader in gemfirexd?

How to write Rowloader JAVA code to inject data from sample.csv file into GenfireXD database.

The GemFireXD distribution includes a JDBCRowLoader source example. Look in the examples directory. In your case you will have to determine which field of your CSV you want to consider as primary keys, parse the CSV and return rows as needed.

You can check IMPORT_DATA_EX and IMPORT_TABLE_EX procedures to load data into GemFireXD.
Since you mentioned csv format IMPORT_DATA_EX might be the recommend way to do it since you can also tweak the number of threads and constraints while loading the data. It's definitely one of the fastest ways to do it but please note that the csv file but be available from the node you're issuing the command.
You might also want to consider starting a peer member with host-data=false.
Reference: http://gemfirexd.docs.pivotal.io/latest/userguide/index.html#reference/system_procedures/derby/rrefimportdataproc_ex.html

What is the best way to store data when a database could not be used?

I am designing a e-Diary application in which the user can store textual data EVERY DAY.
I thought of using a database but now I am looking for other alternatives , thought of encrypted files , but what if they get deleted ?
Could someone provide me ways of doing this.
BTW, I am using Java.(If this is important).
EDIT: Currently I am using files for acheiving this, for login I am using a file in which the username, password are stored in the format username:password, This is not a good approach, so I am looking for some secure approaches.

Serialize your Data and store it using flatfiles.
But why dont you want to use SQlite?
Serializing and storing to the disk is a database aswell.
If you want to serialize it there are a few tutorials and code examples around.
check How to serialize an object into a string
for example.
If you want to use a proper database (which is recommend!) you may check different databases and examples at Java and SQLite

Best file format regarding standard string and integer data?

For my project, I need to store info about protocols (the data sent (most likely integers) and in the order it's sent) and info that might be formatted something like this:
'ID' 'STRING' 'ADDITIONAL INTEGER DATA'
This info will be read by a Java program and stored in memory for processing, but I don't know what would be the most sensible format to store this data in?
EDIT: Here's some extra information:
1)I will be using this data in a game server.
2)Since it is a game server, speed is not the primary concern, since this data will primary be read and utilized during startup, which shouldn't occur very often.
3)Memory consumption I would like to keep at a minimum, however.
4)The second data "example" will be used as a "dictionary" to look up names of specific in-game items, their stats and other integer data (and therefore might become very large, unlike the first data containing the protocol information, where each file will only note small protocol bites, like a login protocol for instance).
5)And yes, I would like the data to be "human-editable".
EDIT 2: Here's the choices that I've made:
JSON - For the protocol descriptions
CSV - For the dictionaries

There are many factors that could come to weigh--here are things that might help you figure this out:
1) Speed/memory usage: If the data needs to load very quickly or is very large, you'll probably want to consider rolling your own binary format.
2) Portability/compatibility: Balanced against #1 is the consideration that you might want to use the data elsewhere, with programs that won't read a custom binary format. In this case, your heavy hitters are probably going to be CSV, dBase, XML, and my personal favorite, JSON.
3) Simplicity: Delimited formats like CSV are easy to read, write, and edit by hand. Either use double-quoting with proper escaping or choose a delimiter that will not appear in the data.
If you could post more info about your situation and how important these factors are, we might be able to guide you further.

How about XML, JSON or CSV ?

I've written a similar protocol-specification using XML. (Available here.)
I think it is a good match, since it captures the hierarchal nature of specifying messages / network packages / fields etc. Order of fields are well defined and so on.
I even wrote a code-generator that generated the message sending / receiving classes with methods for each message type in XSLT.
The only drawback as I see it is the verbosity. If you have a really simple structure of the specification, I would suggest you use some simple home-brewed format and write a parser for it using a parser-generator of your choice.

In addition to the formats suggested by others here (CSV, XML, JSON, etc.) you might consider storing the info in a Java properties file. (See the java.util.Properties class.) The code is already there for you, so all you have to figure out is the properties names (or name prefixes) you want to use.
The Properties class also provides for storing/loading properties in a simple XML format.

Best way to save data in a Java application?

I'm trying to find the best way to save the state of a simple application.
From a DB point-of-view there are 4/5 tables with date fields and relationships off course.
Because the app is simple, and I want the user to have the option of moving the data around (usb pen, dropbox, etc), I wanted to put all data in a single file.
What is the best way/lib to do this?
XML usually is the best format for this (readability & openness), but I haven't found any great lib for this without doing SAX/DOM.

If you want to use XML, take a look at XStream for simple serialization of Java objects into XML. Here is "Two minute tutorial".
If you want something simple, standard Java Properties format can be also a way to store/load some small data.

consider using plain JAXB annotations that come with the JDK:
#XmlRootElement
private class Foo {
#XmlAttribute
private String text = "bar";
}
here's a blog-post of mine that gives more details on this simple usage of JAXB (it also mentiones a more "classy" JAXB-based approach -- in case you need better control over your XML schema, e.g. to guarantee backwards compatibility)

2 other options you might consider -
Hsqldb is a small sql db written in
java. More relevant for your
purposes, it can be configured to
simply write to a csv file as it's
data store, so you could conceivably
use it's text output as a portable
datastore and still use sql, if
that's what you prefer.
A second option might be to write the
datastore directly to a serialized
file either directly or through a
library like prevayler. Very good
performance and simple to implement,
cons are the fragility and opacity of
the format.
But if the data is small enough, xml is probably much less bother.

If you don't need to provide semantic meaning to your data then XML is probably a wrong choice. I would recommend using the fat-free alternative JSON, which is much more naturally built for data structures.