Question

I am trying to write a double[] into a file in HDFS, and later on, I will need to read it back from the file and convert it back to the double[]. Does anyone here know how to do it?

Thanks,

Answer 1

ObjectOutputStream

An ObjectOutputStream writes primitive data types and graphs of Java
objects to an OutputStream. The objects can be read (reconstituted)
using an ObjectInputStream. Persistent storage of objects can be
accomplished by using a file for the stream. If the stream is a
network socket stream, the objects can be reconstituted on another
host or in another process.

Only objects that support the java.io.Serializable interface can be
written to streams. The class of each serializable object is encoded
including the class name and signature of the class, the values of the
object’s fields and arrays, and the closure of any other objects
referenced from the initial objects.

The method writeObject is used to write an object to the stream. Any
object, including Strings and arrays, is written with writeObject.
Multiple objects or primitives can be written to the stream. The
objects must be read back from the corresponding ObjectInputstream
with the same types and in the same order as they were written.

Primitive data types can also be written to the stream using the
appropriate methods from DataOutput. Strings can also be written using
the writeUTF method.

ObjectInputStream

An ObjectInputStream deserializes primitive data and objects
previously written using an ObjectOutputStream.

ObjectOutputStream and ObjectInputStream can provide an application
with persistent storage for graphs of objects when used with a
FileOutputStream and FileInputStream respectively. ObjectInputStream
is used to recover those objects previously serialized. Other uses
include passing objects between hosts using a socket stream or for
marshaling and unmarshaling arguments and parameters in a remote
communication system.

ObjectInputStream ensures that the types of all objects in the graph
created from the stream match the classes present in the Java Virtual
Machine. Classes are loaded as required using the standard mechanisms.

For example, I can save a key generator for encrypting data in this manner:

public static void saveKeyToFile(SecretKey key)
        throws FileNotFoundException, IOException {
    ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(
            "/path/to/mysavedobject"));
    oos.writeObject(key);
    oos.close();
}

public static SecretKey getKeyFromFile(String dir) throws IOException,
        ClassNotFoundException {
    if (dir == null) { 
        dir = "/path/to/mysavedobject";
    }
    SecretKey key = null;
    ObjectInputStream ois = new ObjectInputStream(new FileInputStream(
            dir));
    key = (SecretKey) ois.readObject();
    ois.close();
    return key;
}