CODEWITHSUNDEEP
javaoopgenerics
Zar
Zar

Reputation: 6882

Returning an object of any type

In my cache class, I'm going to have a function which writes the serialized version of an object (undecided type) to a file, something like this (generic function):

public <O> void write(O object) {

   // ...
   serialize(file, object);
   // ...

}

Which works great, however, I'm unable to find a way to create a method which can return any object, like the write() method can take any object. Looking for something like this:

public <O> read() {

   // ...
   O object = unserialize(file);
   // ...

   return object;

}

Any suggestions on how to accomplish this is highly appreciated!

Upvotes: 0

Views: 8025

Answers (6)

user102008
user102008

Reputation: 31303

Yes,

public <O> O read()

is valid and will do what you want (return any type the caller wants). Whether this is a good idea is another matter

Upvotes: 0

Itay Maman
Itay Maman

Reputation: 30723

I'm assuming the caller to read() knows which type it expects to get. This is a reasonable assumption because he is going to assign the return value to some variable:

MyType o = read(...); // Caller knows he's going to get a MyType object

If this assumption is true, then you should change the read() method such that it takes a class object:

public<T> T read(Class<T> t) {
   ... 
}

Then the call site will look as follows:

MyType o = read(MyType.class);

Upvotes: 0

Andr&#233; Stannek
Andr&#233; Stannek

Reputation: 7863

I would do that by making the whole class generic

class Cache <O extends Serializable> {

    public void write(O object) {
        serialize(file, object);
    }

    public O read() {
        O object = (O)unserialize(file);

        return object;
    }
}

Note the cast of the returned object.

Besides that you should use <O extends Serializable> instead of just <O>. This assures that your type parameter id a type that is serializable. This is needed if you want to save (serialize) objects and prevents later errors.

Upvotes: 0

tobias_k
tobias_k

Reputation: 82899

Not sure if I understood the question correctly... You could wrap both methods inside a class and parametrize the class with the object's type, like this:

public class ReaderWriter <T> {
    public ReaderWriter(File file) {...}
    public void write(T object) {...}
    public T read() {...}
}

Upvotes: 0

Peter Lawrey
Peter Lawrey

Reputation: 533492

If you mean

public <O> O read() {

this is almost useless because it is much the same as

public Object read() {

Upvotes: 1

Madara&#39;s Ghost
Madara&#39;s Ghost

Reputation: 174957

You specify the return type of type Object:

public Object function(...)

That way the return type will always be of type Object (since all objects are descendants of Object), so they will be accepted.

Upvotes: 7

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