Casting to dynamic class crashes in method call

Question

I have this code do load any serialized data:

public static <T> T loadData(String filename) {
        T o = null;
        try {
            FileInputStream fi = new FileInputStream(filename);
            ObjectInputStream oi = new ObjectInputStream(fi);
            o = (T) oi.readObject();
            oi.close();
            fi.close();
        } catch (Exception e) {}
        return o;
    }

I call it with this:

ArrayList<String> data = loadData("a.dat");

But in this file I have another serialized object (e.g. a java.util.File object). I expected a error at o = (T) oi.readObject();, which would be catched, but it crashes in the method call. Why?

EDIT: Okay problem solved with the following code:

public static <T> T loadData(Class<T> expectedClass, String filename) {
        T o = null;
        try {
            FileInputStream fi = new FileInputStream(filename);
            ObjectInputStream oi = new ObjectInputStream(fi);
            o = expectedClass.cast(oi.readObject());
            oi.close();
            fi.close();
        } catch (Exception e) {}
        return o;
    }

Whats the exception (message)? – Distjubo Oct 25 '15 at 17:59 — Distjubo, Oct 25 '15 at 17:59

score 3 · Accepted Answer · answered Oct 25 '15 at 17:59

3

T is a generic type, and is "erased" at runtime. So at runtime, T is really just Object, and valid.

Only once you call it and try to coerce it to ArrayList<String> is the actual cast attempted, and fails.

Because of this, your IDE should be showing you an "unchecked" or "unsafe" warning on the cast to a generic type. (This line: o = (T) oi.readObject();)

See also: https://docs.oracle.com/javase/tutorial/java/generics/erasure.html

answered Oct 25 '15 at 17:59

ziesemer

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Thanks i got it finally working with the following code --> see edited question. – Flatron Oct 25 '15 at 18:15
sure, there you go :) – Flatron Nov 08 '15 at 18:20

Casting to dynamic class crashes in method call

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