Why Does It Throw an IOException

Question

This program is run when there is no extant file named "Test Object.to". The output of this program is "IO caught". Why?

import java.io.*;

public class Test 
{
    public static void main (String [] args)
    {
        TestObject testObject = new TestObject("Test Object");
        
        try
        {
            saveTestObject(testObject);
        }
        catch (FileNotFoundException fnf)
        {
            System.out.println("FNF caught");
        }
        catch (IOException io)
        {
            System.out.println("IO caught");
        }
    }
    
    static void saveTestObject(TestObject to)  throws FileNotFoundException, IOException
    {
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(to.name + ".to"));
        oos.writeObject(to);
        oos.close();
    }
}

class TestObject
{
    String name;
    
    TestObject(String s)
    {
        name = s;
    }
}

Answer 1

First of all, this does not throw a FileNotFoundException , since as per the docs , FileOutputStream 's constructor will create the file if it doesn't exist instead of throwing an error. If you print the error, you see:

java.io.NotSerializableException: TestObject

Again, as per the docs , writeObject requires that its argument be Serializable . Since Serializable is just a marker interface, you can just implement it:

class TestObject implements Serializable

and now your code doesn't throw any errors.

Answer 2

好的，我现在看到 TestObject 显然需要实现 Serializable