How to Import Multiple Objects with ObjectInputStream in Java

Learn how to fix the issue of only reading the first object from a file in Java by correctly managing your ObjectOutputStream and reading from the right file.
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This video is based on the question https://stackoverflow.com/q/71003310/ asked by the user 'PeterPefi' ( https://stackoverflow.com/u/13211647/ ) and on the answer https://stackoverflow.com/a/71003675/ provided by the user 'Jules' ( https://stackoverflow.com/u/10885193/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

When working with Java's ObjectInputStream and ObjectOutputStream, you might encounter a frustrating problem: after saving multiple objects to a file, your code only reads the first object. This issue can stem from the way you're handling streams. In this post, we will troubleshoot this problem and provide clear solutions to allow you to import multiple objects successfully.

Understanding the Problem

In the original code provided, the user implemented an ObjectInputStream to read multiple objects from a file, but the output only showed the first object. This means that the code is not correctly set up to read all the objects saved within the file. Let’s break down what went wrong:

Incorrect Scope of Streams: The ObjectOutputStream was instantiated inside a loop for writing to the file, which means that only the last object gets written into the file.

Wrong File Reference: The user attempted to read from "or.rtf" when they saved objects into "qt.rtf". This mismatch led to confusion and additional errors.

Solution: Modifying the Code

Step 1: Change the Scope of ObjectOutputStream

To ensure all objects are serialized into the file correctly, the ObjectOutputStream should be declared outside the loop. This way, you can write all objects in a single file write operation. Here's how the revised code looks:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Update the ObjectInputStream Code

Now, ensure you’re reading from the same file where you wrote the objects. This requires updating the ObjectInputStream setup to match your Saver class. Here’s a modified version of the ObjectInputStream code:

[[See Video to Reveal this Text or Code Snippet]]

Key Points to Remember

Scope Matters: Declare your output streams outside loops when you intend to write multiple objects to the same file.

File Consistency: Always ensure that the input stream corresponds to the same file through which the objects were outputted.

Exception Handling: Be sure to handle EOFException to correctly manage the end of your input stream.

Conclusion

By making these adjustments, you should now be able to read all your saved objects from the file successfully. Understanding the relationship between the ObjectInputStream and ObjectOutputStream, along with proper file management, is crucial for effectively working with serialized objects in Java. Happy coding!

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