为何ObjectInputStream.readObject()返回Object而非写入的具体类型？

Great question! Let’s unpack why this design choice makes sense for Java’s serialization system:

• Flexibility first: support any serializable object
  The whole point of ObjectOutputStream is to let you write any object that implements Serializable—not just your Car class. You could write a String, a custom Dog class, or even a collection of mixed objects to the same stream. Since readObject() has no way of knowing at compile time what type of object you wrote to the file, it has to return the most generic type that can hold any object: Object.

• Java’s static type system constraints
  Java is a statically typed language, which means method return types are fixed at compile time. If readObject() returned Car, how would it handle cases where you wrote a different serializable type? It couldn’t—this would break type safety and the system’s flexibility. Returning Object lets the developer (who knows what they wrote to the stream) cast to the correct type explicitly.

• Historical context
  Serialization was added in Java 1.1, long before generics were introduced in Java 5. Even with modern generics, we can’t make readObject() return a specific type because the actual type of the deserialized object is only known at runtime (when the byte stream is parsed). Generics are a compile-time feature, so they can’t bridge this gap.

• Explicit casting as a safety check
  When you cast (Car) ois.readObject(), you’re telling the compiler: "I’m sure this object is a Car (or a subclass)." If you’re wrong (e.g., someone modified the obj.dat file to hold a different object), you’ll get a ClassCastException at runtime—this is a good thing, because it catches mismatches early instead of letting invalid types slip through silently.

For example, imagine you wrote code like this:

// Write a mix of objects
oos.writeObject(new Car("yellow"));
oos.writeObject("Hello World");
oos.writeObject(new Dog("buddy"));

// Read them back
Car car = (Car) ois.readObject(); // Works
String str = (String) ois.readObject(); // Works
Dog dog = (Dog) ois.readObject(); // Works

If readObject() returned a specific type, this kind of mixed-type serialization would be impossible.

内容的提问来源于stack exchange，提问作者Stefan