Serialization & transient

transient marks a field to be excluded from serialization. On deserialization, it is set to its default value (0, null, false).

transient fields are skipped during serialization and reset to defaults on deserialization. See Q42 for the full explanation.

transient is about serialization (skip this field). volatile is about thread visibility (read/write directly from main memory). They are unrelated concerns.

Serialization converts an object to bytes for storage/network. The class must implement Serializable. Modern code prefers JSON or Protobuf over Java's built-in serialization.

Plain text loses type information and cannot represent complex object graphs. Binary serialization preserves types and references. JSON is the modern compromise: structured, typed, human-readable.

A non-serializable, non-transient field causes NotSerializableException at runtime. Fix by marking it transient, making it Serializable, or implementing custom serialization.

serialVersionUID is a version stamp. If the class changes and the UID does not match the stored bytes, deserialization fails with InvalidClassException. Always declare it explicitly.

ObjectOutputStream.writeObject() serializes. ObjectInputStream.readObject() deserializes. Use try-with-resources for both.

No. Static fields belong to the class, not the object. Serialization saves only instance state. On deserialization, static fields have the class's current value.

Override writeObject() in the child to throw NotSerializableException. This prevents serialization even though the parent is Serializable.

Externalizable gives full control over serialization format. You implement writeExternal/readExternal. Unlike Serializable, it calls the no-arg constructor during deserialization.