Object Class

The java.lang.Object class is the root of the Java class hierarchy. Every class in Java inherits from Object, either directly or indirectly, inheriting a set of core methods.

Object Cloning: Shallow vs. Deep Copy

The clone() method creates a copy of an object:

• To use clone(), a class must implement the Cloneable marker interface. Otherwise, calling super.clone() throws a CloneNotSupportedException.
• Shallow Copy: The default implementation of clone() copies primitive fields, but copies reference pointers for object fields. Both the original and cloned object share references to the same nested sub-objects.
• Deep Copy: To isolate the clone, you must override clone(), call super.clone(), and then explicitly clone all nested mutable reference objects manually.

The Deprecation of finalize()

The finalize() method was designed to perform cleanup before an object is garbage collected. It was deprecated in Java 9 and removed/deprecated-for-removal in modern versions due to serious design flaws:

• Unpredictable Execution: There is no guarantee when (or if) the garbage collector will run finalize, leading to resource leaks.
• Performance Bottleneck: Objects with finalizers slow down garbage collection allocation cycles.
• Security Vulnerability: A finalize method can resurrect objects by reassigning references during GC, bypassing constructor invariants.

Modern Alternative: Use the AutoCloseable interface combined with try-with-resources blocks for predictable resource cleanup.

Common Pitfalls

• Assuming clone() is deep: Using the default clone method and modifying shared nested structures, corrupting the original object.
• Using finalize() for resource cleanup: Relying on finalizers to close database connections or files, causing resource leaks.
• Calling clone() on non-Cloneable: Invoking clone on objects that do not implement Cloneable, throwing runtime exceptions.

Best Practices

• Avoid using the clone() method altogether. Use copy constructors (e.g. public User(User other)) or factory methods to copy objects instead.
• Implement AutoCloseable for classes that hold system resources (sockets, files) to enforce predictable cleanup.
• Override equals(), hashCode(), and toString() to provide proper domain-specific behaviors.

Interview-Relevant Information