Classes and Objects

A Class is a static template or blueprint defining the structure of fields (state) and methods (behavior). An Object is a dynamic, heap-allocated instance of a class, managed at runtime via reference stack pointers.

Stack vs. Heap Memory Layout

When you declare and instantiate an object (e.g. User user = new User();), the JVM splits the memory allocation:

• The Stack Frame: Stores the local reference variable user. The value of this variable is a 64-bit or 32-bit address (under compressed oops) pointing to the heap.
• The Heap Space: Stores the actual object payload, including the object header (mark word + class word) followed by the instance fields.

Object Lifecycle and Garbage Collection

An object's lifecycle progresses through three main phases:

• Creation: Instantiated using the new keyword, which triggers heap allocation, zeroing of fields, and constructor execution.
• Reachability: Active reference paths connect stack frames or static registers to the object.
• GC Eligibility: Once all references pointing to the object are broken or cleared (e.g., set to null or the local frame pops), the object is unreachable and becomes eligible for garbage collection.

Common Pitfalls

• Memory Leaks: Retaining references to unused objects in long-lived collections (like static Lists), preventing garbage collection.
• NullPointerExceptions (NPE): Dereferencing a reference variable that points to null.
• Confusing Class variables with Instance variables: Assuming changes to class fields only affect a single object instance.

Best Practices

• Encapsulate class fields by declaring them private and exposing control via getter and setter methods.
• Break reference paths explicitly by setting large caches to null when they are no longer needed, or use weak references.
• Design classes with minimal, cohesive states to improve testability and maintenance.

Interview-Relevant Information