2.14 Garbage Collection in Java

Learning Objectives

•Explain when an object becomes eligible for garbage collection.
•Describe the role of the garbage collector in the JVM.
•Understand why Java has no manual `free`/`delete` for objects.

Introduction

Java manages memory automatically: once no reachable reference points to an object anymore, it becomes eligible for garbage collection, and the JVM's garbage collector reclaims its memory in the background. Unlike C/C++, Java programmers do not manually free object memory.

Key Concepts

An object becomes eligible for garbage collection once nothing in the reachable program state references it anymore.

The garbage collector runs automatically in the background; its exact timing is not guaranteed or directly controllable.

Setting a reference to `null` can make an object eligible sooner by removing the last reachable reference to it.

Garbage collection prevents most memory leaks common in manually-managed languages, but does not eliminate all forms of them (e.g. objects unintentionally kept reachable via long-lived collections).

Common Mistakes

⚠Calling something like `System.gc()` and assuming it forces immediate collection — it's only a hint to the JVM, not a guarantee.
⚠Assuming Java programs can never leak memory — an object referenced from a long-lived collection that's never cleaned up still leaks, even with garbage collection.

Summary

The JVM's garbage collector automatically reclaims memory for objects that are no longer reachable, freeing Java programmers from manual memory management — though references that outlive their usefulness can still cause leaks.

Previous TopicNext Topic