A memory leak in Java happens when your program holds onto memory it no longer needs. This means objects are kept in memory even though your code will never use them again.

Java uses automatic garbage collection to free up unused memory. However, if your program still has references to objects that are not needed, the garbage collector cannot remove them. Over time, this can fill up the available memory and eventually cause your application to slow down or crash.

Memory leaks are a common cause of the OutOfMemoryError in Java. Recognizing and fixing memory leaks is important for building reliable, efficient applications.

Memory Leak Example: Unclosed Listeners in a GUI Application

A common way memory leaks happen in Java is when objects are unintentionally kept alive by event listeners or references, even though you no longer need them. This often occurs in desktop GUI applications.

Suppose you have a window in a Java application that registers itself as a listener to another object, but never unregisters. Even after you close the window, the listener reference prevents the window from being garbage collected. Over time, this can use up all available memory.

Here’s a beginner-friendly scenario:

• You create a Window object that listens to updates from a DataManager.
• Each time you open a new window, it registers itself as a listener.
• When you close a window, you forget to remove it from the list of listeners.
• The DataManager keeps a reference to every window ever opened, even closed ones, so they are never garbage collected.

This is a memory leak because the unused windows stay in memory forever.

Key points:

• Always unregister listeners when you no longer need them;
• Be careful with long-lived objects holding references to short-lived objects;
• Use tools like profilers to spot memory leaks during development.