What Is an OutOfMemoryError?

An OutOfMemoryError in Java is a runtime error that happens when the Java Virtual Machine (JVM) runs out of memory to allocate for new objects. This means your program has used more memory than the JVM can provide, and it cannot continue running as expected.

This error is not a typical exception that you can easily recover from in your code. Instead, it signals a serious problem, such as creating too many objects, holding onto objects longer than needed, or having memory leaks.

When you see an OutOfMemoryError, your application might crash or behave unpredictably. Understanding why this happens is important for writing efficient and reliable Java programs.

Why OutOfMemoryError Happens

An OutOfMemoryError in Java happens when your application tries to use more memory than the Java Virtual Machine (JVM) can provide. This usually means your code is creating too many objects, holding onto references longer than needed, or processing large data sets that exceed the available memory. When the JVM cannot find enough space to create a new object, it throws an OutOfMemoryError.

How OutOfMemoryError Affects Applications

When an OutOfMemoryError occurs, your application may slow down, behave unpredictably, or even crash. This error stops the program from working correctly because it cannot create new objects or complete important tasks. In many cases, the only way to recover is to restart the application, which can cause data loss or a poor user experience.

Real-World Example: Loading Large Files Into Memory

Imagine you are building a Java application that processes large CSV files uploaded by users. If you attempt to read an entire file into memory using a List<String>, and a user uploads a file that is much larger than your available heap space, your application can throw an OutOfMemoryError.

Scenario:

• You have a server with 512 MB of maximum heap size;
• A user uploads a 2 GB CSV file;
• Your code reads all lines into a single list before processing.

What happens:

• The JVM tries to allocate memory for every line in the file;
• When the memory required exceeds the available heap, the JVM throws an OutOfMemoryError and the application crashes.

Always consider the size of data you load into memory. Use streaming APIs (like BufferedReader or Stream<String>) to process large files line by line, rather than loading the entire file at once.