Why Your Java Classes Shouldn’t Exceed a Thousand Lines—and How to Refactor Them

Promise me, no more thousand‑line classes, okay?

When extending an existing project I encountered a class that was 766 lines long and exposed nearly 40 public interfaces. I rewrote it in tears and decided to write this article to highlight the importance of code refactoring.

Why should a class not be too long?

Unreadable and hard to extend

Reading a huge class takes seconds of scrolling; even the original author can lose track of the logic.

Too many interfaces make extending the class painful; a single change may require modifying thousands of lines.

Redundant code

Redundant code—often produced by copy‑pasting—leads to:

Long, non‑concise methods and classes.

Scatter‑shot modifications, because every duplicated fragment must be updated.

Too many responsibilities

A class with dozens of interfaces almost certainly violates the Single Responsibility Principle. This causes:

Design principle violations.

Ripple‑effect changes (divergent changes) across many callers.

Ripple‑effect modifications across many callees.

Difficulty extending the class.

Triggering “test anger” and “operations anger”.

I already have a class with thousands of lines—what now?

Refactor: Extract redundant code

Extract duplicated code into a separate method, then replace all copies with a method call. This shortens the original method, makes it easier to understand, and centralizes future changes.

Using IntelliJ IDEA to extract redundant code

Identify the duplicated code.

Right‑click → Refactor → Extract → Method (or press Ctrl+Alt+M).

IDEA can detect slight variations and suggest signature changes to cover more duplicates.

Refactor: Change method signature

If the extracted method’s name, parameters, return type, or modifiers are unsatisfactory, use Refactor → Change Signature (shortcut Ctrl+F6). Use a verb‑noun naming style, e.g., Tom.sweepFloor() instead of Tom.sweepFloorWithBroom().

Refactor: Move fields and methods (transfer responsibilities)

Move fields or methods that belong to another class:

Select the member → Refactor → Move → choose target class.

Refactor: Extract a new class

When no existing class can take the responsibilities, create a new class and delegate the relevant fields and methods to it.

Using IntelliJ IDEA to extract a class

Select members → Refactor → Extract → Delegate (or extract Parameter/Object or Method Object if appropriate).

Name the new class and choose a package.

Ensure the extracted methods use the moved fields more often than the remaining code to respect the cohesion principle.

These refactoring steps help keep classes small, maintainable, and aligned with solid design principles.