Why Your 900-Line Class Is a Nightmare and How to Refactor It

Why Classes Shouldn't Be Too Long?

Long classes are hard to read and extend. A 900‑line class with dozens of public interfaces becomes unreadable, difficult to maintain, and prone to bugs.

Problems of Overly Long Classes

Unreadable – scrolling takes seconds; even the original author loses context.

Hard to extend – many interfaces make changes risky and time‑consuming.

Redundant code – copy‑paste leads to duplicated methods and maintenance headaches.

Too many responsibilities – violates the Single Responsibility Principle, causing ripple‑effect changes such as test failures and operational pain.

What If You Already Have Thousands of Lines?

Refactor by extracting redundant code into separate methods.

Extract Redundant Code

Identify duplicated blocks, use IDEA’s Extract Method (Ctrl+Alt+M) to create a reusable method, then replace the copies.

Change Method Signature

Use Change Signature (Ctrl+F6) to rename, adjust parameters, or modify visibility for better clarity.

Move Fields and Methods

Move fields or methods to the class that actually owns the responsibility (Refactor → Move). Decide which member to move first, then move the related method.

Extract a New Class

If no existing class fits, create a new class and delegate the related fields and methods to it. Ensure the extracted method uses the moved fields more often than the remaining code to respect the cohesion principle.

IDEA can automatically detect slight variations and suggest replacements for all duplicated occurrences.