Why Long Classes Are Problematic and How to Refactor Them

When extending an existing project, the author discovered a class with 766 lines and nearly 40 public interfaces, which was painful to modify, prompting a discussion on the importance of code refactoring.

Why a Class Should Not Be Too Long?

Long classes are hard to read because scrolling through hundreds of lines takes time, even for the original author, and they are difficult to extend because many interfaces make changes cumbersome.

Redundant code—often produced by copy‑and‑paste—leads to bloated methods, scattered modifications, and increased maintenance effort.

Having too many responsibilities in a single class violates the Single Responsibility Principle, causing divergent changes, widespread impact of bugs, and making the class hard to extend or test.

What to Do If Your Class Has Thousands of Lines?

Refactor – Extract Redundant Code

Identify duplicate code and extract it into a separate method, replacing the copies with a method call. This shortens the original method, makes it clearer, and centralizes future changes.

In IntelliJ IDEA, use Right‑click → Refactor → Extract → Method or the shortcut Ctrl+Alt+M . The IDE can detect slight variations and suggest signature changes.

Refactor – Change Method Signature

If the extracted method’s name, parameters, return type, or modifiers are unsatisfactory, use Refactor → Change Signature (shortcut Ctrl+F6 ) to rename or adjust it. Method names should follow a verb‑noun pattern.

Refactor – Move Fields and Methods (Transfer Responsibility)

Move fields and methods that belong to another class by selecting them, right‑clicking, and choosing Refactor → Move . Decide which class should own the member based on usage frequency.

Refactor – Extract Class

When members cannot be moved to an existing class, create a new class to take over the responsibilities. Use Refactor → Extract → Delegate to generate a delegate or parameter/method object.

Ensure the extracted members are used more often by the new class than by the original, adhering to the cohesion principle.

Be aware of potential issues, such as extracted methods still accessing unextracted objects; in such cases, further refactoring (e.g., using getters) may be required.

Technical concepts apply across domains; the underlying design principles remain consistent.