Why Overly Long Classes Break Your Code and How to Refactor Them with IDEA

Why Classes Should Not Be Too Long

Classes that grow to hundreds or thousands of lines and expose dozens of public methods become difficult to read and even harder to extend.

Reading: Scrolling through a massive file takes seconds; even the original author can lose the overall picture.

Extending: A large public interface forces changes to affect many callers and large sections of code.

Redundant Code

Redundant (duplicate) code usually originates from copy‑paste. It makes methods and classes overly long and forces scattered modifications whenever a change is required.

Methods and classes become excessively long and less concise.

When a change is needed, every duplicated fragment must be updated, leading to scattered modifications.

Too Many Responsibilities

A class that exposes dozens of interfaces almost always violates the Single Responsibility Principle (SRP). The consequences are:

Violation of SRP – a class should implement only one responsibility; otherwise changes ripple across many callers.

Divergent changes – many callers depend on the class, so a change forces modifications in many places.

Divergent modifications – internal logic changes require updates in all dependent code.

Hard to extend – subclasses or wrappers must implement a huge number of methods, making extension cumbersome.

Increased testing and operational effort.

Refactoring Strategies for Very Large Classes

Extract Redundant Code

Identify duplicated fragments and extract them into a separate method. Replace each duplicate with a call to the new method, which shortens the original method and centralises future changes.

Steps in IntelliJ IDEA:

Locate the duplicated code.

Use Refactor → Extract → Method (shortcut Ctrl+Alt+M).

IDEA detects minor differences (e.g., variable names). Accept signature changes to replace additional occurrences automatically.

If needed, rename or adjust the method signature via Refactor → Change Signature (shortcut Ctrl+F6). Use a verb‑noun name such as cleanUp().

Move Fields and Methods (Transfer Responsibility)

Move members that do not belong to the current class to a more appropriate class.

Decide which member to move and the target class.

Select the field, invoke Refactor → Move, and choose the destination class.

Repeat the same steps for methods.

Extract a New Class

If no existing class can take the moved members, create a new class to hold them.

IntelliJ IDEA workflow:

Select the fields and methods to extract, then choose Refactor → Extract → Delegate (or extract a Parameter Object / Method Object when appropriate).

Provide a meaningful class name and place it in an appropriate package.

Ensure the extracted methods use the extracted fields more often than the remaining methods (cohesion rule). Otherwise the extraction may violate the intimacy principle.

If the extracted method still accesses non‑extracted members, replace direct field access with getters/setters.

Decide which member variable to move before deciding which method to move.