Eliminating Case Analysis

Eliminate Case Analysis is a second rule to find standard protocols for reusable object-oriented softwares.

This rule focuses on utilizing polymorphism as much as possible to improve code’s maintainability.

It tries to remove code like…

if (object instanceof A) {
	A.method();
} else if (object instanceof B) {
	B.method();
}

How? By replacing type check with sending messages (Calling methods) to the target object, and actual type is evaluated there, by dynamic dispatch.

Example

public interface ReportObject {
	// Nothing in common interface
}
 
public class TextReport implements ReportObject {
	public void renderTextContent() {
		System.out.println("Text Rendering");
	}
}
 
public class GraphReport implements ReportObject {
	public void renderGraphicalContent() {
		System.out.println("Graph Rendering");
	}
}
 
public class Renderer {
	public void render(RenderObject report) {
		if (report instanceof TextReport) {
			((TextReport) report).renderTextContent();
		} else if (report instanceof GraphReport) {
			((GraphReport) report).renderGraphicalContent();
		} else {
			System.out.println("Unknown Report Type");
		}
	}
}

• If a new report type (e.g. TableReport) is added, Renderer class’s render method MUST be modified. (No Good)

To tackle this issue,

1. Introduce standard protocol

public interface ReportObject {
	void render();
}

2. Each report classes implement render()

public class TextReport implements ReportObject {
    @Override
    public void render() {
		System.out.println("Text Rendering");
    }
}
 
public class GraphReport implements ReportObject {
    @Override
    public void render() {
		System.out.println("Graph Rendering");
    }
}

3. Renderer class now doesn’t need to check type. It send message to the object instead.

public class Renderer {
    public void render(ReportObject report) {
        report.render(); 
    }
}

Now it enhances maintainability of Renderer, because now it doesn’t need to know all subtypes of ReportObject.

• Even with new ReportObject subtype, Renderer doesn’t need to be changed.
• report.render() call, in runtime, choose the method of actual class, utilizing polymorphism of OOP (Dynamic Dispatch)

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Note

References

• 5.1 Rules for Finding Standard Protocols
• 토비의 봄 TV 1회 - 재사용성과 다이나믹 디스패치, 더블 디스패치