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Effective Java! Limit Source Files to a Single Top-Level Class

kylec32 profile image Kyle Carter ・2 min read

Effective Java Review (36 Part Series)

1) Effective Java Tuesday! Let's Consider Static Factory Methods 2) Effective Java Tuesday! The Builder Pattern! 3 ... 34 3) Effective Java Tuesday! Singletons! 4) Effective Java Tuesday! Utility Classes! 5) Effective Java Tuesday! Prefer Dependency Injection! 6) Effective Java Tuesday! Avoid Creating Unnecessary Objects! 7) Effective Java Tuesday! Don't Leak Object References! 8) Effective Java Tuesday! Avoid Finalizers and Cleaners! 9) Effective Java Tuesday! Prefer try-with-resources 10) Effective Java Tuesday! Obey the `equals` contract 11) Effective Java Tuesday! Obey the `hashCode` contract 12) Effective Java Tuesday! Override `toString` 13) Effective Java Tuesday! Override `clone` judiciously 14) Effective Java Tuesday! Consider Implementing `Comparable` 15) Effective Java Tuesday! Minimize the Accessibility of Classes and Member 16) Effective Java Tuesday! In Public Classes, Use Accessors, Not Public Fields 17) Effective Java Tuesday! Minimize Mutability 18) Effective Java Tuesday! Favor Composition Over Inheritance 19) Effective Java Tuesday! Design and Document Classes for Inheritance or Else Prohibit It. 20) Effective Java Tuesday! Prefer Interfaces to Abstract Classes 21) Effective Java! Design Interfaces for Posterity 22) Effective Java! Use Interfaces Only to Define Types 23) Effective Java! Prefer Class Hierarchies to Tagged Classes 24) Effective Java! Favor Static Members Classes over Non-Static 25) Effective Java! Limit Source Files to a Single Top-Level Class 26) Effective Java! Don't Use Raw Types 27) Effective Java! Elminate Unchecked Warnings 28) Effective Java! Prefer Lists to Array 29) Effective Java! Favor Generic Types 30) Effective Java! Favor Generic Methods 31) Effective Java! Use Bounded Wildcards to Increase API Flexibility 32) Effective Java! Combine Generics and Varargs Judiciously 33) Effective Java! Consider Typesafe Hetergenous Containers 34) Effective Java! Use Enums Instead of int Constants 35) Effective Java! Use Instance Fields Instead of Ordinals 36) Effective Java! Use EnumSet Instead of Bit Fields

Today we tackle a topic similar to the topic from last week where we considered nested classes. This time we consider multiple top-level classes in a single source file. The difference between what we are considering today and what we considered last week is that these classes are at the same level as the "base class". You may not even know that you can have multiple top level classes in a source file (you can only have one top level public class in a source file) and, per this chapter, that's OK because you shouldn't be using this ability.

Let's consider the benefits we get from having multiple top level classes in a file. We don't need to create as many files. It could be used a way to group related classes together. We don't pay extra for additional files so we should be good skipping that benefit and we can use packages to group related files.

So let's look a little more concretely into what problems not following this advice can cause. One that the book doesn't mention is that this is an unexpected ability to take advantage of, because of this, it can be confusing to future developers working in your project when they are looking for where a specific class is defined. On top of this the main benefit that the book mentions is that you can run into cases where you have two classes defining the same class and which instance gets selected depends on how the code was compiled. This is demonstrated in the book by going through different orders of parameters you could pass to the Java compiler on the command line. Now I don't know about you, but I usually am not compiling code on the command line, I'm usually using my IDE so not only does the order affect how the code will execute but we, most of the time, don't have visibilty into the order or parameters passed to the compiler.

To sum it up, having multiple classes defined at the top level in a single source file provides us basically no value but does bring with it predictability and maintainability troubles and thus should be avoided.

Effective Java Review (36 Part Series)

1) Effective Java Tuesday! Let's Consider Static Factory Methods 2) Effective Java Tuesday! The Builder Pattern! 3 ... 34 3) Effective Java Tuesday! Singletons! 4) Effective Java Tuesday! Utility Classes! 5) Effective Java Tuesday! Prefer Dependency Injection! 6) Effective Java Tuesday! Avoid Creating Unnecessary Objects! 7) Effective Java Tuesday! Don't Leak Object References! 8) Effective Java Tuesday! Avoid Finalizers and Cleaners! 9) Effective Java Tuesday! Prefer try-with-resources 10) Effective Java Tuesday! Obey the `equals` contract 11) Effective Java Tuesday! Obey the `hashCode` contract 12) Effective Java Tuesday! Override `toString` 13) Effective Java Tuesday! Override `clone` judiciously 14) Effective Java Tuesday! Consider Implementing `Comparable` 15) Effective Java Tuesday! Minimize the Accessibility of Classes and Member 16) Effective Java Tuesday! In Public Classes, Use Accessors, Not Public Fields 17) Effective Java Tuesday! Minimize Mutability 18) Effective Java Tuesday! Favor Composition Over Inheritance 19) Effective Java Tuesday! Design and Document Classes for Inheritance or Else Prohibit It. 20) Effective Java Tuesday! Prefer Interfaces to Abstract Classes 21) Effective Java! Design Interfaces for Posterity 22) Effective Java! Use Interfaces Only to Define Types 23) Effective Java! Prefer Class Hierarchies to Tagged Classes 24) Effective Java! Favor Static Members Classes over Non-Static 25) Effective Java! Limit Source Files to a Single Top-Level Class 26) Effective Java! Don't Use Raw Types 27) Effective Java! Elminate Unchecked Warnings 28) Effective Java! Prefer Lists to Array 29) Effective Java! Favor Generic Types 30) Effective Java! Favor Generic Methods 31) Effective Java! Use Bounded Wildcards to Increase API Flexibility 32) Effective Java! Combine Generics and Varargs Judiciously 33) Effective Java! Consider Typesafe Hetergenous Containers 34) Effective Java! Use Enums Instead of int Constants 35) Effective Java! Use Instance Fields Instead of Ordinals 36) Effective Java! Use EnumSet Instead of Bit Fields

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Kyle Carter

@kylec32

Backend Architect at MasterControl

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