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

Effective Java Review (90 Part Series)

1 Effective Java Tuesday! Let’s Consider Static Factory Methods
2 Effective Java Tuesday! The Builder Pattern!
… 86 more parts…
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! Eliminate 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 Heterogenous 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
37 Effective Java! Use EnumMap instead of Ordinal Indexing
38 Effective Java! Emulate Extensible Enums With Interfaces.
39 Effective Java! Prefer Annotations to Naming Patterns
40 Effective Java! Consistently Use the Override Annotation
41 Effective Java! Use Marker Interfaces to Define Types
42 Effective Java! Prefer Lambdas to Anonymous Classes
43 Effective Java! Prefer Method References to Lambdas
44 Effective Java! Favor the Use of Standard Functional Interfaces
45 Effective Java! Use Stream Judiciously
46 Effective Java! Prefer Side-Effect-Free Functions in Streams
47 Effective Java! Prefer Collection To Stream as a Return Type
48 Effective Java! Use Caution When Making Streams Parallel
49 Effective Java! Check Parameters for Validity
50 Effective Java! Make Defensive Copies When Necessary
51 Effective Java! Design Method Signatures Carefully
52 Effective Java! Use Overloading Judiciously
53 Effective Java! Use Varargs Judiciously
54 Effective Java! Return Empty Collections or Arrays, Not Nulls
55 Effective Java! Return Optionals Judiciously
56 Effective Java: Write Doc Comments For All Exposed APIs
57 Effective Java: Minimize The Scope of Local Variables
58 Effective Java: Prefer for-each loops to traditional for loops
59 Effective Java: Know and Use the Libraries
60 Effective Java: Avoid Float and Double If Exact Answers Are Required
61 Effective Java: Prefer Primitive Types to Boxed Types
62 Effective Java: Avoid Strings When Other Types Are More Appropriate
63 Effective Java: Beware the Performance of String Concatenation
64 Effective Java: Refer to Objects By Their Interfaces
65 Effective Java: Prefer Interfaces To Reflection
66 Effective Java: Use Native Methods Judiciously
67 Effective Java: Optimize Judiciously
68 Effective Java: Adhere to Generally Accepted Naming Conventions
69 Effective Java: Use Exceptions for Only Exceptional Circumstances
70 Effective Java: Use Checked Exceptions for Recoverable Conditions
71 Effective Java: Avoid Unnecessary Use of Checked Exceptions
72 Effective Java: Favor The Use of Standard Exceptions
73 Effective Java: Throw Exceptions Appropriate To The Abstraction
74 Effective Java: Document All Exceptions Thrown By Each Method
75 Effective Java: Include Failure-Capture Information in Detail Messages
76 Effective Java: Strive for Failure Atomicity
77 Effective Java: Don’t Ignore Exceptions
78 Effective Java: Synchronize Access to Shared Mutable Data
79 Effective Java: Avoid Excessive Synchronization
80 Effective Java: Prefer Executors, Tasks, and Streams to Threads
81 Effective Java: Prefer Concurrency Utilities Over wait and notify
82 Effective Java: Document Thread Safety
83 Effective Java: Use Lazy Initialization Judiciously
84 Effective Java: Don’t Depend on the Thread Scheduler
85 Effective Java: Prefer Alternatives To Java Serialization
86 Effective Java: Implement Serializable With Great Caution
87 Effective Java: Consider Using a Custom Serialized Form
88 Effective Java: Write readObject Methods Defensively
89 Effective Java: For Instance Control, Prefer Enum types to readResolve
90 Effective Java: Consider Serialization Proxies Instead of Serialized Instances

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 (90 Part Series)

1 Effective Java Tuesday! Let’s Consider Static Factory Methods
2 Effective Java Tuesday! The Builder Pattern!
… 86 more parts…
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! Eliminate 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 Heterogenous 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
37 Effective Java! Use EnumMap instead of Ordinal Indexing
38 Effective Java! Emulate Extensible Enums With Interfaces.
39 Effective Java! Prefer Annotations to Naming Patterns
40 Effective Java! Consistently Use the Override Annotation
41 Effective Java! Use Marker Interfaces to Define Types
42 Effective Java! Prefer Lambdas to Anonymous Classes
43 Effective Java! Prefer Method References to Lambdas
44 Effective Java! Favor the Use of Standard Functional Interfaces
45 Effective Java! Use Stream Judiciously
46 Effective Java! Prefer Side-Effect-Free Functions in Streams
47 Effective Java! Prefer Collection To Stream as a Return Type
48 Effective Java! Use Caution When Making Streams Parallel
49 Effective Java! Check Parameters for Validity
50 Effective Java! Make Defensive Copies When Necessary
51 Effective Java! Design Method Signatures Carefully
52 Effective Java! Use Overloading Judiciously
53 Effective Java! Use Varargs Judiciously
54 Effective Java! Return Empty Collections or Arrays, Not Nulls
55 Effective Java! Return Optionals Judiciously
56 Effective Java: Write Doc Comments For All Exposed APIs
57 Effective Java: Minimize The Scope of Local Variables
58 Effective Java: Prefer for-each loops to traditional for loops
59 Effective Java: Know and Use the Libraries
60 Effective Java: Avoid Float and Double If Exact Answers Are Required
61 Effective Java: Prefer Primitive Types to Boxed Types
62 Effective Java: Avoid Strings When Other Types Are More Appropriate
63 Effective Java: Beware the Performance of String Concatenation
64 Effective Java: Refer to Objects By Their Interfaces
65 Effective Java: Prefer Interfaces To Reflection
66 Effective Java: Use Native Methods Judiciously
67 Effective Java: Optimize Judiciously
68 Effective Java: Adhere to Generally Accepted Naming Conventions
69 Effective Java: Use Exceptions for Only Exceptional Circumstances
70 Effective Java: Use Checked Exceptions for Recoverable Conditions
71 Effective Java: Avoid Unnecessary Use of Checked Exceptions
72 Effective Java: Favor The Use of Standard Exceptions
73 Effective Java: Throw Exceptions Appropriate To The Abstraction
74 Effective Java: Document All Exceptions Thrown By Each Method
75 Effective Java: Include Failure-Capture Information in Detail Messages
76 Effective Java: Strive for Failure Atomicity
77 Effective Java: Don’t Ignore Exceptions
78 Effective Java: Synchronize Access to Shared Mutable Data
79 Effective Java: Avoid Excessive Synchronization
80 Effective Java: Prefer Executors, Tasks, and Streams to Threads
81 Effective Java: Prefer Concurrency Utilities Over wait and notify
82 Effective Java: Document Thread Safety
83 Effective Java: Use Lazy Initialization Judiciously
84 Effective Java: Don’t Depend on the Thread Scheduler
85 Effective Java: Prefer Alternatives To Java Serialization
86 Effective Java: Implement Serializable With Great Caution
87 Effective Java: Consider Using a Custom Serialized Form
88 Effective Java: Write readObject Methods Defensively
89 Effective Java: For Instance Control, Prefer Enum types to readResolve
90 Effective Java: Consider Serialization Proxies Instead of Serialized Instances

原文链接：Effective Java! Limit Source Files to a Single Top-Level Class