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## Intent
Aims to simplify methods that collect information by passing a single collection object through various method calls,
allowing them to add results to this collection rather than each method creating its own collection.
Aims to simplify methods that collect information by passing a single collection object through various method calls, allowing them to add results to this collection rather than each method creating its own collection.
## Explanation
### Real-world example
Within a large corporate building, there exists a global printer queue that is a collection of all the printing jobs
that are currently pending. Various floors contain different models of printers, each having a different printing
policy. We must construct a program that can continually add appropriate printing jobs to a collection, which is called
the *collecting parameter*.
Within a large corporate building, there exists a global printer queue that is a collection of all the printing jobs that are currently pending. Various floors contain different models of printers, each having a different printing policy. We must construct a program that can continually add appropriate printing jobs to a collection, which is called the *collecting parameter*.
### In plain words
Instead of having one giant method that contains numerous policies for collecting information into a variable, we can
create numerous smaller functions that each take parameter, and append new information. We can pass the parameter to all
of these smaller functions and by the end, we will have what we wanted originally. This time, the code is cleaner and
easier to understand. Because the larger function has been broken down, the code is also easier to modify as changes are
localised to the smaller functions.
Instead of having one giant method that contains numerous policies for collecting information into a variable, we can create numerous smaller functions that each take parameter, and append new information. We can pass the parameter to all of these smaller functions and by the end, we will have what we wanted originally. This time, the code is cleaner and easier to understand. Because the larger function has been broken down, the code is also easier to modify as changes are localised to the smaller functions.
### Wikipedia says
In the Collecting Parameter idiom a collection (list, map, etc.) is passed repeatedly as a parameter to a method which
adds items to the collection.
In the Collecting Parameter idiom a collection (list, map, etc.) is passed repeatedly as a parameter to a method which adds items to the collection.
### Programmatic example
Coding our example from above, we may use the collection `result` as a collecting parameter. The following restrictions
are implemented:
Coding our example from above, we may use the collection `result` as a collecting parameter. The following restrictions are implemented:
- If an A4 paper is coloured, it must also be single-sided. All other non-coloured papers are accepted
- A3 papers must be non-coloured and single-sided
@@ -85,8 +75,7 @@ public class App {
}
```
We use the `addA4Paper`, `addA3Paper`, and `addA2Paper` methods to populate the `result` collecting parameter with the
appropriate print jobs as per the policy described previously. The three policies are encoded below,
We use the `addA4Paper`, `addA3Paper`, and `addA2Paper` methods to populate the `result` collecting parameter with the appropriate print jobs as per the policy described previously. The three policies are encoded below,
```java
public class App {
@@ -162,12 +151,9 @@ public class App {
}
```
Each method takes a collecting parameter as an argument. It then adds elements, taken from a global variable, to this
collecting parameter if each element satisfies a given criteria. These methods can have whatever policy the client
desires.
Each method takes a collecting parameter as an argument. It then adds elements, taken from a global variable, to this collecting parameter if each element satisfies a given criteria. These methods can have whatever policy the client desires.
In this programmatic example, three print jobs are added to the queue. Only the first two print jobs should be added to
the collecting parameter as per the policy. The elements of the `result` variable after execution are,
In this programmatic example, three print jobs are added to the queue. Only the first two print jobs should be added to the collecting parameter as per the policy. The elements of the `result` variable after execution are,
| paperSize | pageCount | isDoubleSided | isColour |
|-----------|-----------|---------------|----------|
@@ -185,26 +171,19 @@ which is what we expected.
Use the Collecting Parameter design pattern when
- When multiple methods produce a collection of results and you want to aggregate these results in a unified manner.
- In scenarios where reducing the number of collections created by methods can improve memory efficiency and
performance.
- When you're refactoring large methods that perform multiple tasks, including the collection of results from various
operations.
- In scenarios where reducing the number of collections created by methods can improve memory efficiency and performance.
- When you're refactoring large methods that perform multiple tasks, including the collection of results from various operations.
## Tutorials
Tutorials for this method are found in:
- [Refactoring To Patterns](http://www.tarrani.net/RefactoringToPatterns.pdf) by Joshua Kerivsky
- [Smalltalk Best Practice Patterns](https://ptgmedia.pearsoncmg.com/images/9780134769042/samplepages/013476904X.pdf) by
Kent Beck
- [Smalltalk Best Practice Patterns](https://ptgmedia.pearsoncmg.com/images/9780134769042/samplepages/013476904X.pdf) by Kent Beck
## Known uses
Joshua Kerivsky gives a real-world example in his book 'Refactoring to Patterns'. He gives an example of using the
Collecting Parameter Design Pattern to create a `toString()` method for an XML tree. Without using this design pattern,
this would require a bulky function with conditionals and concatenation that would worsen code readability. Such a
method can be broken down into smaller methods, each appending their own set of information to the collecting parameter.
See this in [Refactoring To Patterns](http://www.tarrani.net/RefactoringToPatterns.pdf).
Joshua Kerivsky gives a real-world example in his book 'Refactoring to Patterns'. He gives an example of using the Collecting Parameter Design Pattern to create a `toString()` method for an XML tree. Without using this design pattern, this would require a bulky function with conditionals and concatenation that would worsen code readability. Such a method can be broken down into smaller methods, each appending their own set of information to the collecting parameter. See this in [Refactoring To Patterns](http://www.tarrani.net/RefactoringToPatterns.pdf).
Other examples include:
@@ -223,23 +202,18 @@ Benefits:
Trade-offs:
- Increases coupling between the caller and the methods being called since they must agree on the collection to use.
- May introduce side effects in methods if not carefully managed, as methods are no longer self-contained in their
result handling.
- May introduce side effects in methods if not carefully managed, as methods are no longer self-contained in their result handling.
## Related patterns
- [Composite](https://java-design-patterns.com/patterns/composite/): Can be used in tandem with Collecting Parameter
when dealing with hierarchical structures, allowing results to be collected across a composite structure.
- [Visitor](https://java-design-patterns.com/patterns/visitor/): Often used together, where Visitor handles traversal
and operations on a structure, and Collecting Parameter accumulates the results.
- [Command](https://java-design-patterns.com/patterns/command/): Commands may utilize Collecting Parameter to aggregate
results from multiple operations executed by the command objects.
- [Composite](https://java-design-patterns.com/patterns/composite/): Can be used in tandem with Collecting Parameter when dealing with hierarchical structures, allowing results to be collected across a composite structure.
- [Visitor](https://java-design-patterns.com/patterns/visitor/): Often used together, where Visitor handles traversal and operations on a structure, and Collecting Parameter accumulates the results.
- [Command](https://java-design-patterns.com/patterns/command/): Commands may utilize Collecting Parameter to aggregate results from multiple operations executed by the command objects.
## Credits
- [Refactoring To Patterns](http://www.tarrani.net/RefactoringToPatterns.pdf) by Joshua Kerivsky
- [Smalltalk Best Practice Patterns](https://ptgmedia.pearsoncmg.com/images/9780134769042/samplepages/013476904X.pdf) by
Kent Beck
- [Smalltalk Best Practice Patterns](https://ptgmedia.pearsoncmg.com/images/9780134769042/samplepages/013476904X.pdf) by Kent Beck
- [Wiki](https://wiki.c2.com/?CollectingParameter)
- [Refactoring: Improving the Design of Existing Code](https://amzn.to/3TVEgaB)
- [Clean Code: A Handbook of Agile Software Craftsmanship](https://amzn.to/4aApLP0)