In software engineering, the template method pattern is a behavioral design pattern that defines the program skeleton of an algorithm in a base class, but allows its subclasses to provide concrete implementation for some or all of the algorithm.
The template method pattern is a powerful technique for code reuse and for defining an algorithm's structure, but it should be used with care. In particular, template methods should be used only when it is appropriate to define an algorithm in terms of its steps, and when those steps can be reasonably expected to be implemented differently by different subclasses.
The template method pattern is a behavioral design pattern that defines the program skeleton of an algorithm in a base class, but allows its subclasses to provide concrete implementation for some or all of the algorithm.
The template method pattern is a powerful technique for code reuse and for defining an algorithm's structure, but it should be used with care. In particular, template methods should be used only when it is appropriate to define an algorithm in terms of its steps, and when those steps can be reasonably expected to be implemented differently by different subclasses.
In the template method pattern, the algorithm's steps are implemented in methods that are either abstract or have a default implementation. These methods are called template methods. The template methods define the overall structure of the algorithm, while the concrete implementation details are provided by the methods that are either abstract or have a default implementation.
The template method pattern is often used in frameworks, where the framework defines the overall structure of an algorithm or application, while the concrete details are provided by the user of the framework.
The template method pattern should be used when you want to define an algorithm in terms of its steps, and when those steps can be reasonably expected to be implemented differently by different subclasses.
In particular, the template method pattern is useful when:
You need to define an algorithm that can be decomposed into a series of steps.
It is expected that different subclasses will implement one or more of the steps in the algorithm differently.
The order in which the steps are executed is important.
You want to encapsulate the implementation details of the algorithm in a base class, so that they can be easily changed without affecting the subclasses.
The template method pattern can be implemented in Kotlin using either inheritance or delegation.
When using inheritance, the base class defines the overall structure of the algorithm, while the concrete implementation details are provided by the subclasses.
To implement the template method pattern using inheritance, we need to:
Define an abstract base class that contains the template method.
Define concrete subclasses that override the methods in the base class to provide the desired implementation.
For example, consider an application that needs to log messages to a file. We can define a base class that contains the template method for logging messages, and concrete subclasses that override the methods to provide the desired implementation.
abstract class Logger {
fun log(message: String) {
openFile()
writeFile(message)
closeFile()
}
abstract fun openFile()
abstract fun writeFile(message: String)
abstract fun closeFile()
}
class FileLogger: Logger() {
override fun openFile() {
// ...
}
override fun writeFile(message: String) {
// ...
}
override fun closeFile() {
// ...
}
}
class ConsoleLogger: Logger() {
override fun openFile() {
// ...
}
override fun writeFile(message: String) {
// ...
}
override fun closeFile() {
// ...
}
}
In the example above, the base class defines the template method log()
that contains the overall structure of the algorithm. The log()
method calls the openFile()
, writeFile()
, and closeFile()
methods, which are implemented differently by the subclasses.
When using delegation, the base class defines the overall structure of the algorithm, while the concrete implementation details are provided by a delegate object.
To implement the template method pattern using delegation, we need to:
Define a base class that contains the template method.
Define a delegate class that contains the methods that will be called by the template method.
Create an instance of the delegate class in the base class, and pass it to the template method.
For example, consider an application that needs to log messages to a file. We can define a base class that contains the template method for logging messages, and a delegate class that contains the methods to be called by the template method.
class Logger(val delegate: LoggerDelegate) {
fun log(message: String) {
delegate.openFile()
delegate.writeFile(message)
delegate.closeFile()
}
}
class FileLoggerDelegate: LoggerDelegate {
override fun openFile() {
// ...
}
override fun writeFile(message: String) {
// ...
}
override fun closeFile() {
// ...
}
}
class ConsoleLoggerDelegate: LoggerDelegate {
override fun openFile() {
// ...
}
override fun writeFile(message: String) {
// ...
}
override fun closeFile() {
// ...
}
}
In the example above, the base class defines the template method log()
that contains the overall structure of the algorithm. The log()
method calls the openFile()
, writeFile()
, and closeFile()
methods of the delegate object. The concrete implementation details are provided by the delegate classes.
The template method pattern has the following advantages:
It encapsulates the implementation details of an algorithm in a base class, so that they can be easily changed without affecting the subclasses.
It allows you to define the overall structure of an algorithm in a base class, while allowing subclasses to provide concrete implementation for some or all of the steps.
It promotes code reuse by allowing you to define common code in a base class that can be reused by all subclasses.
It is easy to extend by allowing you to add new steps to the algorithm without affecting the existing code.
The template method pattern has the following disadvantages:
It can lead to code duplication if the same algorithm is needed in multiple places with only minor variations. In such cases, it is usually better to extract the common code into a separate function or method.
It can make the code harder to understand and maintain if the algorithm is too complex. In such cases, it is usually better to refactor the code to make it more readable and maintainable.
The template method pattern is a powerful technique for code reuse and for defining an algorithm's structure, but it should be used with care. In particular, template methods should be used only when it is appropriate to define an algorithm in terms of its steps, and when those steps can be reasonably expected to be implemented differently by different subclasses.