1. “Don’t call us, we’ll call you”
The Template Pattern
By Konstantinos Liolios
For CSPP51023
5/7/2019
Template Pattern

2. Intent It is a behavioral pattern
Define a parent class and leave the implementation of some methods to subclasses but guide this process unlike the strategy pattern
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Template Pattern

3. MOTIVATION Code reusability and polymorphism
Factor out common elements
Do not reinvent the wheel
Most likely you have used it already w/o knowing
Every time you implement methods in subclasses you are using the Template Pattern
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Template Pattern

4. Implementation Issues
Operations which must be overridden by a subclass should be made abstract
If the template method itself should not be overridden by a subclass, it should be made final
To allow a subclass to insert code at a specific spot in the operation of the algorithm, insert “hook” operations into the template method. These hook operations may do nothing by default.
Try to minimize the number of operations that a subclass must override, otherwise using the template method becomes tedious for the developer
In a template method, the parent class calls the operations of a subclass and not the other way around. This is an inverted control structure that's sometimes referred to as "the Hollywood principle," as in, "Don't call us, we'll call you".
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Template Pattern

5. Criticism Communicates intent poorly
Difficult to compose functionality
Difficult to comprehend program flow
Difficult to maintain
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Template Pattern

6. Problem
You open a database handle many times in many of your problems and you slightly change the same code every time you connect to a different RDBMS (Oracle, Postgress etc.)
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Template Pattern

7. Solution Factor out the common methods to a parent abstract class
Create a template class implementing only the use-case specific methods and inherit the “concrete ones”
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Template Pattern

8. (4) Template Methods Concrete Methods
Complete methods that do some basic functionality and will be inherited by all derived classes (ec. do a handshake with a server app or release a filehandle)
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Template Pattern

9. (4) Template Methods Abstract Methods
Methods that need to be implemented on the derived classes
For example: Draw an isosceles circle in a jpanel, or draw a right triangle
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Template Pattern

10. (4) Template Methods Hook Operations
Methods that “may” be overidden in the derived classes
For example: Write a default abstract method.
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Template Pattern

11. (4) Template Methods
Finally methods that combine all the previous ones
For example implement the algorithm
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Template Pattern

12. Solution Description/UML
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Template Pattern

13. The Template Way 5/7/2019 Template Pattern import java.sql.*;
public class OpenDatabaseHandle {
abstract class DatabaseHandle(String connectionString, String userID, String Password){
public ResultSet result;
public final ResultSet Connect(connectionString, defaultUserID, defaultPassword){
OpenConnection();
ExecuteQuery();
ReturnResultSet(); };
abstract void OpenConnection(){
String defaultConnectionString= + connectionString;
Connection con = DriverManager.getConnection(defaultConnectionString, userID, password); }
public void ExecuteQuery(String sqlQuery) {
Statement select = con.createStatement();
result = select.executeQuery (“sqlQuery");
select.close();
con.close();
public ResultSet ReturnResultSet()
return result;
class MysqlHandle extends DatabaseHandle {
Public MysqlHandle(String connectionString, String userID, String password) {
super(connectionString);
public OpenConnection()
String MySQLconnectionString=“jdbc:msql://” + connectionString;
Connection con = DriverManager.getConnection(“MySQLconnectionString”, userID, password);
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Template Pattern

14. 5/7/2019
Template Pattern