Template Method Pattern

Definition: define the skeleton of an algorithm in a a method, deferring some steps to subclasses. Template Method pattern lets subclasses redefine certain steps of an algorithms without changing the algorithm's structure. 

Class diagram:

The OO Principles: 
- The Hollywood Principles: Dont call us, we'll call you.
The template method in supper class tell the subclasses that, dont call us, we'll call you. The supper class is on high level component, it has control over the algorithm, and call the subclasses only when they're need for an implementation of template method. 
The client will depend on the abstraction supper class rather than the concrete subclasses, which reduce the dependencies of overall system.

Advantages:

- There is no code duplication. (the reusable code is pulled to the parent class)

- Flexibility lets subclasses decide how to implement steps in an algorithm.

Disadvantages:

- Difficult for debug and maintenance.

Relation with other pattern:
- Factory method pattern: Subclasses decide which concrete classes to instantiate.  this pattern is a specialization of Template Method.
- Strategy pattern: Encapsulate interchangeable behaviors and use delegation to decide which behavior is use.
- Template method: Subclasses decide how to implement the steps in an algorithm.

What problem does this pattern solve?
Two different components have significant similarities, but demonstrate no reuse of common interface or implementation. If a change common to both components becomes necessary, duplicate effort must be expended.

Use case:

The template method pattern is a technique that defines the steps required for some action, implementing the boilerplate steps, and leaving the customizable steps as abstract. Subclasses can then implement this abstract class and provide a concrete implementation for the missing steps.

public abstract DatabaseQuery {

    public void execute() {
        Connection connection = createConnection();
        executeQuery(connection);
        closeConnection(connection);
    } 

    protected Connection createConnection() {
        // Connect to database...
    }

    protected void closeConnection(Connection connection) {
        // Close connection...
    }

    protected abstract Results executeQuery(Connection connection, String query);
}