What is the Template Method pattern?

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creational

Template Method

Define the skeleton of an algorithm in a base class, deferring specific steps to subclasses.

Several processes share a shape. Importing CSV, XML and JSON all mean: open the file, read it, validate, save, close.

Only the reading step really differs. But if you write each importer separately, you copy the other four steps three times. Fix a bug in one and you must remember the others.

Template Method puts the fixed order in a base class and lets subclasses fill in only the parts that vary.

What the interviewer is probing.

1. "What does Template Method define?" *Probing:* the purpose. *Stalls:* "A template." *Moves up:* the skeleton of an algorithm in a base method, deferring specific steps to subclasses without letting them change the order.

2. "How does it compare with Strategy?" *Probing:* the comparison. *Stalls:* "They are the same." *Moves up:* Template Method varies steps through inheritance at compile time; Strategy swaps the whole algorithm through composition at runtime.

3. "What is a hook method?" *Probing:* the detail. *Stalls:* "An override." *Moves up:* an optional step with an empty or default implementation a subclass may override — unlike an abstract step it must.

4. "What is the criticism?" *Probing:* honesty. *Stalls:* "None." *Moves up:* inheritance couples subclasses to the base class, so a base change ripples — composition is usually preferred now.

A recipe — the process is always prep → cook → plate. What you prep, cook, and plate varies by dish.

Key Concepts

The base class has one method that runs the steps in order. This is the template method, and it is usually final so subclasses cannot change the sequence.

final

Steps that are always the same are implemented in the base class. Steps that differ are abstract, and each subclass supplies its own version.

You can also add optional hooks: methods with an empty default body that subclasses may override if they need to.

The trade-off is that this uses inheritance, so a subclass is tied to its parent. Strategy solves a similar problem with composition, which is more flexible.

java

public abstract class DataMiner {
    public final void mine(String path) {
        openFile(path);
        String raw = extractData();
        String[] parsed = parseData(raw);
        analyzeData(parsed);
        closeFile();
    }
    protected abstract void openFile(String path);
    protected abstract String extractData();
    protected abstract void closeFile();
    protected String[] parseData(String raw) { return raw.split("\n"); }
    protected void analyzeData(String[] data) { System.out.println(data.length); }
}