Template Method pattern introduction with Java

May 01, 2021

This article’s code is basically from here. I will show Template Method pattern, which is one of the design patterns.

Abstract Class

public abstract class AbstractDisplay { // Abstract class
    public abstract void open();        // abstract method1 open
    public abstract void print();       // abstract method2 print
    public abstract void close();       // abstract method3 close
    public final void display() {
        open();                             // first, open
        for (int i = 0; i < 5; i++) {       // print 5 times
            print();
        }
        close();                            // last, close
    }
}

Main Class to utilize abstract and concrete class

public class Main {
    public static void main(String[] args) {
        AbstractDisplay d1 = new CharDisplay('H');
        AbstractDisplay d2 = new StringDisplay("Hello, world.");
        d1.display();
        d2.display();
    }
}

Concrete Class Sample1 charDisplay

public class CharDisplay extends AbstractDisplay {
    private char ch;
    public CharDisplay(char ch) {
        this.ch = ch;
    }
    public void open() {
        System.out.print("<<");
    }
    public void print() {
        System.out.print(ch);
    }
    public void close() {
        System.out.println(">>");
    }
}

Sample1 output

<<HHHHH>>

Concrete Class Sample2 stringDisplay

public class StringDisplay extends AbstractDisplay {
    private String string;
    private int width;
    public StringDisplay(String string) {
        this.string = string;
        this.width = string.getBytes().length;
    }
    public void open() {
        printLine();
    }
    public void print() {
        System.out.println("|" + string + "|");
    }
    public void close() {
        printLine();
    }
    private void printLine() {
        System.out.print("+");
        for (int i = 0; i < width; i++) {
            System.out.print("-");
        }
        System.out.println("+");
    }
}

sample2 output

+---------------+
|Hello, world.|
|Hello, world.|
|Hello, world.|
|Hello, world.|
|Hello, world.|
+---------------+

Pros

Logic can be shared.
We do not have to write same code.
Easy to test / Less code to fix.

Cons

Difficult to decide how much should be standardized and left to individual implementation
If you don’t design to cover all the patterns that could be used, you will end up with a template that is difficult to apply.