DemoIoC_SimpleCalculator_IoC1.java

//
// Generic part
//
public abstract class SimpleCalculator {
    
    private static PrintStream log = System.out;
    private static Scanner cs = new Scanner(System.in);
    
    protected int num1, num2;
    protected int result;
    
    public void takeAction() {
        log.println("Enter 1st number:");
        String num1 = cs.nextLine();
        log.println("Enter 2nd number:");
        String num2 = cs.nextLine();
        
        this.num1 = Integer.parseInt(num1);
        this.num2 = Integer.parseInt(num2);
        
        this.result = doCalculate(this.num1, this.num2);
        
        printResult();
    }
    
    protected abstract int doCalculate(int num1, int num2);
    protected abstract void printResult();
}

//
// Tailored part
//
public class Add extends SimpleCalculator {
    
    private static PrintStream log = System.out;

    protected int doCalculate(int num1, int num2) {
        return num1 + num2;
    }

    protected void printResult() {
        log.format("The ADD result: %d + %d = %d", this.num1, this.num2, this.result);
    }
}

public class Sub extends SimpleCalculator {
    
    private static PrintStream log = System.out;

    protected int doCalculate(int num1, int num2) {
        return num1 - num2;
    }

    protected void printResult() {
        log.format("The SUB result: %d - %d = %d", this.num1, this.num2, this.result);
    }
}

// Main entrypoint
public class Main {

    private static PrintStream log = System.out;
    private static Scanner cs = new Scanner(System.in);
    
    public static void main(String[] args) {
        log.println("!~ SIMPLE CALCULATOR ~!");
        log.println("Choose arithmetic operations (+ or -):");
        
        String operation = cs.nextLine();
        if (operation.equals("+")) {
            Add addInst = new Add();
            addInst.takeAction();
        } else if (operation.equals("-")) {
            Sub subInst = new Sub();
            subInst.takeAction();
        } else {
            log.println("Not supported!");
        }
    }
}