Question

a Composite class representing arithmetic expression.

Participants:

• Expression – interface. Operations: double calculate(), boolean isTree().
• Constant – represent a number. Leaf node.
• Plus, Minus, Div, Mul – binary operations. Use abstract BinaryOperation class to implement common functionality

Q. Create a code implementing a pattern, Implementing Composite implies multiple class. By my estimate, at least 3. In minimal Composite implementation, classes have two methods (or a constructor and 1 method). extensive coding is not required. (using Java)

Answer 1

public interface Arithmetic {
    double compute();
    default void appendChild(Arithmetic arithmetic) {}
    default void removeChild(Arithmetic arithmetic) {}
}

Next, we have Leaf nodes, which each represent a singular operation.

public class Addition implements Arithmetic {
    private final double x;
    private final double y;

    public Addition(double x, double y) {
        this.x = x;
        this.y = y;
    }

    @Override
    public double compute() {
        return x + y;
    }
}

public class Subtraction implements Arithmetic {
    private final double x;
    private final double y;

    public Subtraction(double x, double y) {
        this.x = x;
        this.y = y;
    }

    @Override
    public double compute() {
        return x - y;
    }
}

And finally, a Composite node, which represents multiple operations.

public class CompositeAddition implements Arithmetic {
    private final List<Arithmetic> operations = new ArrayList<>();

    public CompositeAddition(Arithmetic... arithmetics) {
        operations.addAll(Arrays.asList(arithmetics));
    }

    @Override
    public double compute() {
        return operations.stream().mapToDouble(Arithmetic::compute).sum();
    }

    @Override
    public void appendChild(Arithmetic arithmetic) {
        operations.add(arithmetic);
    }

    @Override
    public void removeChild(Arithmetic arithmetic) {
        operations.remove(arithmetic);
    }
}

I leave the remaining arithmetic types as an exercise for the reader. These few classes are sufficient for a demonstration.

public class Main {
    public static void main(String... args) {
        Arithmetic fivePlusTwo = new Addition(5,2);
        Arithmetic fiveMinusTwo = new Subtraction(5,2);
        Arithmetic sevenPlusThree = new CompositeAddition(fivePlusTwo, fiveMinusTwo);
        System.out.println(sevenPlusThree.compute());
    }
}

Let me know if you have any doubts or if you need anything to change. 

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Thank You!
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