How can I access a method of a class from a generic method

I'm working on a tiny exercise java program that calculates circle and square (classes) area, that implements surface (interface) which has a method called area(). A requirement is that I have to implement a class called SumArea that has a generic method called calcArea() that receives Circle circ[] and Square square[] arrays and executes area calculation.

Program structure:

-> UseSumArea.java (main method)
-> Surface.java (interface)
-> Square.java (class that implements Surface.java)
-> Circle.java (class that implements Surface.java)
-> SumArea.java (class that executes calcArea() method)

UseSumArea.java

public class UseSumArea {
        public static void main(String[] args) {
            Square square[] = { new Square(2.0), new Square(5.0) };
            Circle circ[] = { new Circle(3.0), new Circle(2.0) };
            Surface surf[] = new Surface[square.length + circ.length];
            surf[0] = square[0];
            surf[1] = square[1];
            surf[2] = circ[0];
            surf[3] = circ[1];
            SumArea sum = new SumArea();
            System.out.println("Square's sum area = " + sum.calcArea(square));
            System.out.println("Circle's sum area = " + sum.calcArea(circ));
            System.out.println("Surface's sum area = " + sum.calcArea(surf));
        }
    
    }

Surface.java

public interface Surface {
    public double area();
}

Square.java

public class Square implements Surface {

private double area;
private double side;

public Square(double l) {
    this.side = l;
    area();
}

@Override
public double area() {
    return this.area = (this.side)*(this.side);
}

public double getArea() {
    return area;
}

public void setArea(double area) {
    this.area = area;
}

public double getSide() {
    return side;
}

public void setSide(double side) {
    this.side = side;
}

}

Circle.java

public class Circle implements Surface {

private double area;
private double radius;

public Circle (double r) {
    this.radius = r;
    area();
}

@Override
public double area() {
    return area = (((this.radius)*(this.radius))*(Math.PI));
}

public double getRadius() {
    return radius;
}

public void setRadius(double raio) {
    this.raio = raio;
}

public double getArea() {
    return area;
}

public void setArea(double area) {
    this.area = area;
}

}

SumArea.java

public class SumArea {

private double area;

public <T> double calcArea(T[] t) { //generic method that receives Square and Circle arrays
    double arrayArea = 0;
    for (T a : t) {
        arrayArea = arrayArea+(a.area()); 
    }
    return this.area = arrayArea;
}
}

My doubt is over this SumArea's code snippet:

arrayArea= arrayArea+(a.area());

How can I access the area() method of each Circle and Square objects inside this generic method?