Java 接口练习题 为什么不能run?

package Fourup;

public interface Shape {
    public abstract double area(double a) ;
    
}

package Fourup;

public class Square implements Shape{
public double area(double a) {
    double s;
    s=a*a;
    return s;
    }

}

package Fourup;

public class Circle implements Shape {
    public double area(double a) {
        double s;
        s=(Math.PI)*a*a;
        return s;
    }
    
}
package Fourup;

public class Tests {
    public static void man(String []args) {
        Square s = new Square();
        Circle c = new Circle();
        System.out.println(s.area(2.0));
        System.out.println(c.area(3.0));
    }
}
 

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