Main.java:10: error: interface expected here class Oval implements Shape{ ^ Main.java:5: error: Main.Shape is not abstract and does not override abstract method perimeter() in Main.Shape class Shape{ ^ Main.java:6: error: abstract methods cannot have a body abstract double area(){}; ^ Main.java:7: error: abstract methods cannot have a body abstract double perimeter(){}; ^ Main.java:23: error: cannot find symbol return PI*a*b; ^ symbol: variable PI location: class Main.Oval Main.java:26: error: cannot find symbol return 2*PI*b+4*(a-b); //L=2πb+4(a-b) ^ symbol: variable PI location: class Main.Oval 6 errors

这段代码在编译时出现了6个错误，其中有以下几个问题：

1. 在 `class Oval implements Shape` 中，应该实现接口而不是继承类。所以应该改为 `class Oval implements Shape`。

2. 在抽象类中，抽象方法不能有方法体。所以需要去掉 `abstract double area(){};` 和 `abstract double perimeter(){};` 中的花括号。

3. 在 `Oval` 类中，计算面积和周长时出现了错误。在 Java 中，π的值可以使用 `Math.PI` 来表示，而不是直接使用 `PI`。

修改后的代码如下所示：

interface Shape {
    double area();
    double perimeter();
}

class Rectangle implements Shape {
    double length;
    double width;

    Rectangle(double l, double w) {
        length = l;
        width = w;
    }

    public double area() {
        return length * width;
    }

    public double perimeter() {
        return 2 * (length + width);
    }
}

class Oval implements Shape {
    double a;
    double b;

    Oval(double x, double y) {
        a = x;
        b = y;
    }

    public double area() {
        return Math.PI * a * b;
    }

    public double perimeter() {
        return 2 * Math.PI * b + 4 * (a - b); // L=2πb+4(a-b)
    }
}

public class Main {
    public static void main(String[] args) {
        Shape rect = new Rectangle(5, 10);
        Shape oval = new Oval(5, 10);

        System.out.println("Rectangle Area: " + rect.area());
        System.out.println("Rectangle Perimeter: " + rect.perimeter());

        System.out.println("Oval Area: " + oval.area());
        System.out.println("Oval Perimeter: " + oval.perimeter());
    }
}