java的抽象类的三角形周长_怎么使用Java把抽象类改为接口计算三角形，长方形，圆形的面积？...

对于第一个问题，计算三角形周长的代码如下：

public abstract class Triangle {
    protected double a, b, c;

    public Triangle(double a, double b, double c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }

    public abstract double getPerimeter();
}

public class ConcreteTriangle extends Triangle {
    public ConcreteTriangle(double a, double b, double c) {
        super(a, b, c);
    }

    @Override
    public double getPerimeter() {
        return a + b + c;
    }
}

// 使用方法
Triangle t = new ConcreteTriangle(3, 4, 5);
double perimeter = t.getPerimeter();
System.out.println("三角形周长为：" + perimeter);

对于第二个问题，把抽象类改为接口，可以定义一个`Shape`接口，其中包含计算面积的方法`getArea()`，然后分别实现三个形状的类，并实现`Shape`接口，代码如下：

public interface Shape {
    double getArea();
}

public class Triangle implements Shape {
    private double a, b, c;

    public Triangle(double a, double b, double c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }

    @Override
    public double getArea() {
        double p = (a + b + c) / 2;
        return Math.sqrt(p * (p - a) * (p - b) * (p - c));
    }
}

public class Rectangle implements Shape {
    private double width, height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public double getArea() {
        return width * height;
    }
}

public class Circle implements Shape {
    private double radius;

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

    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}

// 使用方法
Shape t = new Triangle(3, 4, 5);
double area = t.getArea();
System.out.println("三角形面积为：" + area);

Shape r = new Rectangle(3, 4);
area = r.getArea();
System.out.println("长方形面积为：" + area);

Shape c = new Circle(3);
area = c.getArea();
System.out.println("圆形面积为：" + area);