1．求解用java写（如三角形，矩型，圆）的的周长，面积，要求用到继承，多态，抽象类，接口，内部类等。

//抽象的形状类
public abstract class Shape{ }
//接口
public interface IDisplay{
void display(); //显示图形的基本信息
double getArea(); //计算面积
double getGirth(); //计算周长
}
//三角形类
public class Triangle extends Shape implements IDisplay{
protected double a;
protected double b;
protected double c;
public Triangle(double a, double b, double c){
this.a = a; this.b = b; this.c = c;
}
@Override public double getArea() {
double s = (a + b + c) / 2;
return Math.sqrt(s*(s-a)*(s-b)*(s-c));
}
@Override public double getGirth() {
return this.a + this.b + this.c;
}
@Override public void display() {
System.out.println("三角形");
System.out.println("边长：" + a + ", " + b + ", " + c);
}
}
//矩形类
public class Rectangle extends Shape implements IDisplay {
protected double width; protected double height;
public Rectangle(double width, double height){
this.width = width;
this.height = height;
}
@Override public double getArea() {
return this.width * this.height;
}
@Override public double getGirth() {
return 2 * ( this.width + this.height);
}
@Override public void display() {
System.out.println("矩形");
System.out.println("宽：" + this.width + ", 高：" + this.height);
}
}
//圆类
public class Circle extends Shape implements IDisplay {
protected double radius;
public Circle(double radius){
this.radius = radius;
}
@Override public double getArea() {
return Math.PI * this.radius * this.radius;
}
@Override public double getGirth() {
return 2 * Math.PI * this.radius;
}
@Override public void display() {
System.out.println("圆");
System.out.println("半径：" + this.radius);
}
}