*做Triangle类，Trapezium类，Circle类，实现相应功能。
1.Triangle类有返回周长和面积属性，并且有Boolean属性可以根据用户输入判断该三边是否可以构成三角形，如不能则重新输入。
2.Trapezium类可以返回面积。
3.Circle类可以返回周长及面积*

Triangle类

public class Triangle {
    private double length1;
    private double length2;
    private double length3;
    private double perimeter;
    private double area;
    public Triangle() {
        
    }
    public Triangle(double length1,double length2,double length3) {
        this.length1=length1;
        this.length2=length2;
        this.length3=length3;
    }
    public double getperimeter() {
        boolean flag=true;
        if(flag==islength()) {
        perimeter=length1+length2+length3;
        System.out.println("三角形周长是"+perimeter);
        }
        else {
            System.out.println("该三边不能构成三角形,无法输出周长");
        }
        return perimeter;
    }
    public double getarea() {
        boolean flag=true;
        if(flag==islength()) {
        area=0.5*length1+length2+length3;
        System.out.println("三角形面积是"+area);
        }
        else {
            System.out.println("该三边不能构成三角形，无法输出面积");
        }
        return area;
    }
    public void setlength(double length1,double length2,double length3) {
        this.length1=length1;
        this.length2=length2;
        this.length3=length3;
    }
    public boolean islength() {
        if(((length1+length2)>length3)&&((length1+length3)>length2)&&((length2+length3)>length1)) {
            return true;
        }
        else {
            return false;
        }
    }
}

Trapezium类

public class Trapezium {
    private double uplength;
    private double downlength;
    private double high;
    private double area;
    public Trapezium() {
        
    }
    public Trapezium(double uplength,double downlength,double high){
        this.uplength=uplength;
        this.downlength=downlength;
        this.high=high;
    }
    public double getarea() {
        area=(uplength+downlength)*high/2;
        System.out.println("梯形的面积是"+area);
        return area;
    }
}

Circle类

public class Circle  {
    private double radius;
    private double perimeter;
    private double area;
    public Circle() {
        
    }
    public Circle(double radius) {
        this.radius=radius;
    }
    public double getperimeter() {
        perimeter=2.0*3.14*radius;
        System.out.println("圆的周长是"+perimeter);
        return perimeter;
    }
    public double getarea() {
        area=3.14*radius*radius;
        System.out.println("圆的面积是"+area);
        return area;
    }
}

测试类

import java.util.Scanner;
public class Test  {
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println("请输入三角形三个边长");
        Scanner sc=new Scanner(System.in);
                boolean flag1=true;
                boolean flag3;
                do{
        double length1=sc.nextDouble();
        double length2=sc.nextDouble();
        double length3=sc.nextDouble();
                Triangle t1=new Triangle(length1,length2,length3);
                flag3=t1.islength();
                if(flag1==flag3){
                    t1.getperimeter();
                    t1.getarea();
                }
                else{
                    System.out.println("该三边不能构成三角形,请重新输入三边");
                }
                }while(flag3==false);
        System.out.println("请输入梯形上底，下底，高");
        double uplength=sc.nextDouble();
        double downlength=sc.nextDouble();
        double high=sc.nextDouble();
                Trapezium tr1=new Trapezium(uplength,downlength,high);
        tr1.getarea();
        System.out.println("请输入半径");
        double radius=sc.nextDouble();
                Circle c1=new Circle(radius);
        c1.getperimeter();
        c1.getarea();
        sc.close();
    }
}

新手学习，请多指教。