JAVA及其简单：根据用户的选择计算矩形、圆形、或三角形面积。如图

import java.util.*;
import java.lang.*;
public class Main{

public static final double PI = 3.14;

public static void main(String []args){

Scanner scan = new Scanner(System.in);

System.out.println("形状类型:\n"+"1.长方形\n"+"2.圆形\n"+"3.三角形\n");
System.out.println("请输入num:");

try{
int num = Integer.parseInt(scan.nextLine());
double result = 0.0;
double height = 0.0;
double width = 0.0;

switch(num){
case 1:
System.out.print("请输入height:");
height = Double.parseDouble(scan.nextLine());
System.out.print("请输入width:");
width = Double.parseDouble(scan.nextLine());
result = height*width;
case 2:
System.out.print("请输入radius:");
width = Double.parseDouble(scan.nextLine());
result = width*width*PI;
case 3:
System.out.print("请输入height:");
height = Double.parseDouble(scan.nextLine());
System.out.print("请输入width:");
width = Double.parseDouble(scan.nextLine());
result = height*width/2;
}
System.out.println("Area:"+result);
}catch(Exception e){
System.err.print("Invalid Parameter");
}

}

}

我这儿有一个求多种形状体积的代码，给你作为参考一下