构建之法1：编写一个生成四则运算题目的程序

import java.util.*;
public class ArithmeticTopic {
    private static Random random = new Random();

    public ArithmeticTopic() {
        // TODO Auto-generated constructor stub
    }
    private String createNumber() {
        int number1 =1+random.nextInt(100);
        int point1 = 0;
        String numberString="";
        if(random.nextBoolean()) {
            point1 = number1;
            number1 = 1+random.nextInt(point1);
            if(point1==number1) point1=0;
        }
        if(point1>0) {
            numberString = ""+number1+"/"+point1;
        }else {
            numberString = "" +number1;
        }
        return numberString;
    }
    private String topicString() {
        char operator = ‘+‘;
        switch(random.nextInt(3))  {
            case 0 : operator = ‘+‘;    break;
            case 1 : operator = ‘-‘;    break;
            case 2 : operator = ‘*‘;    break;
            case 3 : operator = ‘/‘;    break;
            default: break;
        };
        return createNumber()+operator+createNumber()+"=";
    }
    public static void main(String[] args) {
        ArithmeticTopic topic = new ArithmeticTopic();
        for(int i=0;i<10;i++)
        System.out.println(topic.topicString());
    }
}

程序思路：

　　　产生真分数：先产生一个1-100的随机整数y, 依照随机布尔值判断是否再产生一个1-y的的随机整数x作为分子，形成x/y，如果x/y=1则直接输出x.

　　   产生四则运算表达式： 数x1,x2,字符串中间插入随机运算符，最后加上等号输出到命令行.