java - Java 中的递归以及类(class)对象和先决条件

Question

美好的一天，有没有比使用 switch 语句更有效的方法来解决这个递归问题。在我的类(class)类中，我有一个类(class)和先决条件的递归关联，然后在 PreReqs 类中设置 preReqs。当用户进入一个有 preReqs 的类(class)时，如何打印出该类(class)的所有 preReqs？现在我正在使用 switch 语句并单独打印每个 preReq，但是有没有更好的方法仍然使用递归来做到这一点？

举个例子:如果用户输入该类(class)，所有的 preReqs 也将打印出来。

ACS-3947 Algorithm Design
prereq: ACS-2947 Data Structures and Algorithms
ACS-2947 Data Structures and Algorithms
prereq: ACS-1904 Programming Fundamentals II
ACS-1904 Programming Fundamentals II
prereq: ACS-1903 Programming Fundamentals I
ACS-1903 Programming Fundamentals I
no prereq

类(class)类别:

import java.util.*;
public class Course
{  
    protected String courseNumber; 
    protected String courseName;
    protected Course prerequisite; 

    public Course(){
        courseNumber = courseName =  "Unknown"; 
        prerequisite= null; 
    }

    public Course (String cn, String num){
        this.courseNumber=num;
        this.courseName=cn; 
    }

    public String getCourseNumber(){
        return courseNumber;
    }
    public String getCourseName(){
        return courseName;
    }
    public Course getPreReq(){
        return prerequisite;
    }

    public void setCourseNumber(String courseNumber){
        this.courseNumber=courseNumber;
    }
    public void setCourseName(String courseName){
        this.courseName=courseName;
    }
    public void setPreReq(Course pr){
        prerequisite =pr; 
    }

}

PreReq 类:

import java.util.*;
import java.io.*;
public class Prereqs 
{
    public static void main (String [] args){
        Scanner kb = new Scanner (System.in);

        Course nineteen03 = new Course ("Programming Fundamentals I","ACS-1903");
        Course nineteen04 = new Course ("Programming Fundamentals II"," ACS-1904");
        Course two47 = new Course ("Data Structures and Algorithms","ACS-2947 "); 
        Course three47 = new Course ("Algorithm Design","ACS-3947");
        Course two09 = new Course ("Internet Programming","ACS-2909");
        Course three09 = new Course ("Advanced Internet Programming ","ACS-3909");

        nineteen04.setPreReq(nineteen03);
        two47.setPreReq(nineteen04);
        three47.setPreReq(two47);
        two09.setPreReq(nineteen03);
        three09.setPreReq(nineteen03); 
        System.out.println("Enter course number with the format: AAA-999");
        String input = kb.next(); 

         validate(input);
    }

    public static void course(Course nineteen04, Course nineteen03,Course two47, Course three47, Course two09, Course three09, String input  ){
        Course c1 = nineteen04.getPreReq();
        Course c2 = two47.getPreReq();
        Course c3 = three47.getPreReq(); 
        Course c4 = two09.getPreReq();
        Course c5 = three09.getPreReq();

        switch (input){
            case "ACS-1904":
            System.out.println(nineteen04.getCourseName()+" "+nineteen04.getCourseNumber());
            System.out.println("preReq: " + c1.getCourseName()+ " "+ c1.getCourseNumber());

        }

    }

    public static String validate (String input)
    {   
        String arg = input;

        boolean valid = arg.length()==7;

        if (!valid){
            throw new IllegalArgumentException("Not the correct format: AAA-999");           
        }
        valid = arg.charAt(3) == '-';  
        if(!valid) {
            throw new IllegalArgumentException("Not the correct format: AAA-999");
        }
        for(int i=0; i < 3 && valid; i++){
            valid = ((i == 3 && Character.isLetter(arg.charAt(i))));
        }

        for(int i=3; i < 3 && valid; i++){
            valid = ((i==6 && Character.isDigit(arg.charAt(i))));
        }
        return arg;
    }
}

Answer 1

递归方法需要包含终止递归的条件。您的类(class)列表及其先决条件让我想起了一个链接列表，其中每门类(class)都指向其先决条件。当我们到达没有先决条件的类(class)时，该列表终止。以下代码是您的Course类，添加了一个主方法(从您的 Prereqs 类导入)和我命名为 requirements() 的递归方法。我还添加了方法toString()使类(class)列表及其先决条件的显示更加“人类可读”。您可以通过更改传递给方法 requirements() 的初始调用的类(class)来进行实验。 .

public class Course {
    protected String courseNumber;
    protected String courseName;
    protected Course prerequisite;

    public Course(){
        courseNumber = courseName =  "Unknown";
        prerequisite= null;
    }

    public Course (String cn, String num){
        this.courseNumber=num;
        this.courseName=cn;
    }

    public String getCourseNumber(){
        return courseNumber;
    }

    public String getCourseName(){
        return courseName;
    }

    public Course getPreReq(){
        return prerequisite;
    }

    public void setCourseNumber(String courseNumber){
        this.courseNumber=courseNumber;
    }

    public void setCourseName(String courseName){
        this.courseName=courseName;
    }

    public void setPreReq(Course pr){
        prerequisite =pr;
    }

    public String toString() {
        return courseNumber + " " + courseName;
    }

    private static void requirements(Course c) {
        if (c == null) {
            return;
        }
        else {
            System.out.println(c);
            requirements(c.getPreReq());
        }
    }

    public static void main(String[] args) {
        Course nineteen03 = new Course ("Programming Fundamentals I","ACS-1903");
        Course nineteen04 = new Course ("Programming Fundamentals II"," ACS-1904");
        Course two47 = new Course ("Data Structures and Algorithms","ACS-2947 "); 
        Course three47 = new Course ("Algorithm Design","ACS-3947");
        Course two09 = new Course ("Internet Programming","ACS-2909");
        Course three09 = new Course ("Advanced Internet Programming ","ACS-3909");

        nineteen04.setPreReq(nineteen03);
        two47.setPreReq(nineteen04);
        three47.setPreReq(two47);
        two09.setPreReq(nineteen03);
        three09.setPreReq(nineteen03);

        requirements(three09);
    }
}

运行上面的代码会显示以下内容:

ACS-3909 Advanced Internet Programming 
ACS-1903 Programming Fundamentals I