Question

我需要根据以下方法实现equals方法。

 public class Database{

 .........

//A helper method to look for a patron using 
//id. If found, it returns it, otherwise, it returns null.

public Patron lookUp(int id) {
    int val = patrons.indexOf(id);
    if(val!=-1){return patrons.get(val);}
    else
        return null;}}

equals方法是patron类，我为它编写了以下代码。

public class Patron{

.........

//Need to implement this in order to have a 
//method such as indexOf of ArrayList work properly

public boolean equals(Object o) {
   if(!(o instanceof Patron)){
       return false;}
   else{
        return true;}}

所以基本上我需要使用indexOf方法检查是否存在具有提供的id号的顾客。为此，我们在Patron类中创建了equals方法。但它不起作用，总是返回-1。

以下是两个班级。

 import java.util.*;
 public class Database{
// Declare an ArrayList of patrons
ArrayList<Patron> patrons;

// Constructor 
public Database()
{
    patrons = new ArrayList<Patron>();
}

// [5 points] A method that adds a patron object. It 
//returns false if the patron already exists, 
//otherwise it returns true. You should use the 
//lookUp method below.
public boolean add( Patron p  ){
    if(lookUp(p.id)==null){
        patrons.add(p);     
        return true;
    }
    else
        return false;
}

// [5 points] A helper method to look for a patron using 
//id. If found, it returns it, otherwise, it returns null.
public Patron lookUp(int id) {
    int val = patrons.indexOf(id);
    if(val!=-1){
        return patrons.get(val);
    }
    else
        return null;}}

import java.util.*;
public class Patron{

//Data members: id, name, a list of checked out books
int id;
String name;
ArrayList<Book> booksCheckedOut;

// [3 points] Constructor
public Patron(int id, String name)
{ this.id = id;
  this.name = name;
  booksCheckedOut = new ArrayList<Book>();
}

//A method to return the id
public int getID() {
    return id;
}

// [5 points] Need to implement this in order to have a 
//method such as indexOf of ArrayList work properly
public boolean equals(Object o) {
   if(!(o instanceof Patron)){
       return false;
    }
   else{
        return true;
}
}       

// [2 points] A method to check out a book by adding it 
//to the ArrayList above
public void addBook(String bookTitle, double bookPrice){
    Book b = new Book(bookTitle,bookPrice);
    booksCheckedOut.add(b);
}

// [5 points] List all books in the ArrayList
public void showStatement() {
    for(Book val: booksCheckedOut){
        System.out.println(val);}}}

Answer 1

如果您希望ArrayList<Patron>的{{1}}返回-1以外的任何内容，则必须向其传递Patron的实例。

传递int，就像你在这里一样

int val = patrons.indexOf(id);

将始终返回-1。

如果您希望能够按标识符查找Patron，请使用HashMap<Integer,Patron>代替List。

或者您可以将Patron lookUp(int id)更改为Patron lookUp(Patron p)，然后在p方法中将p.id更改为add。

PS，我假设您在equals类中的Patron实现不完整，因为当您向true实例传递Patron时，它始终返回10:00 AM, 31 January 2017。 / p>

{JAVA}为我的lookUp方法实现equals（Object o）方法

1 个答案: