添加新元素时清空数组

Question

我已经实现了视频租赁服务。

VideoStore class 维护 Video 个对象的数组。

当我尝试将新视频添加到清单时，它成功插入了新元素。但是之前添加的所有元素都不再可以访问了。

为什么会这样？

public class Video {

    String videoName;
    boolean checkout;
    int rating;
    
    Video (String name){
        this.videoName=name;
    }
    
    String getName() {
        return videoName;
    }
    void doCheckout() {
        this.checkout=true;
    }
    void doReturn() {
        this.checkout=false;
    }
    void receiveRating(int rating) {
        this.rating=rating;
    }
    int getRating() {
        return rating;
    }
    boolean getCheckout() {
        return checkout;
    }
}

public class VideoStore {

    Video[] store=null;
    
    int checkVideo(String name) {
        int count =0;
        if(store==null) {
            count =0;
        }else
            for(Video video:store) {
                if(video!=null) {
                if(video.getName().equals(name))
                    count=1;
            }}
            return count;
    }
    
    void addVideo(String name) {
        int s;
        
        Video video = new Video(name);
        if(store==null)
            s=0;
        else {
            s=store.length;
        }
        
        Video[] store2 = new Video[s+1];
        store2[s]=video;
        store=store2;
    }
    
    void doCheckout(String name) {
        if(store == null || store.length==0) {
            System.out.println("Store is empty");
            return;
        }else
            for(Video video:store) {
                if(video!=null) {
                if(video.getName().equals(name))
                    video.doCheckout();
            }}
            
    }
    
    void doReturn(String name) {
        if(store == null || store.length==0) {
            System.out.println("Store is empty");
            return;
        }else
        
            for(Video video:store) {
                if(video!=null) {
                if(video.getName().equals(name))
                    video.doReturn();
            }}
    }
    void receiveRating(String name, int rating) {
        if(store == null || store.length==0) {
            System.out.println("Store is empty");
            return;
        } else
        
            for(Video video:store) {
                if(video!=null) {
                if(video.getName().equals(name))
                    video.receiveRating(rating);
            }}
        
    }
    void listInventory() {
        if(store == null || store.length==0) {
            System.out.println("Store is empty");
            return;
        }else
        
            for(Video video:store) {
                if(video!=null) {
                System.out.print("---------------------------------------------------------------------------");
                System.out.printf("\n\t%-20s\t|\t%-10s\t|\t%-15s\n", "Video Name","Checkout Status","Rating");
                System.out.printf("\n\t%-20s\t|\t%-10s\t|\t%-15s\n", video.getName(), video.getCheckout(),video.getRating());
                System.out.println("---------------------------------------------------------------------------");
            }
            }
    }
    
}

import java.util.Scanner;
public class VideoLauncher {
    
    public static void main(String[] args) {
        
        Scanner sc =new Scanner(System.in);
        int input=0;
        
        VideoStore vStore =new VideoStore();
        
        while(true) {
            System.out.println("\n1. Add Videos : "+"\n"+"2. Check Out Video : "+"\n"+"3. Return Video : "+
                       "\n"+"4. Receive Rating : "+"\n"+"5. List Inventory : "+"\n"+"6. Exit : ");
            System.out.print("Enter Your Choice (1..6) : ");
            if(!sc.hasNextInt()) {
                sc.next();
                input=0;
            } 
            String name;
            input =sc.nextInt();
            sc.nextLine();
            
            switch(input) {
            case 1:
                System.out.print("Enter the name of the video you want to add : ");
                name =sc.nextLine();
                
                vStore.addVideo(name);
                System.out.println("Video '" +name+"' added successfully.");
                
                break;
                
            case 2:
                System.out.print("Enter the name of the video you want to check out : ");
                name = sc.nextLine();if(vStore.checkVideo(name)==0) {
                    System.out.println("Video does not exist.");
                }else {
                
                    vStore.doCheckout(name);
                System.out.println("Video '"+name+"' checked out successfully.");
                }
                break;
                
            case 3:
                System.out.print("Enter the name of the video you want to return : ");
                name = sc.nextLine();
                if(vStore.checkVideo(name)==0) {
                    System.out.println("Video does not exist.");
                }else {
                    vStore.doReturn(name);
                System.out.println("Video '"+name+"' returned successfully.");
                }
                break;
                
            case 4:
                System.out.print("Enter the name of the video you want to rate : ");
                name = sc.nextLine();
                if(vStore.checkVideo(name)==0) {
                    System.out.println("Video does not exist.");
                }else {
            
                    System.out.print("Enter the rating for this video : ");
                    int rating = sc.nextInt();
                    vStore.receiveRating(name, rating);
                    System.out.println("Rating '"+rating+"' has been mapped to the Video '"+name+"'.");
                }
                break;
                
            case 5:
                vStore.listInventory();
                break;
            case 6:
                sc.close();
                System.out.print("Exiting...!! Thanks for using the application.");
                return;
            default:
                System.out.println("Please enter correct choice.");
    }}
    }
}

Answer 1

if try to insert one more video... the previous element is deleted from inventory

这是预料之中的，因为在方法 addVideo() 中，您不是在制作副本，而是在创建长度更长的 new array然后向其中添加一个新的 Video 对象。那将是 store.

中的唯一元素

要保留之前添加的所有元素，您必须创建 copy of the existing array。其中一种方法是使用 Arrays 实用程序 class.

的静态方法 copyOf()

    public void addVideo(String name) {
        int s;

        Video video = new Video(name);
        if (store == null)
            s = 0;
        else {
            s = store.length;
        }
        Video[] store = Arrays.copyOf(store, s + 1);
        store[s]=video;
    }

Answer 2

看看addVideo方法：

void addVideo(String name) {
    int s;
    
    Video video = new Video(name);
    if(store==null)
        s=0;
    else {
        s=store.length;
    }
    
    Video[] store2 = new Video[s+1];

    // This array only has a single item in it: the new video is at the end of the
    // array, and the rest of the array is empty.
    // Be sure to preserve the original content.
    //
    // Copying the entire array every time you add a video is not
    // very efficient, though, because it means
    // that adding a new video will always be an O(n) operation, so you may want to
    // consider an alternative implementation here.
    store2[s]=video;

    // You replace the entire array here
    store=store2;
}

在此方法中，您每次都将原始数组替换为仅包含您刚刚添加的视频的数组。

此外，正如我在代码注释中提到的，数组在这里是一种不太理想的数据结构，因为这意味着添加或签出视频都是 O(n) 操作。您可能需要考虑使用散列 table。

添加新元素时清空数组

Array is cleared when adding a new element

java

arrays

class

object

flow-control