java - 由于 NullPointerException 导致的错误

Question

类测试

package test;
import java.io.IOException;
import org.jsoup.*;
import org.jsoup.nodes.Document;
import org.jsoup.nodes.Element;
import org.jsoup.select.Elements;
import java.util.Scanner;
public class test {
    public static void main(String[] args) throws IOException {
        Scanner in = new Scanner(System.in);
        String  url, artistIn, artist, base;
        SongUrl[] obj = new SongUrl[100];
        base = "http://lyrics.wikia.com/";
        int count = 0;
        System.out.println("Enter an artist");
        artistIn = in.nextLine();
        artist = artistIn.replace(' ', '_');
        url = "http://lyrics.wikia.com/api.php?func=getSong&artist=" + artist + "&fmt=html";
        Document doc = Jsoup.connect(url).get();
        Elements a = doc.select("a[href]");
        for(Element alink : a){
            obj[count] = new SongUrl();
            obj[count].urlSong = alink.toString();
            count++;
            obj[count].serialNo = count;      //Faced NPE on this line
            System.out.println(count + " " + alink.html());
        }
    }
}

SongUrl 类

public class SongUrl {
    int serialNo;
    String urlSong;
}

我在线上遇到了 NullPointerExceptionobj[count].serialNo = 计数； 。令人惊讶的是，如果发生 NPE，它应该发生在它之前的行 obj[count].urlSong = alink.toString(); 上。我对出了什么问题一无所知。

Answer 1

您已经增加了计数，现在它指向数组中的下一项null

  obj[count].urlSong = alink.toString();
  count++;                      // this is causing the issue
  obj[count].serialNo = count;  // obj[count] is null  

在 for 循环末尾增加计数。

<小时/>

我建议您在声明下方进行初始化，以避免出现此类 NullPointerException

 SongUrl[] obj = new SongUrl[100];
 for(int i=0;i<obj.length;i++){
        obj[i] = new SongUrl();
 }

更好用ArrayList

保持成员私有(private)并使用getter和setter方法。了解更多关于 Encapsulation

public class SongUrl {
    private int serialNo;
    private String urlSong;
    // getter & setter
}