如何从参数中的文件加载数据并将其作为Java中的列表返回？

如何解决如何从参数中的文件加载数据并将其作为Java中的列表返回？

我正在尝试从参数中的“ feedsfile”加载数据，并将其作为列表返回。到目前为止，我已通过.add方法将“ feedsfile”添加到列表“ loadFeed”中，并使用扫描仪进行了尝试，但这似乎不起作用。

它说：“列表类型中的add（Feed）方法不适用于参数（文件）。”

@Override
public List<Feed> loadSubscribedFeeds(File feedsFile) {
    List<Feed> loadFeed = new ArrayList<>();
    
    try (Scanner s = new Scanner(new FileReader(feedsFile))) {
        while (s.hasNext()) {
            loadFeed.add(feedsFile);
        }
    } 
    catch (FileNotFoundException e) {
        e.printStackTrace();
    }
    return loadFeed;
}

这是Feed类

package de.uk.java.feader.data;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
import com.rometools.rome.feed.synd.SyndEntry;
import com.rometools.rome.feed.synd.SyndFeed;

import de.uk.java.feader.utils.FeaderUtils;

public class Feed implements Serializable,Comparable<Feed> {

private static final long serialVersionUID = 1L;

private String url;
private String title;
private String description;
private String publishedDateString;
private List<Entry> entries;


public Feed(String url) {
    super();
    this.url = url;
    this.entries = new ArrayList<Entry>();
    this.title = "";
    this.description = "";
    this.publishedDateString = "";
}

/**
 * Creates an instance of a Feed and transfers the feed
 * data form a SyndFeed object to the new instance.
 * @param url The URL string of this feed
 * @param sourceFeed The SyndFeed object holding the data for this feed instance
 */
public Feed(String url,SyndFeed sourceFeed) {
    this(url);
    setTitle(sourceFeed.getTitle());
    setDescription(sourceFeed.getDescription());
    
    if (sourceFeed.getPublishedDate() != null)
        setPublishedDateString(FeaderUtils.DATE_FORMAT.format(sourceFeed.getPublishedDate()));

    
    for (SyndEntry entryTemp : sourceFeed.getEntries()) {
        Entry entry = new Entry(entryTemp.getTitle());
        entry.setContent(entryTemp.getDescription().getValue());
        entry.setLinkUrl(entryTemp.getLink());
        entry.setParentFeedTitle(getTitle());
        if (entryTemp.getPublishedDate() != null) {
            entry.setPublishedDateString(FeaderUtils.DATE_FORMAT.format(entryTemp.getPublishedDate()));
        }
        addEntry(entry);
    }
}

public String getUrl() {
    return url;
}

public void setTitle(String title) {
    this.title = title != null ? title : "";
}

public String getTitle() {
    return title;
}

public void setDescription(String description) {
    this.description = description != null ? description : "";
}

public String getDescription() {
    return description;
}

public void setPublishedDateString(String publishedDateString) {
    this.publishedDateString = publishedDateString != null ? publishedDateString : "";
}

public String getPublishedDateString() {
    return publishedDateString;
}

/**
 * Returns a short string containing a combination of meta data for this feed
 * @return info string
 */
public String getShortFeedInfo() {
    return getTitle() + " [" +
            getEntriesCount() + " entries]: " + 
            getDescription() +
            (getPublishedDateString() != null && getPublishedDateString().length() > 0
                ? " (updated " + getPublishedDateString() + ")"
                : "");
}

public void addEntry(Entry entry) {
    if (entry != null) entries.add(entry);
}

public List<Entry> getEntries() {
    return entries;
}

public int getEntriesCount() {
    return entries.size();
}

@Override
public boolean equals(Object obj) {
    return (obj instanceof Feed)
        && ((Feed)obj).getUrl().equals(url);
}

@Override
public int hashCode() {
    return url.hashCode();
}

@Override
public String toString() {
    return getTitle();
}

@Override
public int compareTo(Feed o) {
    return getPublishedDateString().compareTo(o.getPublishedDateString());
}

}

解决方法

问题所在处loadFeed.add（feedsFile）...您没有使用扫描仪进行读取。

尝试一下

While(s.hasNextLine()){
  loadFeed.add(s.nextLine());
}

,

如果读取文件返回的对象是String，为什么要尝试从文件feed中加载对象？该列表应包含字符串而不是提要对象。 您可能需要将文件内容转换为真实对象，看看how to convert a String to Object in JAVA

尝试此操作或类似方法：

import java.nio.charset.StandardCharsets;
import java.nio.file.Paths;
import java.nio.file.Files;
import java.util.stream.Collectors;

@Override
public List < Feed > loadSubscribedFeeds(File feedsFile) {

  List < String > list = Files.lines(Paths.get(feedsFile.getAbsolutePath()),StandardCharsets.ISO_8859_1).collect(Collectors.toList());

  List < Object > objs = new ArrayList < >();

  try {
    for (String s: list)
    objs.add(Class.forName(s).newInstance());
    //IMPORTANT! s should contain not only feed for example but com.mypackage.feed
  } catch(Exception ex) {
    System.out.println(ex.getMessage());
  }

  List < Feed > listFeed = new ArrayList < >();

  for (Object l: objs) {
    if (l instanceof Feed)
        listFeed.add((Feed) l);
  }
  return listFeed;
}

如果只需要文件内容的字符串表示形式而不是实际对象，请考虑将返回类型更改为List<String>并返回list，您已经完成了。而且，如果您想继续使用代码从文件中读取字符串（而不是像我那样使用lambda），请像已经建议的那样使用nextLine（和hasNextLine()），并更改{{1} }到List<Feed>。

使用您已经编写的内容的完整代码如下：

List<String>

其中文件包含的内容类似于：

public List<Feed> loadSubscribedFeeds(File feedsFile) throws IOException {

    List<String> list = new ArrayList<>();

    try (Scanner s = new Scanner(new FileReader(feedsFile))) {
        while (s.hasNextLine()) {
            list.add(s.nextLine());
        }
    }
    catch (FileNotFoundException e) {
        e.printStackTrace();
    }

    List<Object> objs = new ArrayList<>();

    try {
        for (String s : list)
            objs.add(Class.forName(s).newInstance());
        //IMPORTANT! s should contain not only feed for example but com.mypackage.feed
    }catch(Exception ex){
        System.out.println(ex.getMessage());
    }
     
    List<Feed> listFeed = new ArrayList<>();

    for (Object l : objs){
        if (l instanceof Feed)
            listFeed.add((Feed)l);
    }

    return listFeed;
}

对于Subfeed，Feed的子类。