Java：ClassCastException-无法将java.lang.Class强制转换为

Question

So I get a,

ClassCastException: java.lang.Class cannot be cast to com.glostrode.Management.Link

The line on which the exception occurs on the line indicated in the following method (line 32 of the file):

public Link getLink(String name) {
    txtFile f = new txtFile("plugins/PrimeManager/linksList.txt");
    List<String> lines = f.getLines();
    int i;
    txtFile f2;
    for(i=0; i<lines.size(); i++){
        f2 = new txtFile("plugins/PrimeManager/links/"+lines.get(i)+".txt");
        if(f2.getLines().isEmpty()) {
            return null;
        }
        Object o = f2.getObject();
        Link li = (Link) o;// THIS LINE HERE
        if(li.name == lines.get(i)){
            return li;
        }
    }
    return null;
}

The getLines() method returns a List<String> containing the lines in the file specified in the initialisation of the txtFile object. The getObject() method is as follows:

public Object getObject(){
        try {
            FileInputStream i = new FileInputStream(this.file);
            ObjectInputStream o = new ObjectInputStream(i);
            Object r = o.readObject();
            o.close();
            i.close();
            return r;
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
        return null;
    }
}

From my current understanding, the object returned by o.readObject() should be castable to its original form (the file that is read contains a Link object).

I used the following method to add it to the file:

public void addObject(Object obj){
    try {
        FileOutputStream f = new FileOutputStream(this.file);
        ObjectOutputStream o = new ObjectOutputStream(f);
        o.writeObject(obj);
        o.flush();
        o.close();
        f.close();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

txtFile constructor:

public txtFile(String path) {
        this.path = path;
        this.file = new File(path);
        if(!this.file.exists()){
            try {
                this.file.createNewFile();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

The source code: linksCommand.java:

package com.glostrode.Management;

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.bukkit.ChatColor;
import org.bukkit.command.Command;
import org.bukkit.command.CommandExecutor;
import org.bukkit.command.CommandSender;

import com.glostrode.Management.FileIO.txtFile;
//import com.glostrode.Management.FileIO.txtFile;


public class linksCommand implements CommandExecutor {

    public Link getLink(String name) {
        txtFile f = new txtFile("plugins/PrimeManager/linksList.txt");
        List<String> lines = f.getLines();
        int i;
        txtFile f2;
        for(i=0;i<lines.size();i++){
            f2 = new txtFile("plugins/PrimeManager/links/"+lines.get(i)+".txt");
            if(f2.getLines().isEmpty()) {
                return null;
            }
            Link li = f2.getObject() != null ? (Link) f2.getObject() : null;
            if(li.name == lines.get(i)){
                return li;
            }
        }
        return null;
    }
    public List<String> getLinks() {
        int i;
        List<String> a = new ArrayList<String>();
        txtFile f = new txtFile("plugins/PrimeManager/linksList.txt");
        if(f.getLines().isEmpty()){
            return null;
        }
        for(i=0;i<f.getLines().size();i++){
            Link b = this.getLink(f.getLines().get(i));
            if(b!=null) {
                a.add(b.name+": "+b.link);
            }
        }
        return a;
    }
    public void addLink(Link l){
        txtFile f = new txtFile("plugins/PrimeManager/linksList.txt");
        f.addLine(l.name);
        txtFile lf = new txtFile("plugins/PrimeManager/links/"+l.name+".txt");
        lf.addObject(l.getClass());
    }
    public void deleteLink(String name){
        txtFile f = new txtFile("plugins/PrimeManager/links/"+name+".txt");
        f.delete();
    }
    public void exists() throws IOException{
        File f = new File("plugins/PrimeManager/links.json");
        if(!(f.exists())){
            f.createNewFile();
            BufferedWriter w = new BufferedWriter(new FileWriter(f));
            w.write("[{\"name\":\"example\",\"link\":\"https://www.google.com/\"}]");
            w.flush();
            w.close();
        }
    }

    public linksCommand() {
    }

    @Override
    public boolean onCommand(CommandSender sender, Command cmd, String label, String[] args) {
        if(args.length>0 || (args.length != 0 && args[0] == null)){
            Link a = (Link)this.getLink(args[0]);
            if(a==null){
                sender.sendMessage(ChatColor.RED+"Link not found!");
                return true;
            }
            sender.sendMessage(ChatColor.AQUA+a.name+": "+a.link);
            return true;
        }else {
            List<String> lines = this.getLinks();
            if(lines == null || lines.isEmpty()){
                sender.sendMessage(ChatColor.RED+"There are no links to display.");
                return true;
            }
            int i;
            for(i=0;i<lines.size();i++) {
                sender.sendMessage(ChatColor.AQUA+lines.get(i));
            }
            return true;
        }
    }
}

txtFile.java:

package com.glostrode.Management.FileIO;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.ArrayList;
import java.util.List;

public class txtFile {
    BufferedReader reader;
    BufferedWriter writer;
    File file;
    int i;
    String path;
    public txtFile(String path) {
        this.path = path;
        this.file = new File(path);
        if(!this.file.exists()){
            try {
                this.file.createNewFile();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
    public boolean getExistence(){
        return this.file.exists();
    }
    public void delete(){
        this.file.delete();
    }
    public void initiateReader(){
        try {
            this.reader = new BufferedReader(new FileReader(this.file));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
    public void initiateWriter(){
        try {
            this.writer = new BufferedWriter(new FileWriter(this.file,true));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    public boolean includes(String item){
        if(this.getLines().size()==0){
            return false;
        }else{
            if(this.getLines().contains(item)){
                return true;
            }return false;
        }
    }
    public String getLineWithSubstring(String item){
        List<String> a = this.getLines();
        int i;
        for(i=0;i<a.size();i++){
            if(a.get(i).contains(item)){
                return a.get(i);
            }
        }return "EMPTY_LINE";
    }
    public String getLineAtPos(int pos){
        this.initiateReader();
        int i;
        for(i = 0;i<this.getLines().size();i++){
            if(i == pos){
                return this.getLines().get(i);
            }
        }return null;
    }
    public List<String> getLines(){
        String ln;
        this.initiateReader();
        try {
            List<String> res = new ArrayList<String>();
            for(this.i = 0;(ln = this.reader.readLine()) != null;this.i++){
                res.add(ln.trim());
            }
            this.reader.close();
            return res;
        } catch (IOException e) {
            e.printStackTrace();
            return null;
        }
    }
    public void addObject(Object obj){
        try {
            FileOutputStream f = new FileOutputStream(this.file);
            ObjectOutputStream o = new ObjectOutputStream(f);
            o.writeObject(obj);
            o.flush();
            o.close();
            f.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    public Object getObject(){
        try {
            FileInputStream i = new FileInputStream(this.file);
            ObjectInputStream o = new ObjectInputStream(i);
            Object r = o.readObject();
            o.close();
            i.close();
            return r;
        } catch (IOException | ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return null;
    }
    public void close() {
        try {
            this.writer.close();
            this.reader.close();
        }catch(IOException e) {
            e.printStackTrace();
        }
    }
    public void replaceLine(int pos, String t){
        List<String> previous = this.getLines();
        previous.set(pos, t);
        this.removeAll("");
        this.addLines(previous);
    }
    public void addLine(String str){
        try {
            this.initiateWriter();
            this.writer.append(str);
            this.writer.newLine();
            this.writer.flush();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    public void addLines(List<String> str){
        this.initiateWriter();
        int i;
        for(i=0;i<str.size();i++){
            try {
                this.writer.newLine();
                this.writer.append(str.get(i));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }try {
            this.writer.flush();
            this.writer.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    public void removeLine(String str){
        List<String> fileContents = this.getLines();
        this.file.delete();
        int i;
        for(i = 0;i<fileContents.size();i++){
            if(i == fileContents.indexOf(str)){
                fileContents.remove(i);
                i = fileContents.size();
            }
        }
        for(i=0;i<fileContents.size();i++){
            this.addLine(fileContents.get(i));
        }
    }
    public void removeAll(String except){
        this.initiateWriter();
        try {
            this.writer.write(except);
            this.writer.flush();
            this.writer.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } 
    }
}

Answer 1

Okay,
The problem is with how you called your addObject method.
Since you called it with Link.class it returns Class object only.
Which can not be cast to Link on Link li = (Link) o

Instead just pass Link object.

addObject(l)

Where l is Link's object

Answer 2

lf.addObject(l.getClass());

You write a Class object, you get a Class object.

Do this instead:

lf.addObject(l);