将 ArrayList 保存到文本文件

Question

我一直在尝试将 ArrayList 保存到文件中。我可以看到它正在创建文本文件，但文本文件中没有任何内容，只是空白。

这里是 ArrayList 的主要代码，带有保存选项的开关。

static int input, selection, i = 1;
static ArrayList<Animals> a;

// Main Method
public static void main(String[] args){

    // Create an ArrayList that holds different animals
    a = new ArrayList<>();
    a.add(new Animals(i++, "Bear", "Vertebrate", "Mammal"));
    a.add(new Animals(i++, "Snake", "Invertebrate", "Reptile"));
    a.add(new Animals(i++, "Dog", "Vertebrate", "Mammal"));
    a.add(new Animals(i++, "Starfish", "Invertebrates", "Fish"));

    while (true) {
        try {
            System.out.println("\nWhat would you like to do?");
            System.out.println("1: View List\n2: Delete Item\n3: Add Item\n4: Edit Item\n5: Save File\n0: Exit");
            selection = scanner.nextInt();
            if(selection != 0){
                switch (selection) {
                    case 1:
                        ViewList.view();
                        Thread.sleep(4000);
                        break;
                    case 2:
                        Delete.deleteItem();
                        Thread.sleep(4000);
                        break;
                    case 3:
                        Add.addItem();
                        Thread.sleep(4000);
                        break;
                    case 4:
                        Edit.editItem();
                        Thread.sleep(4000);
                        break;
                    case 5:
                        Save.saveToFile("animals.txt", a);
                        Thread.sleep(4000);
                        break;

这是我为处理文件而写的。

import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.ArrayList;

public class Save extends ALProgram{
     public static void saveToFile(String fileName, ArrayList list){
            Path filePath = Paths.get(fileName);
            try{
                System.out.println("File Saved");
                Files.write(filePath, list, Charset.defaultCharset());
            }catch(IOException e){
                e.printStackTrace();
            }

     }
}

这里是动物课

class Animals {

public int id;
public String type, vertebrate, aclass;

public Animals(int id, String type, String vertebrate, String aclass) {
    this.id = id;
    this.type = type;
    this.vertebrate = vertebrate;
    this.aclass = aclass;

}

public int getID() {
    return id;
}

public String getType() {
    return type;
}

public String getVert() {
    return vertebrate;
}

public String getaclass() {
    return aclass;
}

}

Answer 1

有两个变化：

1. 您的类需要实现 CharSequence 才有资格传递给 Files.write。

2. 您需要重写 toString 方法来指定保存时内容的外观。 我可以看到以上两个更改后的输出。

       class Animals implements CharSequence {
   
           public int id;
           public String type, vertebrate, aclass;
   
   public Animals(int id,String type,String vertebrate,String aclass) {
   this.id = id;
   this.type = type;
   this.vertebrate = vertebrate;
               this.aclass = aclass;
           }
   
           public int getID() {
               return id;
           }
   
           public String getType() {
               return type;
           }
   
           public String getVert() {
               return vertebrate;
           }
   
           public String getaclass() {
               return aclass;
           }
   
           @Override
           public int length() {
               return toString().length();
           }
   
           @Override
           public char charAt(int index) {
               return toString().charAt(index);
           }
   
           @Override
           public CharSequence subSequence(int start, int end) {
               return toString().subSequence(start, end);
           }
   
           /* (non-Javadoc)
            * @see java.lang.Object#toString()
            */
           @Override
           public String toString() {
               return "Animals [id=" + id + ", type=" + type + ", vertebrate=" + vertebrate + ", aclass=" + aclass + "]";
           }
   
   
           }
   

Answer 2

所以，第一个，你不能通过将它转换为字符串来保存它。 您需要获取每个元素，构建一个字符串，然后将其写入文件。 这是一个例子：

public static void saveToFile(String fileName, ArrayList<Animal> list){

StringBuilder sb = new StringBuilder();

for(int i=0; i<=list.size(); i++) {
Animal lAn = list.get(i);
sb.Append("Animal ID: "+lAn.getID()+"; Animal type: "+lAn.getType()+"; Animal vert: "+lAn.getVert()+"; Animal aclass: "+lAn.getaclass()+"\r\n");
}

try (PrintStream out = new PrintStream(new FileOutputStream(fileName))) { out.print(sb.toString()); }

} catch(IOException e){ e.printStackTrace(); } }

试试看。也可能存在一些错误/拼写错误，因为我是用手机编写的。

Answer 3

你只能写Iterable<? extends CharSequence>，所以像下面这样改变你的代码。

请确保覆盖Animal类的toString方法

    List<Animal> animals = new ArrayList<>();
    Animal a1 = new Animal(1L, "XYZ", "A2B");
    Animal a2 = new Animal(2L, "ABC", "IJK");
    animals.add(a1);
    animals.add(a2);
    List<String> strings = new ArrayList<>();
    for (Animal animal : animals) {
        strings.add(animal.toString());
    }
    Files.write(Paths.get("output.out"), strings, Charset.defaultCharset());