Question

我正在尝试使此循环将文件中的每一行放入一个对象，然后将每个对象存储在arrayList中。出于某种原因，循环正确存储了输入的前三行，但是当到达最后一行时，数组列表中的每个项目都变为最后一行中的值。我在下面粘贴了代码的相关部分，有什么建议吗？

while (lineIteration != null) {
   //store polynomial objects from Polynomial class in an array list.
    ta.setText(ta.getText() + lineIteration + "\n"); //prints the input to 
                                                     // the text area
    Polynomial objLine = new Polynomial(lineIteration); //store a line in the 
                                                        // objLine object
    System.out.println(objLine);
    polyArrayList.add(objLine); //add the object to the arrayList
    lineIteration = inBR.readLine(); //read the next lineIteration.
    }

完整代码如下。 / ***** CLASS Main.java ***** /

import java.util.Iterator;

public class Polynomial implements Iterable, Comparable{

    private static String poly;

    public Polynomial (String x){ // constructor that accepts the input and stores it in a polynomial.
        poly = x;
        return;
    }

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

    private static class Node<String>{

        private String data;
        private Node<String> next;
        public Node (String data, Node<String> next){
            this.data = data;
            this.next = next;
        }
    }
    @Override
    public Iterator iterator() {
        return null;
    }

    @Override
    public int compareTo(Object o) {
        return 0;
    }
}

/****Polynomial.java****/

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.io.*;
import java.awt.event.ActionListener;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.logging.Logger;

public class Main extends JFrame implements ActionListener {
    JMenuBar mb;
    JMenu file;
    JMenuItem open;
    JTextArea ta;
    static ArrayList<Polynomial> polyArrayList = new ArrayList<>(); //ArrayList of objects Polynomial
    Main(){
        open = new JMenuItem("Open File");
        open.addActionListener(this);
        file = new JMenu("File");
        file.add(open);
        mb = new JMenuBar();
        mb.setBounds(0,0,800,20);
        mb.add(file);
        ta = new JTextArea(800,800);
        ta.setBounds(0,20,800,800);
        add(mb);
        add(ta);
    }
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == open) {
            JFileChooser fc = new JFileChooser();
            int i = fc.showOpenDialog(this);
            if (i == JFileChooser.APPROVE_OPTION) {
                File selectedFile = fc.getSelectedFile();
                String filepath = selectedFile.getPath();
                try {
                    BufferedReader inBR;
                    inBR = new BufferedReader(new FileReader(selectedFile));
                    String lineIteration = inBR.readLine();
                    while (lineIteration != null) {
                        //store polynomial objects from Polynomial class in an array list.
                        ta.setText(ta.getText() + lineIteration + "\n"); //prints the input to the text area
                        Polynomial objLine = new Polynomial(lineIteration); //store a line in the objLine object
                        System.out.println(objLine);
                        polyArrayList.add(objLine); //add the object to the arrayList
                        lineIteration = inBR.readLine(); //read the next lineIteration.
                    }
                } catch (FileNotFoundException fileNotFoundException) {
                    System.out.println("File not Found");
                } catch (IOException ioException) {
                    System.out.println("IO Exception");
                }
                System.out.println(polyArrayList.get(0));
                System.out.println(polyArrayList.get(1));
                System.out.println(polyArrayList.get(2));
                System.out.println(polyArrayList.get(3));
            }
        }
    }
    public static void main(String[] args){
        System.out.println("Start");

        Main om = new Main();
        om.setSize(500,500);
        om.setLayout(null);
        om.setVisible(true);
        om.setDefaultCloseOperation(EXIT_ON_CLOSE);
    }


}

Answer 1

poly变量具有static修饰符，表示该变量在该类的所有实例之间共享。当在Polynomial对象的另一个实例中将变量设置为新值时，这也将反映在所有其他Polynomial对象中。

由于文件的最后一行是在变量上设置的最新值，因此它将是所有Polynomial对象上设置的值。只需删除变量上的static。

Answer 2

您必须从poly中的Polynomial中删除静态变量：

public class Polynomial implements Iterable, Comparable {

  private String poly;

  public Polynomial(String poly) { // constructor that accepts the input and stores it in a polynomial.
    this.poly = poly;
}

static关键字表示所有实例将在它们之间共享完全相同的值。即使您没有看到，在每次迭代中，您都将所有实例中的poly值设置为相同的值。您可以进一步了解static关键字here。

循环将相同的对象添加到我的arrayList的所有元素中

2 个答案: