我制作了一个小的java swing应用程序来存储文件students.txt中的学生列表。
但是我在将许多学生保存到文件中时遇到了问题：
当我点击“保存”按钮时，只存储一个学生记录。如果我在文本字段中输入新数据并再次单击“保存”按钮，则仅保存新创建的学生，覆盖旧记录，而我希望将新学生附加到文件中的现有学生列表中。
这是我的代码：
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.util.StringTokenizer;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.JButton;
import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
public class studentTest extends JFrame {
JPanel p1, p2, p3, p4, p5, p6;
JTextField f1, f2, f3, f4;
JLabel l1, l2, l3, l4;
JButton b1, b2, b3;
public studentTest() {
setBounds(100, 150, 300, 400);
b1 = new JButton("Save");
b1.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
List<Student> Students = new ArrayList();
Student newStudents = new Student();
newStudents.setStudentFName(f1.getText());
newStudents.setStudentLName(f2.getText());
newStudents.setGPA(f4.getText());
newStudents.setID(f3.getText());
Students.add(newStudents);
try{
PrintWriter pw = new PrintWriter(new FileOutputStream("/Users/Yukki/Desktop/Students.txt"));
pw.println(newStudents+"\n");
pw.close();
} catch (FileNotFoundException ex) {
}
}
});
b2 = new JButton("First Record");
b3 = new JButton("Next");
l1 = new JLabel(" FirstName :");
l2 = new JLabel(" LastName :");
l3 = new JLabel(" ID :");
l4 = new JLabel(" GPA :");
f1 = new JTextField(10);
f2 = new JTextField(10);
f3 = new JTextField(10);
f4 = new JTextField(10);
p1 = new JPanel(new GridLayout(1, 2));
p1.add(l1);
p1.add(f1);
p2 = new JPanel(new GridLayout(1, 2));
p2.add(l2);
p2.add(f2);
p3 = new JPanel(new GridLayout(1, 2));
p3.add(l3);
p3.add(f3);
p4 = new JPanel(new GridLayout(1, 2));
p4.add(l4);
p4.add(f4);
p5 = new JPanel(new GridLayout(1, 1));
p5.add(b1);
p6 = new JPanel(new GridLayout(1, 2));
p6.add(b2);
p6.add(b3);
setLayout(new GridLayout(6, 1));
add(p1);
add(p2);
add(p3);
add(p4);
add(p5);
add(p6);
}
public static void main(String[] args) {
(new studentTest()).show();
}
}
答案 0 :(得分：1)
“newStudents”只是一个Student实例。迭代列表并编写每个项目。 （列表名称应以简单的字母btw开头）。此外，您只需在列表中添加一名学生。
for (Student student: students) {
pw.println(student);
}
您可以覆盖student的toString（）以获得有意义的字符串表示。
答案 1 :(得分：1)
虽然您可以尝试多种解决方案，但我建议您这样做。尝试使用Java Architecture for XML Binding (JAXB)，因为它允许您定义将Student实例输出到XML的方法。
从那里，它应该允许您将多个实例添加到同一个文件。
可能会建议您对对象进行序列化，但就个人而言，我会避免使用它。它只是用于短期存储物体，通常用于通过电线传输，但这只是我的意见......
以示例更新...
这是基于JAXB exmaple: Marshalling and Unmarshalling list or set of objects和JAXB Hello World Example
中的示例首先，我们需要定义Student类和JAXB注释。这允许我们描述如何编组（翻译）对象的实例
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
@XmlRootElement
public class Student {
String name;
int age;
int id;
// This is a requirement of JAXB...
public Student() {
}
public Student(int id, String name, int age) {
this.name = name;
this.age = age;
this.id = id;
}
public String getName() {
return name;
}
@XmlElement
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
@XmlElement
public void setAge(int age) {
this.age = age;
}
public int getId() {
return id;
}
@XmlAttribute
public void setId(int id) {
this.id = id;
}
}
接下来，我们需要创建一个包含对象列表的简单包装类。这是必需的，以便JAXB API可以更轻松地访问内部数据......
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlRootElement;
@XmlAccessorType (XmlAccessType.FIELD)
@XmlRootElement
public class Students implements Iterable<Student> {
private List<Student> students;
public Students() {
this.students = new ArrayList<>(25);
}
public Student get(int index) {
return students.get(index);
}
public void add(Student student) {
students.add(student);
}
public void remove(Student student) {
students.remove(student);
}
public int size() {
return students.size();
}
@Override
public String toString() {
return "List-o-Students: " + size();
}
@Override
public Iterator<Student> iterator() {
return students.iterator();
}
}
然后，最后，我们需要测试它......
import java.io.File;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;
import org.jaxb.test.Student;
import org.jaxb.test.Students;
public class JAXBExample {
public static void main(String[] args) {
Students students = new Students();
students.add(new Student(100, "Bob", 29));
students.add(new Student(101, "John", 19));
students.add(new Student(102, "Joe", 39));
students.add(new Student(103, "Jane", 25));
try {
File file = new File("file.xml");
JAXBContext jaxbContext = JAXBContext.newInstance(Students.class);
Marshaller jaxbMarshaller = jaxbContext.createMarshaller();
// output pretty printed
jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
jaxbMarshaller.marshal(students, file);
jaxbMarshaller.marshal(students, System.out);
} catch (JAXBException e) {
e.printStackTrace();
}
try {
File file = new File("file.xml");
JAXBContext jaxbContext = JAXBContext.newInstance(Students.class);
Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
students = (Students) jaxbUnmarshaller.unmarshal(file);
System.out.println(students);
} catch (JAXBException e) {
e.printStackTrace();
}
}
}
哪些输出......
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<students>
<students id="100">
<age>29</age>
<name>Bob</name>
</students>
<students id="101">
<age>19</age>
<name>John</name>
</students>
<students id="102">
<age>39</age>
<name>Joe</name>
</students>
<students id="103">
<age>25</age>
<name>Jane</name>
</students>
</students>
List-o-Students: 4
更新了可更新示例
以下允许您动态更新XML加班。它几乎与读，写，读相同，但提供了一些额外的功能......
try {
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();
File xml = new File("file.xml");
Document document = db.parse(xml);
// Create a new context...
JAXBContext jc = JAXBContext.newInstance(Students.class);
// Create a new Binder from the context
Binder<Node> binder = jc.createBinder();
// Load the document
Students students = (Students) binder.unmarshal(document);
// Update the student at position 0
Student student = students.get(0);
student.setAge(100);
// Add a new student
students.add(new Student(200, "Harry", 65));
// Update the document
binder.updateXML(students);
// Write the contents back to the file..
FileOutputStream fos = null;
try {
TransformerFactory tf = TransformerFactory.newInstance();
Transformer t = tf.newTransformer();
fos = new FileOutputStream(xml);
t.transform(new DOMSource(document), new StreamResult(fos));
} catch (TransformerFactoryConfigurationError | TransformerException exp) {
exp.printStackTrace();
} finally {
try {
fos.close();
} catch (Exception e) {
}
}
} catch (ParserConfigurationException | SAXException | IOException | JAXBException exp) {
exp.printStackTrace();
}
这可能看起来有点复杂，但这意味着您现在可以动态修改基础文档。
根据您的需求，我只是在开始时阅读现有列表，更新列表，最后，当您准备好时，将其写回来......但那只是我...
最后，一个完全可运行的例子......
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import javax.xml.bind.Binder;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerException;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.TransformerFactoryConfigurationError;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
import org.jaxb.test.Student;
import org.jaxb.test.Students;
import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.xml.sax.SAXException;
public class JAXBExample {
public static void main(String[] args) {
Students students = new Students();
students.add(new Student(100, "Bob", 29));
students.add(new Student(101, "John", 19));
students.add(new Student(102, "Joe", 39));
students.add(new Student(103, "Jane", 25));
write(students);
students = null;
students = read();
for (Student student : students) {
System.out.println(student);
}
update();
students = null;
students = read();
for (Student student : students) {
System.out.println(student);
}
}
public static void write(Students students) {
System.out.println("----> Write");
try {
File file = new File("file.xml");
JAXBContext jaxbContext = JAXBContext.newInstance(Students.class);
Marshaller jaxbMarshaller = jaxbContext.createMarshaller();
// output pretty printed
jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
jaxbMarshaller.marshal(students, file);
// jaxbMarshaller.marshal(students, System.out);
} catch (JAXBException e) {
e.printStackTrace();
}
}
public static Students read() {
System.out.println("----< Read");
Students students = null;
try {
File file = new File("file.xml");
JAXBContext jaxbContext = JAXBContext.newInstance(Students.class);
Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
students = (Students) jaxbUnmarshaller.unmarshal(file);
System.out.println(students);
} catch (JAXBException e) {
e.printStackTrace();
}
return students;
}
public static void update() {
System.out.println("----> Update");
try {
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();
File xml = new File("file.xml");
Document document = db.parse(xml);
JAXBContext jc = JAXBContext.newInstance(Students.class);
Binder<Node> binder = jc.createBinder();
Students students = (Students) binder.unmarshal(document);
Student student = students.get(0);
student.setAge(100);
students.add(new Student(200, "Harry", 65));
binder.updateXML(students);
FileOutputStream fos = null;
try {
TransformerFactory tf = TransformerFactory.newInstance();
Transformer t = tf.newTransformer();
fos = new FileOutputStream(xml);
t.transform(new DOMSource(document), new StreamResult(fos));
} catch (TransformerFactoryConfigurationError | TransformerException exp) {
exp.printStackTrace();
} finally {
try {
fos.close();
} catch (Exception e) {
}
}
} catch (ParserConfigurationException | SAXException | IOException | JAXBException exp) {
exp.printStackTrace();
}
}
}