在文件中保存和加载列表

Question

我目前正在准备考试并正在执行以下任务：

如何将 ArrayList 传递给存储列表数据的“保存”方法和将数据传回的另一个“加载”方法？

class Person {

    private String firstname;

    private String lastname;

    private String sortname;

public Person(String firstname, String lastname) {

        this.firstname = firstname;

        this.lastname = lastname;

        updateSortname();

//getter 和 setter..

根据任务我应该使用这些方法：

public static List<Person> load(String filename) throws IOException {

        return ??;

}

public static Person load(DataInputStream in) throws IOException {

        return ??;

}

public static void save(String filename, List<Person> list) throws IOException {

}

public static void save(DataOutputStream out, Person person) throws IOException {

}

public static List<Person> unserialize(String filename) throws IOException, ClassNotFoundException {

        return ??;

}

public static void serialize(String filename, List<Person> persons) throws IOException {

}

这是应该产生以下输出的主要方法：

[威利·旺卡（WonkaWilly）、查理·巴克特（BucketCharlie）、乔爷爷（JoeGrandpa）]

[威利·旺卡（WonkaWilly）、查理·巴克特（BucketCharlie）、乔爷爷（JoeGrandpa）]

[威利·旺卡（WonkaWilly）、查理·巴克特（BucketCharlie）、乔爷爷（JoeGrandpa）]

public class PersonTest {

public static void main(String[] args) throws IOException, ClassNotFoundException {

    List<Person> persons = new ArrayList<>();

    persons.add(new Person("Willy", "Wonka"));

    persons.add(new Person("Charlie", "Bucket"));

    persons.add(new Person("Grandpa", "Joe"));

    System.out.println(persons);

    Person.save("persons.sav", persons);

    persons = Person.load("persons.sav");

    System.out.println(persons);

    Person.serialize("persons.ser", persons);

    persons = Person.unserialize("persons.ser");

    System.out.println(persons);

}

}

它应该看起来像这样。但我不知道如何为 ArrayLists 做这件事。

public static void save(String filename , Graph graph ) throws IOException{

try (ObjectOutputStream out = new ObjectOutputStream(new BufferedOutputStream (new FileOutputStream (filename)))) {

out.writeObject (graph);

}

}

Answer 1

由于您需要Person对象的输出，因此我们需要重写toString()类Person。[威利·旺卡（WonkaWilly）、查理·巴克特（BucketCharlie）、乔爷爷（JoeGrandpa）]class Person {//Respective Constructor, Getter & Setter methods/* Returns the string representation of Person Class.&nbsp;&nbsp;* The format of string is firstName lastName (lastNameFirstName)*/&nbsp; @Override&nbsp; public String toString() {&nbsp;&nbsp; &nbsp; return String.format(firstName + " " + lastName + "("+ lastName + firstName + ")");&nbsp;&nbsp; }&nbsp;}有许多方法可以将对象写入文件。这是与PrintWriter将对象保存到文件public static void save(String filename, List<Person> list) throws IOException {&nbsp;PrintWriter pw = new PrintWriter(new FileOutputStream(fileName));&nbsp;for (Person person : list) {&nbsp; &nbsp; pw.println(person.toString());&nbsp; &nbsp;}&nbsp;pw.close();}或者使用序列化// 你可以使用序列化机制。要使用它，您需要执行以下操作：将Person类声明为实现Serializable：public class Person implements Serializable {&nbsp; &nbsp; ...&nbsp; @Override&nbsp; public String toString() {&nbsp;&nbsp; &nbsp; &nbsp;return String.format(firstName + " " + lastName + "("+ lastName + firstName + ")");&nbsp;&nbsp; }&nbsp;}将您的列表写入文件：public static void save(String filename, List<Person> list) throws IOException {&nbsp;FileOutputStream fos = new FileOutputStream(filename);&nbsp;ObjectOutputStream oos = new ObjectOutputStream(fos);&nbsp;oos.writeObject(list);&nbsp;oos.close();}从文件中读取列表：public static List<Person> load(String filename) throws IOException {&nbsp;FileInputStream fis = new FileInputStream(filename);&nbsp;ObjectInputStream ois = new ObjectInputStream(fis);&nbsp;List<Person> list = (List<Person>) ois.readObject();&nbsp;ois.close();&nbsp;return list;}

Answer 2

你可以尝试这样的事情：public static void save(String filename , ArrayList<Person> persons) throws IOException{&nbsp; &nbsp; try (ObjectOutputStream out = new ObjectOutputStream(new BufferedOutputStream (new FileOutputStream (filename)))) {&nbsp; &nbsp; &nbsp; &nbsp; for(int i = 0; i < persons.size; i++){&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;out.writeObject(persons.get(i));&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp;}}