在Java中尝试异常，try / catch块和单例

Question

I am trying to learn to work with error handling/throwing exceptions in Java. I have a class, UserDatabase, holding a collection of students, and it should also save the collection to a file.

What I'm fairly sure that I haven't done correctly, is the methods for file handling. Method public boolean saveDatabase should not throw exceptions and be handled in a try-catch, and use the encode method from the student class on Student objects, to write every object as a line in the file. This one is compiling, but it looks off to me. In the book it says write the method public boolean saveDatabase(); and as you can see I changed the header for it to make sense to me. This, mainly because I don't know how to write a method with the header ending with ();

Method public boolean loadDatabase should also handle possible IO errors and return false if one occurs. For every line in the field it should create a student object by the constructor public Student(String encodedStudent) from the sudent class. If a line in the file cannot be decoded as a student it should throw a new exception, DatabaseFormatException (this is a seperate class). This one is also listed as public boolean loadDatabase(); in the book. Let's face it, this one is completely off. I have no idea what to do and I have been working with it for hours and hours, reading books, reading online, I am lost.

Here's my code:

   /**
* This method should not throw exceptions.
* By using a try/catch block, we anticipate possible failures.
* We recognize that these actions might fail and throw errors.
*/
   public boolean saveDatabase() throws IOException {
   //This method is using the encode method on student objects and should
   //write each object as a line in the file. 
   String encode = null;
   boolean saved;
   try {

       encode = null;
   userdatabase.saveDatabase();
   saved = false;
}
   catch (IOException e) {
       System.out.println("Error");
       saved = false;
    }
    finally {
        if(encode.equals(students)) {
        //System.out.println("Students" + students)?;
        saved = true;
    }
    } 
   return saved;
}

   /**
    * Method loadDatabase should handle possible IO errors, and return false
    * if one occurs. Otherwise, it should return true and create a new 
Student object
    * by using the constructor public Student(String encodedStudent).
    * If a line cannot be decoded as a student, the method should throw a 
new
    * exception "DatabaseFormatException".
    * 
    */
     public boolean loadDatabase() throws IOException,DatabaseFormatException {
   //Attempting to use the String encodedStudent constructor from Student class
   String encodedStudent = null;
   boolean loaded;
   try {
       //Attempting to create possible IO errors returning false if they occur
       enco dedStudent = null;
       //UserDatabase.loadDatabase(encodedStudent);
       loaded = false;
    }
    catch(IOException e) {
        if (encodedStudent == null) {
            System.out.println("Error");
            loaded = false;
        }
    }
    //Trying a for each loop to throw a DatabaseFormatException

   for(Student student : students) {
        //This is supposed to throw a DatabaseFormatException if a 
        //line in the file cannot be decoded as a student!
       if(student.getName.isEmpty() && this.course.isEmpty()) {
            throw new DatabaseFormatException(
            "No student found");
        }
    }

Answer 1

Your code should be

public boolean saveDatabase() {
   try {
        // maybe do some more work...
        userdatabase.saveDatabase();
        return true;
    } catch (IOException e) {
        return false;
    }
}

Simply return true/false depending on wether an exception occurred or not. Drop the saved since you no longer need it. And drop the encode since you did not need it in the first place and never assigned a value to it.