澄清如何在Java中引发和捕获自定义异常

Question

So I have this program that is basically recreating SQL in java. I have this "driver" class that sorta controls the whole thing. It calls and instantiates various classes and methods to make it all work. With in this I have this method

public void checkCommand(String input) {

for (Command c : commands) {
    if (c.matches(input)) {
        try {
            c.execute();
        } catch (MyException e) {
            System.out.println(e.getMessage());
        }
        return;
    }
}
System.out.println("That was not a valid command.");

Execute is a method that will get called on every command entered. So every thing goes through this at some point. Within the DefineTableCommand class, execute contains

database.addTable(tableName, fieldList);

database is a singleton instance of the Object I'm using to to store all the tables. The addTable command calls a few other things and so on. Within these, I have some methods that could go wrong, like the table name already exists or something along those line.

public void addTable(String tableName, String fieldList) throws MyException {
    Table table;
    table = new Table(tableName, fieldList);
    if (instance.checkForDuplicates(tableName)) {
       throw new MyException(
            "There was a table with that name already present.");
    } else {
         instance.tableCollection.put(tableName, table);
    }
}

Now, What I want to happen, and what I thought would happen, was that the exception would be thrown from addTable, the exception would be passed up the stack to the catch in the checkCommand. This does not seem to be the case and I'm not sure I understand why.

To explain further. The user is given a prompt in which they enter various commands (define table emp having fields (name varchar) for example) if there is an issue with their command, they should see an error message, but the program should keep running. so lets say they enter a table with the same name as a table already present, they should be given an error message and the program continues with no new table. Instead no error message is printed, but the table still isn't added.

Answer 1

Make sure that part of the signature of execute() is:

    execute() throws MyException

And in its implementation doesn't catch MyException like:

    //eg
    void execute() throws MyException
    {
      database.addTable(tableName, fieldList);
     //any code
    }

If this is the case.

Answer 2

确保在Command类execute（）方法引发MyException之前没有捕获它