Java中的抽象类-创建对象

Question

I'm writing a code in Java using abstract classes for the first time and I have a problem. I have a list (array) of users, each of them is an object of userClass. I have created an abstract class 'account' and 2 classes which extends it 'basicAccount' and 'premiumAccount' and I would like to create basic or premium account for every user depending on an input. My problem is that I don't know how to implement userClass, so it would create object of basic or premium class depending on a string 'accountType' Example:

public class userClass {
private String email;
private String accountType;
private account account;

public userClass(String email, String accountType){
    this.setEmail(email);
    this.setAccountType(accountType);

    if(accountType.equals("Basic")){
        basicAccount account = new basicAccount();
    }
    else if(accountType.equals("Premium")){
        premiumAccount account = new premiumAccount();
    }
}

Could you please help me? Can I put those if's in userClass method?

Answer 1

A very important concept in Object-oriented Programming is polymorphism . Polymorphism allows you to use an object A as if it were an instance of another type B if A and B follow any one of these rules

• B is a class and A's type is a subclass of B
• B is an interface and A's type implements B

In your case, the first rule applies. Both basicAccount and premiumAccount are a subclass of account . Thus, you can use your premium account and basic account objects as account objects! So just ASSIGN the newly created object to the account field!

if(accountType.equals("Basic")){
    basicAccount account = new basicAccount();
    this.account = account; // This works! Don't worry!
}
else if(accountType.equals("Premium")){
    premiumAccount account = new premiumAccount();
    this.account = account;
}

A side effect, though, is that after using basicAccount and premiumAccount objects as account s, you cannot access the methods of basicAccount and premiumAccount by directly accessing this.account .

If you really want to do so, try this method:

Say you have a doStuff method defined in basicAccount and you want to call it:

this.account.doStuff();

That does not work because this.account is of type account and account does not define a method called doStuff .

What you need to do is check whether the account field is of type basicAccount :

if (this.account instanceof basicAccount)

In the if statement, cast this.account to basicAccount and call the method:

((basicAccount)this.account).doStuff();

Answer 2

Something like this would probably do it:

public class UserClass{
    Account account;

    public UserClass(String accountType){
         if(accountType.equals("Basic"){
              this.account = new basicAccount();
         } else {
              this.account = new premiumAccount();
         {
    }
}