为单个obj创建通用类，列表 <obj> 并且有一个objs？

Question

With the following class:

public class Foobar<T> implements Serializable {

    private Boolean isTrue;
    private String randomMessage;
    private T data;

}

How when constructing an object of Foobar here can I decide that my data variable might be either the following:

• A singular object
• A list of cars
• A car and some has-a objects

How can this class support something like that? where I can throw those different scenarios at it to handle it robustly? Is this an attempt to be too abstract, should I just create multiple classes?

Answer 1

This is a candidate for inheritance.

Create an abstract class:

public abstract class Foobar<T> implements Serializable { ...

Then create three different classes for each type of content:

public class FoobarSingle<T> extends Foobar<T> {
    T data;
    ...

public class FoobarList<T> extends Foobar<T> {
    List<T> data;
    ...

public class FoobarComplex<T> extends Foobar<T> {
    T data;
    Object anotherField;
    ...

Another option is to make a single class Foobar<T> and a abstract container class for the data it holds:

public class FoobarSingle<T> extends Foobar<T> {
    FoobarContainer<T> data;
    ...

FoobarContainer may then be subclassed to hold different kinds of data.

Either way, you must be prepared to handle different kinds of data depending on the actual class, which you can use instanceof operator to.

There is the option of having private fields in a single Foobar class for different types of data and then access them by overloaded methods:

setData(T car) { ... }
setData(List<T> car) { ... }
setData(T car, Object otherProperty) { ... }

But this approach is prone to errors, since you must yourself manage situations where one type of content is written and other type is attempted to read.