可以在Java中为变量分配其他类型吗？

Question

I was wondering if it's possible in Java to have a variable that can potentially be different types? To be specific, I have two different objects of different class, both of them are tables. So let's say we have:

TableTypeA t1;
TableTypeB t2;

Let's say that both classes have a method called getSelectedRow(), is it possible for me to make a third variable so that I can do the following:

SomeType t3;
t3 = t1; 
t3.getSelectedRow();

t3 = t2;
t3.getSelectedRow();

TableTypeA and TableTypeB do not extend the same class, the closest common superclass they have do not have the getSelectedRow() method. Seeing how similarly they function, one of them should probably be extending the other or their classes should've been merged together, but let's say a structural change like that is not an option, is what I'm suggesting possible to do?

Thanks.

Edit: I think I will add an interface for these, thank you very much for all of the quick responses!

Answer 1

If the classes implement an interface that has getSelectedRow() , sure.

Such a change isn't structural/organizational in the sense that re-organizing a class hierarchy is.

Answer 2

It's possible, if TableTypeA and TableTypeB both implement an interface called SomeType (rather than deriving from a base class). Eg:

interface SomeType {
    Row getSelectedRow();
}

class TableTypeA implements SomeType {
    Row getSelectedRow() {
        // ...
    }
}

class TableTypeB implements SomeType {
    Row getSelectedRow() {
        // ...
    }
}

If not, then no, you have to go via reflection .

Answer 3

It is possible with an interface:

public interface Table {
    Row getSelectedRow();
}

public class TableTypeA implements Table {
    Row getSelectedRow() {    // Must be defined

    }
}

public class TableTypeB implements Table {
    Row getSelectedRow() {    // Must be defined

    }
}

Without a change to the architecture of the classes, it is possible to use the instanceof operator and typecasts.

Answer 4

... but let's say a structural change like that is not an option, is what I'm suggesting possible to do?

Without a change to the types involved, it is not possible to do that. The best you can hope for is to write some ugly code using instanceof and type casts, and hide it inside a method. Reflection is an alternative, but that is uglier and more expensive (IMO).

Answer 5

Without a common Interface or super Class, no, not in a straight forward way. The compiler will complain about the assignment and request an explicit cast. The explicit cast would cause a ClassCastException .

An alternative would be to use reflection.

Answer 6

Only if they share an interface. This isn't like JavaScript.

Answer 7

不，Java不支持鸭子类型 。

Answer 8

if you need this behaviour AND you cannot change the super classes/implement interfaces then all you can do is create wrapper classes.

interface Table{ Row getSelectedRow(); }

class WrapperA extends TableTypeA implements Table{
  // copy all constructors and forward to super()
}

class WrapperB extends TableTypeB implements Table{
  // same
}

Answer 9

If you don't control the original classes, Adapters can come in handy http://en.wikipedia.org/wiki/Adapter_pattern

public interface TableAdapter {
    Row getSelectedRow();
}

public class TableTypeAAdapter implements TableAdapter {
    private TableTypeA tableTypeA;
    public TableTypeAAdapter(TableTypeA tableTypeA) {
        this.tableTypeA = tableTypeA;
    }

    public Row getSelectedRow() {
        tableTypeA.getSelectedRow();
    }
}

And a similar class for TableTypeB

Then:

TableAdapter table = new TableTypeAAdapter(tableTypeA);
table.getSelectedRow();

table = new TableTypeBAdapter(tableTypeB);
table.getSelectedRow();