创建并处理通用类型列表

Question

I have a list of values that I need to store that can be of any type (mostly int, double, date, ...) at runtime. Later I need to process these. What is the way I should approach this problem?

This is what I have come up with, and it appears to work, but I am not sure if this is the correct way to approach this problem.

    ArrayList<Object> list = new ArrayList<Object>();

    list.add("hello");
    list.add(1);
    list.add(0.5);

    for (int i = 0; i < list.size(); i++)
    {
        Object obj = list.get(i);

        if (obj instanceof Integer)
        {
            System.out.println((double)obj);
        }
        else if (obj instanceof String)
        {
            System.out.println(obj.toString());
        }
        else
        {
            System.out.println("Unable to deal with type.");
        }
    }

Answer 1

I realize your example is probably simplified, but I'd introduce a wrapper for all of them to represent whatever is common between them in your world. Here's an example:

interface PrintableThing {
   string specialToString();
}

class IntPrintable implements PrintableThing {
    private final int int_;

    public IntPrintable(int a) { int_ = a; }
    public specialToString() { return ((double)int_).toString(); }
}

And so on. Now you can just iterate over a list of ArrayList<PrintableThing> and ask each element to do it's specialToString method.

Answer 2

Two ways of doing it I can think of:

1. As your provided code shows, check every object with instanceof in else-if's.

2. Create an abstract class (or interface) with a set of methods you will apply to objects, eg toString() (which – in many cases – is already implemented the way you want it to be).

Answer 3

additionally you can create a custom javabean class to hold these data types . Like this :

class data{
    private Double double1;
    private int int1;
    private String1;
}

and you can make a list of this class type :

List<data> l = new List<data>();

In my opinion that makes more sense that your current implementation.

Answer 4

Usually you will use the generic Type to make sure that a Collection holds only that Type of elements. So it is not necessary to say that the elements of ArrayList have the type Object because that's normal.

And when you convert to other types, make sure you use the type you tested with instanceof otherwise you can encounter a ClassCastException . What I mean is oyur line where you converting an Object of Integer to an primitive double.

Otherwise it's OK

And one hint, instead of using:

for (int i = 0; i < list.size(); i++)

you can use

for (Object obj : list)