如何在Java中比较两个值

Question

I have a container which can persist values of different types implementing the Comparable interface. I need to compare those values as follows:

UpperLowerContainer values;
//initializing the container
Comparable<?> upper = (Comparable<?>) values.getUpper();
Comparable<?> lower = (Comparable<?>) values.getLower();
if (upper.compareTo(lower) < 0){ //This is not compiled
    //do some
}

The code is not compiled because of the obvious reason. The capture of the lower 's wildcard cannot be cast to the capture of upper 's wildacrd and vice versa.

But how can I solve that issue? Any idea?

UPD: The type of the values stored in the container are the same. There is a validation before storing them into it.

Answer 1

What does your class UpperLowerContainer look like? You could use type arguments, so that you do not need to use wildcards and casts. For example:

public class UpperLowerContainer<T extends Comparable<T>> {

    private final T lower;
    private final T upper;

    public UpperLowerContainer(T lower, T upper) {
        this.lower = lower;
        this.upper = upper;
    }

    public T getLower() {
        return lower;
    }

    public T getUpper() {
        return upper;
    }
}

And then:

Comparable<T> upper = values.getUpper();
Comparable<T> lower = values.getLower();
if (upper.compareTo(lower) < 0) {
    // do something
}

The reason why it doesn't work is because the compiler can't be sure that the two wildcards in these lines refer to the same type:

Comparable<?> upper = (Comparable<?>) values.getUpper();
Comparable<?> lower = (Comparable<?>) values.getLower();

If you call compareTo on one of those comparables, you have to pass it a value of the same type. However, because type type is unknown, there's no way for the compiler to check if you pass a value of the right type, so you get an error.

Answer 2

Given code will not compile until you will not specify type of object. You can not use wild cards as you are trying to use. class declaration should be:

public class UpperLowerContainer<T extends Comparable<T>>

and you should use like

Comparable<T> upper = values.getUpper();
Comparable<T> lower = values.getLower();

now you code

if (upper.compareTo(lower) < 0) {
    // do something
}

will compile