Compare two typical types in Java?

Question

I have the following method:

public <U, V> boolean isEqual(List<U> a, List<V> b) { // check if U == V } 

I want to check if the tags U and V the same classes.

Answer 1

You cannot do this because of erasing styles , it's that simple.

Consider the following:

 public static void main(String[] args) { List<? extends Number> l1 = Arrays.asList(1L, 2, 3L); List<? extends Number> l2 = Arrays.asList(1); isEqual(l1, l2); } public static <U, V> boolean isEqual(List<U> a, List<V> b) { // is U == V here? } 

Here U == V ? l1 contains instances of Long and Integer , but l2 contains one instance of Integer .

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I assume from your comment:

The first condition must be that their type is the same

what you need is the only type of U In this case, use the following signature:

 public static <U> boolean isEqual(List<U> a, List<U> b) { } 

nor will the above code compile anymore.

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What you can also do is add 2 parameters that take classes:

 public static <U, V> boolean isEqual(List<U> a, List<V> b, Class<U> uClass, Class<V> vClass) { if (!uClass.equals(vClass)) { // classes are different } } 

In this case, you can print a message if the specified classes do not match.

Answer 2

If you create your own class, you may require that the Class<T> be included in the constructor, as shown here

Example:

 public class SomeClass<T> { private final Class<T> clazz; public SomeClass(Class<T> clazz) { this.clazz = clazz; } public Class<T> getParam() { return clazz; } } 

Now you can call SomeClass#getParam() to get the declared type.

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There is also a way to do this with reflection.

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All of this suggests that you have to make weird workarounds to this because of Type Erasure . Basically, at runtime, Java sees all generics as Object , so when compiling your List may be List<Integer> or List<Boolean> , but at run time they are both List<Object> .

Answer 3

If you are trying to compare the contents of two lists, you should not perform type comparisons yourself. Instead, you should do this:

  public static <U, V> boolean isEqual(List<U> a, List<V> b) { if (a.size() != b.size()) return false; for (int i = 0; i < a.size(); i++) if (!a.get(i).equals(b.get(i))) return false; return true; } 

Thus, you rely on types U and V to be able to handle equals () correctly. Here are some guidelines for implementing equals (): http://www.javaworld.com/article/2072762/java-app-dev/object-equality.html

What I suppose you want to do is quickly return in case the types are different. But with the implementation that I gave you, you will get the same behavior - you will return to the first iteration.

Answer 4

Don't rely on

 a.getClass().equals(b.getClass()) 

This is not true, it will check if both types have ArrayList and not String, Integer, etc.

Try it only

 a.get(0).getClass().equals(b.get(0).getClass()) 

Make sure you check for a null state, otherwise you will encounter a NullPointerException here.

Answer 5

I think you would like to check if == b is in place of U == V. In this case, you need to write your own comparison method to compare an instance of class U and V.

Answer 6

Assuming a and b are not null, not null,

 a.get(0).getClass().equals(b.get(0).getClass()) 

gotta do the trick.