Compare object type

Question

I don't know how to perform type checking on newEntry, I want to make sure that it is of type MyTable (without creating an object of MyTable).

public static boolean add(String table, Object newEntry)
{
    boolean result; 
    if (table.equals("MyTable") && newEntry.getClass() == MyTable.getClass())
    {
         ...
    }   
}

My problem is:

newEntry.getClass() == MyTable.getClass().

Note: MyTable is a class name, not an object.

newEntry instanceof Table ? – AllTooSir Apr 14 '13 at 19:41 — AllTooSir, Apr 14 '13 at 19:41

score 5 · Answer 1 · answered Apr 14 '13 at 19:38

5

Basically what you want is:

isAssignableFrom

Take a look at: http://www.ralfebert.de/blog/java/isassignablefrom/

So, in your case, you want:

MyTable.class.isAssignableFrom(newEntry.getClass())

answered Apr 14 '13 at 19:38

JHollanti

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Thanks, that got the job done. However, I'm wondering why instanceOf wouldn't work. Instance of was my first approach . . – Don Code Apr 14 '13 at 20:03
It has to do with which class you mention first and which one last. For example, newEntry.getClass().isAssignableFrom(MyTable.class) wouldn't probably work for you either. Try flipping the arguments and the instanceof should start to work as well. – JHollanti Apr 14 '13 at 20:06

score 3 · Answer 2 · edited May 23 '17 at 12:23

3

Use instanceof operator .. Refer to the JLS for more documentation

Check this famous answer What is the difference between instanceof and Class.isAssignableFrom(...)?

edited May 23 '17 at 12:23

Community

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1

answered Apr 14 '13 at 19:40

Srujan Kumar Gulla

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score 2 · Answer 3 · answered Apr 14 '13 at 19:40

2

instanceof is your friend:

if (table.equals("MyTable") && newEntry instanceof MyTable)

It is actually a shorthand for the isAssignableFrom method, but it's much easier to type :)

answered Apr 14 '13 at 19:40

gaborsch

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@syb0rg hurrying towards 3000 :D – gaborsch Apr 14 '13 at 19:42
1

@syb0rg kind of you, made my day complete :) now do the cleanup :) – gaborsch Apr 14 '13 at 19:51

score 2 · Answer 4 · edited May 23 '17 at 12:01

Compare with instanceof.

if (newEntry instanceof MyTable) {
    // do something
  }

In this example, the condition is true if newEntry is an instance of MyTable, or if newEntry is an instance of a superclass of MyTable.

Change your statement to this to make it work properly:

if (table.equals("MyTable") && newEntry instanceof MyTable)

You could also use isAssignableFrom() to compare them. The reason you might want to do this is because with instanceof, you have to know the class you are comparing before you compile your program. With isAssignableFrom(), you can change the class you are comparing to during run-time.

if (table.equals("MyTable") && MyTable.class.isAssignableFrom(newEntry.getClass()))

score 0 · Answer 5 · edited Apr 14 '13 at 20:03

0

You can use MyTable.class to retrieve the class name.

edited Apr 14 '13 at 20:03

Hugo Dozois

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answered Apr 14 '13 at 19:37

akostadinov

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@JHollanti, because answer needs to be at least 30 characters – akostadinov Apr 14 '13 at 19:55

Compare object type

5 Answers5