Passing Class object as a key in HashMap in java

Question

How can we set Class object as a key in HashMap. Suppose I have one class "Student" with two fields "rollNo" and "name". I need to pass object of this class as key in HashMap.How can I do that. Thanks

Answer 1

No need to override anything. it will work

Answer 2

then when you wanna use it. lets just say your value is a string

HashMap<Student, String> map =
new HashMap<>();

Student student = new Student ();
map.put (student, "Student1");

Answer 3

Student student1 = new Student ();
Student student2 = new Student ();

map.put (student1, "Test1");
map.put (student2, "Test2");
map.put (student1, "Test3");

the map now has 2 entries, since student1 was detected as a duplicate, it overrides the first one. so now map.get (student1); will give you "Test3"

Answer 4

another example

Student student1 = new Student ();

map.put(student1, "Test1");
Student student2 = student1;

map.put (student2, "Test2");

remember student1 is just a reference to the object. so setting student2 = student1 also points student2 to the same reference as student1, therefore the second map.put () overrides the first, and now the only entry in the map is Key: student2, Value: Test2

Answer 5

please check this:

http://stackoverflow.com/questions/9440380/using-an-instance-of-an-object-as-a-key-in-hashmap-and-then-access-it-with-exac

Answer 6

ok yea then you should override equals() and hashcode ()

Answer 7

Many thanks Edward.. but have u tried your solutions or u r really agree with overriding equals and hasCode method.. I really need to understand it properly ?

Answer 8

i havent tried it so dont trust me 100%. but it would make sense to override hashcode and equals

Answer 9

just use the class name as the key so
HashMap <Student, Value>

Answer 10

Do u think it would work... i mean do I need to override.. equals and hasCode method in Student class ?

Answer 11

But still i have one doubt regarding HashMap behaviour..

As it wont allow duplicate keys so in this case how it would identify the key is duplicate..

I mean passing multiple object of Student class as a key..

got my point