LinkedHashMap program is giving is not generic; it cannot be parameterized error

Question

I have made a Student class and it in not getting initialized with LinkedHashMap. Giving compile time error :

The type LinkedHashMap is not generic; 
it cannot be parameterized with arguments <Student, Integer>.

CODE :

StudentLinkedMap :

 public class LinkedHashMap {

public static void main(String[] args) {

Map <Student,Integer> students = new  LinkedHashMap<Student,Integer>();

Student s1 = new Student(1,2,"Dhwani","Pandya");
Student s2 = new Student(2,2,"Priyam ","Parekh");
Student s3 = new Student (3,2,"Sucheta","Shrivastava");
Student s4 = new Student(3,6,"Nirali","Rokadia");
Student s5 = new Student(2,6,"Kajari","Agrawal");

students.put(s1,2);
students.put(s2,4);
students.put(s3,6);
students.put(s4,5);
students.put(s5,7);

for(Map.Entry lhs:students.entrySet()){
    System.out.println(lhs.getKey()+ "  "+ lhs.getValue());
}

}

}

Student Class:

  public class Student {

private int rollno;
private  String firstname;
private  String lastname;
private  int std;

 public Student(int rollno, int std,String firstname, String lastname){

    this.firstname=firstname;
    this.lastname=lastname;
    this.rollno=rollno;
    this.std=std;

    }

@Override
public String toString(){
    return ("\troll no="+ this.rollno + "\tstandard="+ 
 this.std+"\tfirstname="+this.firstname+"\tlastname="+this.lastname);
 }


 }

score 2 · Answer 1 · answered Sep 09 '17 at 13:59

2

You have named your class LinkedHashMap so the name shadows java.util.LinkedHashMap. You can either change the name of your class, or use the fully qualified class name.

Map<Student,Integer> students = new java.util.LinkedHashMap<>();

answered Sep 09 '17 at 13:59

Elliott Frisch

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after making the changes gives the error - LinkedHashMap cannot be resolved to a type – Sep 09 '17 at 14:02
But i am getting the same 6 outputs - roll no=2 standard=6 firstname=Kajari lastname=Agrawal 2 roll no=2 standard=6 firstname=Kajari lastname=Agrawal 4 roll no=2 standard=6 firstname=Kajari lastname=Agrawal 6 roll no=2 standard=6 firstname=Kajari lastname=Agrawal 5 roll no=2 standard=6 firstname=Kajari lastname=Agrawal 7 – Sep 09 '17 at 14:06
That is a runtime error, seems like your compiler error is fixed. You are using `Student` as a key to the `Map`, but you aren't overriding `hashCode`... do so (and don't forget to [override `equals`](https://stackoverflow.com/a/27609/2970947) as well). – Elliott Frisch Sep 09 '17 at 14:15
. why is overriding hashcode and equals so important – Sep 09 '17 at 14:27
Because a `Map` (or `Set`) doesn't allow duplicate keys, so the first thing it does is get the hash of instances to determine where to store it. Once it has that it compares with any values already there, if they're equal it stores the new instance and evicts the old one. – Elliott Frisch Sep 09 '17 at 14:32
I am unable to create equals method. can you help me define it. i created the hashcode method please check if that is correct. I posted in the answer section. – Sep 09 '17 at 14:56

LinkedHashMap program is giving is not generic; it cannot be parameterized error

1 Answers1