I have written a code which has Student class and student objects are used as keys as follows,
public class ExampleMain01 {
private static class Student{
private int studentId;
private String studentName;
Student(int studentId,String studentName){
this.studentId = studentId;
this.studentName = studentName;
}
@Override
public int hashCode(){
return this.studentId * 31;
}
@Override
public boolean equals(Object obj){
boolean flag = false;
Student st = (Student) obj;
if(st.hashCode() == this.hashCode()){
flag = true;
}
return flag;
}
@Override
public String toString(){
StringBuffer strb = new StringBuffer();
strb.append("HASHCODE ").append(this.hashCode())
.append(", ID ").append(this.studentId)
.append(", NAME ").append(this.studentName);
return strb.toString();
}
public int getStudentId() {
return studentId;
}
public String getStudentName() {
return studentName;
}
} // end of class Student
private static void example02() throws Exception{
Set<Student> studentSet = new HashSet<Student>();
studentSet.add(new Student(12, "Arnold"));
studentSet.add(new Student(12, "Sam"));
studentSet.add(new Student(12, "Jupiter"));
studentSet.add(new Student(12, "Kaizam"));
studentSet.add(new Student(12, "Leny"));
for(Student s : studentSet){
System.out.println(s);
}
} // end of method example02
private static void example03() throws Exception{
Map<Student, Integer> map = new HashMap<Student,Integer>();
Student[] students = new Student [] {
new Student(12, "Arnold"),
new Student(12, "Jimmy"),
new Student(12, "Dan"),
new Student(12, "Kim"),
new Student(12, "Ubzil")
};
map.put(students[0], new Integer(23));
map.put(students[1], new Integer(123));
map.put(students[2], new Integer(13));
map.put(students[3], new Integer(25));
map.put(students[4], new Integer(2));
Set<Map.Entry<Student, Integer>> entrySet = map.entrySet();
for(Iterator<Map.Entry<Student, Integer>> itr = entrySet.iterator(); itr.hasNext(); ){
Map.Entry<Student, Integer> entry = itr.next();
StringBuffer strb = new StringBuffer();
strb.append("Key : [ ").append(entry.getKey()).append(" ], Value : [ ").append(entry.getValue()).append(" ] ");
System.out.println(strb.toString());
}
} // end of method example03
public static void main(String[] args) {
try{
example02();
example03();
}catch(Exception e){
e.printStackTrace();
}
}// end of main method
} // end of class ExampleMain01
In the above code in Student class the hashcode and equals are implemented as follows,
@Override
public int hashCode(){
return this.studentId * 31;
}
@Override
public boolean equals(Object obj){
boolean flag = false;
Student st = (Student) obj;
if(st.hashCode() == this.hashCode()){
flag = true;
}
return flag;
}
Now when I compile and run the code,
the code in method example02 gives an output as
HASHCODE 372, ID 12, NAME Arnold
i.e the Set holds only one object,
What I understood that as the key of all the objects has the same hashcode hence only single object lies in the bucket 372. Am I right ?
Also the method example03() give output as
Key : [ HASHCODE 372, ID 12, NAME Arnold ], Value : [ 2 ]
From the above method we can see that as the key returns the same hashcode, the Hashmap only holds the single key value pair.
So my question is where does the collision happens ?
Can a key can point to multiple values ?
Where does the linkedlist concept comes while searching for value of respective key ?
Can anybody please explain me the above things with respect to the examples I have shared ?
