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7) Suppose there is Student class like
class student {
int age;
string name;
}
We want to store these objects in a HashMap. Do we need to
override any methods in Student class? If any which ones
and why? what if i just override equals or just hashcode?
what will be the results in both the cases?
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Answer / manoranjan
Logically no need to override any method but problem with
this design will be that we can not find the objects stored
in the hashmap. So to overcome this problem we need to
override equlas() and hashcode() mathod of Object
class.According to Sun java guide line if equals() method is
teeling that two objects are equal then their hashcode must
be same.But reverse is not true.
| Is This Answer Correct ? | 10 Yes | 0 No |
Answer / lucky singh
It is a good practice to override equals() and hashcode()
methods if you are dealing with collections. Both of the
functions are used while searching for a particular object
in a hashmap. In general, equals() method is used to
determine if two objects are meaningfully equal or not. And
hashcode() method is used to determine if two objects are
meaningfully unequal or not.
| Is This Answer Correct ? | 2 Yes | 0 No |
Answer / krishna
There is no need to override equals() and hashCode() methods. But if we want to eliminate duplicate objects in the HashMap, you should override both the methods. Either one is not enough.
| Is This Answer Correct ? | 0 Yes | 0 No |
Answer / punch
Their is NO need to override any methods of student class,
bcoz in the ket of hashmap we are using String and equal
method and hashcode method of String class is already
overrided . So NO need to override any methods and take help
of String class of the same.
| Is This Answer Correct ? | 1 Yes | 7 No |
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Question 7 [8] Consider the following class and answer the questions below it: public class StackWithGuard extends Stack { public StackWithGuard(int size) { super(size); } synchronized public boolean isEmpty() { return super.isEmpty(); } synchronized public boolean isFull() { return super.isFull(); } synchronized public int getSize() { return super.getSize(); } synchronized public void push(Object obj) { try { while (isFull()) { wait(); } } catch (InterruptedException e) {} super.push(obj); COS2144/102 11 notify(); } synchronized public Object pop() { try { while (isEmpty()) { wait(); } } catch (InterruptedException e) {} Object result = super.pop(); notify(); return result; } public static void main(String args[]) { StackWithGuard stack = new StackWithGuard(5); new Producer(stack, 15).start(); new Consumer(stack, 15).start(); } } Note: The Stack class is provided in the Appendix. Note also: The following questions all refer to the pop() method of the StackWithGuard class given above. 7.1 What does the synchronized keyword ensure for this method? (2) 7.2 Why is a while loop used to test whether the stack is empty? In other words, why wouldn't the following if statement be sufficient? if (isEmpty()) { wait(); } (2) 7.3 Why is the result of popping (provided by the inherited pop() method) stored in a temporary variable? In other words, why wouldn't the following statement be sufficient? return super.pop(); (2) 7.4 Why is the while loop placed in a try-catch structure? (2) Appendix The LinkedQueue class: public class LinkedQueue implements Queue { private Node first, last; private int count; public LinkedQueue() { first = last = null; count =0; } public int size() { return count; } public boolean isEmpty() { return (count == 0); 12 } public void enqueue(Object o) { Node node = new Node(); node.element = o; node.next = null; node.prev = last; if (last != null){ last.next = node; } else { last = first = node; } last = node; count++; } public void dequeue() { if ((first!= null) & (first.next!=null)) { first = first.next; first.prev = null; count--; } else { first = last = null; count--; } } public Object front() { return first; } } class Node { Object element; Node next, prev; } The Stack class: public class Stack { protected Object rep[]; protected int top = -1; protected int size = 0; protected int count = 0; public Stack(int size) { if (size > 0) { this.size = size; rep = new Object[size]; } } public boolean isFull() { return (count == size); } public boolean isEmpty() { return (count == 0); } public int getSize() { return size; } public void push(Object e) { if (e != null && !isFull()) { COS2144/102 13 top++; rep[top] = e; count ++; } } public Object pop() { Object result = null; if (!isEmpty()) { result = rep[top]; top--; count--; } return result; } }
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