Equals and Hash Code Questions

You must have a browser that supports JavaScript and DHTML to take this test properly, otherwise you will not be able to check your answers. Also, read this article for better understanding of the equals and hashCode methods. You may also have a look at this cartoon based on the same subject.

Q. 1 What will happen when you compile and run the following code?

	int a = 133;
	Integer b = new Integer(133);
	System.out.println(b.equals(a));

Options
a. It will print : true
b. It will print : false
c. The code will not compile
d. None of these

Q. 2 a and b are two objects, and a.equals(b) returns true. Also, a.hashCode() returns 1234, what will be returned by b.hashCode()? Assume that both implement equals and hashCode methods correctly.

Options
a. 1234
b. It can not be determined
c. It will return an integer that represents internal address of b.
d. None of these

Q. 3 a and b are two objects, and a.equals(b) returns false. Also, a.hashCode() returns 1234, what will be returned by b.hashCode()? Assume that both implement equals and hashCode methods correctly.

Options
a. 1234
b. It can not be determined
c. An integer value other than 1234
d. It will return an integer that represents internal address of b.
e. None of these

Q. 4 Examine the given code carefully and state if it overrides the equals method correctly.

1.	import java.util.Date;
2.	public final class Birthday
3.	{
4.		private Date bday;
5.
6.		public boolean equals(Object obj)
7.		{
8.			if(this == obj)
9.				return true;
10.			if(obj instanceof Birthday)
11.			{
12.				Birthday bd = (Birthday) obj;
13.				return bday.equals(bd.bday);
14.			}
15.			else if(obj instanceof Date)
16.			{
17.				return bday.equals(obj);
18.			}
19.			else
20.				return false;
21.		}
22.
23.		// other methods
24.	}

Options
a. The code does override the equals method correctly.
b. Can not be determined from the given code.
c. The code does not override the equals method correctly.
d. None of these.

Q. 5 What will happen when you compile and run the following code?

	String s1 = "Manish";
	String s2 = s1;
	s1 = null;
	System.out.println(s2.equals(s1));

Options
a. It will print : true
b. It will print : false
c. The code will not compile
d. None of these

Q. 6 Select the most appropriate implementation of the hashCode method that can be inserted at line 15. Assume that the equals method is implemented correctly in this class.

1.	public class HashCode
2.	{
3.		private int amount;
4.		private int rate;
5.		private double total;
6.
7.		//only one constructor
8.		public HashCode(int a, int r)
9.		{
10.			amount = a;
11.			rate = r;
12.			total = amount + ((amount * rate) / 100);
13.		}
14.
15.		// what should be inserted here?
16.
17.		// equals method not shown
18.	}

Options

      public int hashCode()
      {
       	int hash = 7;
       	hash = 31 * hash + amount;
       	hash = 31 * hash + rate;
       	long bits = Double.doubleToLongBits(total);
       	hash = 31 * hash + (int)(bits ^ (bits >>> 32));
       	return hash;
      }

      public int hashCode()
      {
       	int hash = 7;
       	hash = 31 * hash + amount;
       	hash = 31 * hash + rate;
       	hash = 31 * hash + (int)(Double.doubleToLongBits(total));
       	return hash;
      }

      public long hashCode()
      {
       	int hash = 1;
       	hash = 31 * hash + amount;
       	return hash;
      }

      public int hashCode()
      {
       	int hash = 1;
       	hash = 31 * hash + amount;
       	hash = 31 * hash + rate;
       	return hash;
      }

Q. 7 What will be printed when the following code is compiled is run? Type only the output that will printed in correct case without any quotes or spaces.
```
	Object i = new Integer(7);
	Object f = new Float(7);
	Object d = new Double(7);
	System.out.println(i.equals(f) == d.equals(f));
```

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