OCA Java SE 7 Programmer I Study guide - Chapter 2 - Self Test - Question 8

[Devaney Marcondes]

Hi.
About the question 8 of the Self test, chapter 2 say:
You need to update a value of a hash table (that is, HashMap) where the primary key must equal a specified string. Which statements would you need to use in the implementation of this algorithm?
A. Iteration statement
B. Expression statement
C. Conditional statement
D. Transfer of control statement
Answer: A, B, and C. Iteration, expression, and conditional statements would be used to implement the algorithm.
The following code segment demonstrates the use of these statements by programmatically replacing the ring on the little finger of a person’s left hand. The statements are prefaced by comments that identify their types.

import java.util.HashMap;
public class HashMapExample {
 public static void main(String[] args){
    HashMap<String,String> leftHand = new HashMap<String,String>();
    leftHand.put("Thumb", null);
    leftHand.put("Index finger", "Puzzle Ring");
    leftHand.put("Middle finger", null);
    leftHand.put("Ring finger", "Engagement Ring");
    leftHand.put("Little finger", "Pinky Ring");

    //Iteration statement
    for(String s : leftHand.keySet()){
        //Conditional statement
        if(s.equals("Little finger")){
            System.out.println(s + " had a " + leftHand.get(s));
            //Expression Statement
            leftHand.put("Little finger", "Engineer's Ring");
            System.out.println(s + " has an " + leftHand.get(s));
        }
    }
 }
}

In my opinion this question is a little confuse, because if I know the key I can update a HashMap without the support of Iteration statement and without the Conditional statement as below:

import java.util.HashMap;
public class HashMapExampleZ {
 public static void main(String[] args){
    HashMap<String,String> leftHand = new HashMap<String,String>();
    leftHand.put("Thumb", null);
    leftHand.put("Index finger", "Puzzle Ring");
    leftHand.put("Middle finger", null);
    leftHand.put("Ring finger", "Engagement Ring");
    leftHand.put("Little finger", "Pinky Ring");

    //Expression Statement
    leftHand.put("Little finger", "Engineer's Ring");
    System.out.println("Has an " + leftHand.get("Little finger"));

 }
}

If I must use the Iteration statement and the Conditional statement I can use the Transfer of control statement also.

What do you think?

Thanks

[Roel De Nijs]

Hi Devenay,

In my opinion you are correct! It's actually the power of a map to be able to access/change a value by its key using an expression statement. You don't need an iteration or conditional statement at all. The first example is simply bad programming, your 2nd example is how it should be done

So that should be added as an erratum to the book.

Kind regards,
Roel

[Sika Annas]

I'm curious, why is there HashMap in this book? They're not listed in the OCA Java 7 exam objectives.

[Roel De Nijs]

Sika Annas wrote:I'm curious, why is there HashMap in this book? They're not listed in the OCA Java 7 exam objectives.

That's true! From the Collections framework you should only know ArrayList for the OCAJP7

[Vince Silvey]

For the same question Devaney addresses at the beginning of this thread, I selected "A" (Iteration statement) and "B" (Expression statement) as answers. I did this because I could think of a way to use only a "while" as shown in the code below, and come up with output identical to the author's:

import java.util.HashMap;
public class HashMapWhile {
	static String s;
	public static void main(String[] args){
		HashMap<String,String> leftHand = new HashMap<String,String>();
		leftHand.put("Thumb", "No Ring");
		leftHand.put("Index finger", "Puzzle Ring");
		leftHand.put("Middle Finger", "No Ring");
		leftHand.put("Ring finger", "Engagement Ring");
		leftHand.put("Little finger", "Pinky Ring");
		
		// Iteration Statement + Expression
		while (leftHand.get(s).equals("Little finger")) {
				System.out.println(s + " had a " + leftHand.get(s));
				leftHand.put("Little finger", "Engineer's Ring");
				System.out.println(s + " has an " +leftHand.get(s));
		}
	}
}

I had to remove any references to "null" values and define "static String s" at the beginning of the class, but again, I got identical output and didn't use a conditional.

[Roel De Nijs]

Vince Silvey wrote:I had to remove any references to "null" values and define "static String s" at the beginning of the class, but again, I got identical output and didn't use a conditional.

Your solution gives me a NullPointerException which is not identical to the output of HashMapExampleZ.