Thinking about using our OCA 8 book to study for the Java Foundations Junior Associate exam? It covers most of the topics. See what other topics you need to learn and where to read about that. One of those topics is comparing two Strings using compareTo(). Luckily there are only two things you need to know.

What does compareTo() return.

Scenario	Result
The two Strings are equal	0
The first String sorts earlier alphabetically	a negative number
The first String sorts later alphabetically	a positive number

Let’s look at some examples:

String first = "abc";
String second = "def";
System.out.println(first.equals(first));  // true
System.out.println(first.compareTo(first)); // 0

System.out.println(first.equals(second));  // false
System.out.println(first.compareTo(second)); // -3
System.out.println(second.compareTo(first)); // 3

Don’t worry. You don’t have to know the answer is -3. You just have to know that it is negative or positive or zero.

How is alphabetically defined?
For the exam, you need to know that numbers sort before letters and uppercase sorts before lowercase. What do you think the output of this code is?

public class PlayTest {

	public static void main(String[] args) {

		String nums = "123";
		String uppercase = "ABC";
		String lowercase = "abc";
		printComparison(nums, nums);
		printComparison(nums, uppercase);
		printComparison(nums, lowercase);
		System.out.println();
		printComparison(uppercase, nums);
		printComparison(uppercase, uppercase);
		printComparison(uppercase, lowercase);
		System.out.println();
		printComparison(lowercase, nums);
		printComparison(lowercase, uppercase);
		printComparison(lowercase, lowercase);
	}

	private static void printComparison(String one, String two) {
		int result = one.compareTo(two);
		if (result == 0) {
			System.out.println("0");
		} else if (result < 0) {
			System.out.println("negative");
		} else {
			System.out.println("positive");
		}
	}

The answer is:

0
negative
negative

positive
0
negative

positive
positive
0

Make sure you can fill this in by yourself. You should also know the space sorts before letters. For example, printComparison(“ABC”, “A C”); prints out a positive number.

Summary

There’s not much you have to memorize. The key facts are:

• The method name is compareTo()
• Numbers sort before capital letters which sort before lowercase letters
• Spaces sort before letters

Practice Questions

Question 1

What does the following output? (Choose all that apply)

String first = "MOO";
String second = "moo";
System.out.println(first.compareTo(second));
System.out.println(first.equals(second));

A: A negative number

B: Zero

C: A positive number

D: true

E: false

F: none of the above

Question 2

What does the following output? (Choose all that apply)

String first = "moo";
String second = " moo";
System.out.println(first.compareTo(second));
System.out.println(first.equals(second));

A: A negative number

B: Zero

C: A positive number

D: true

E: false

F: none of the above

Question 3

What does the following output? (Choose all that apply)

String first = "MOO";
String second = "MOO";
System.out.println(first.compareTo(second));
System.out.println(first.equals(second));

A: A negative number

B: Zero

C: A positive number

D: true

E: false

F: none of the above

Question 4

What does the following output? (Choose all that apply)

String first = "moo";
String second = "MOO";
System.out.println(first.compare(second));
System.out.println(first.equals(second));

A: A negative number

B: Zero

C: A positive number

D: true

E: false

F: none of the above

Question 5

What does the following output? (Choose all that apply)

String first = "MOO";
String second = "1";
System.out.println(first.compareTo(second));
System.out.println(first.equals(second));

A: A negative number

B: Zero

C: A positive number

D: true

E: false

F: none of the above

The answers are posted here.

Thinking about using our OCA 8 book to study for the Java Foundations Junior Associate exam? It covers most of the topics. See what other topics you need to learn and where to read about that. One of those topics is iterating through a list.

Since Java 5, the most common way to iterate though a list (if you don’t need the loop index) is:

List<String>  list = Arrays.asList("sheep", "deer", "rat");
for (String name : list) {
   System.out.println(name);
}

Before Java 5 came along, there was another way:

List list = Arrays.asList("sheep", "deer", "rat");
 Iterator it = list.iterator();
 while (it.hasNext()) {
   String name = (String) it.next();
   System.out.println(name);
 }

But we are told to use generics in new code which would give us:

List<String> list = Arrays.asList("sheep", "deer", "rat");
 Iterator<String> it = list.iterator();
 while (it.hasNext()) {
   String name = it.next();
   System.out.println(name);
 }

Why would you do this? Shrug. For reading old code I guess. But it is on the test so no time like the present to learn this idiom. See what is wrong here?

// DOES NOT COMPILE
 List<String> list = Arrays.asList("sheep", "deer", "rat");
 Iterator<String> it = list.iterator();
 while (it.next()) {
   String name = it.hasNext();
   System.out.println(name);
 }

This one reverses the order of hasNext/next. Remember that you have to check that the iterator has a next element before getting it. Now what do you think is the problem with this?

// BAD
 List<String> list = Arrays.asList("sheep", "deer", "rat");
 Iterator<String> it = list.iterator();
 System.out.println(it.next());

If the list is empty, this code throws an exception. Probably not what you had in mind. Instead, you should write:

 List<String> list = Arrays.asList("sheep", "deer", "rat");
 Iterator<String> it = list.iterator();
 if (it.hasNext()) System.out.println(it.next());

Summary

There’s not much you have to memorize. The key facts are:

• Call hasNext() in an if statement or while loop
• Call next() once you know there is a next element
• If the Iterator doesn’t use generics, you must cast unless you want Object

Practice Questions

Question 1

Which correctly fill in the blanks?

 List<String> list = Arrays.asList("a", "b", "c");
 Iterator<String> it = list.iterator();
 while (it._____()) {
   String name = it.______();
   System.out.println(name);
 }

A: hasNext, next

B: next, hasNext

C: The code does not compile with either A or B

D: The code throws an exception after being completed with A or B

Question 2

Which correctly fill in the blanks?

 List<String> list = Arrays.asList("a", "b", "c");
 Iterator it = list.iterator();
 while (it._____()) {
   String name = it.______();
   System.out.println(name);
 }

A: hasNext, next

B: next, hasNext

C: The code does not compile with either A or B

D: The code throws an exception after being completed with A or B

Question 3

Which correctly fill in the blanks?

 List list = Arrays.asList("a", "b", "c");
 Iterator<String> it = list.iterator();
 while (it._____()) {
   String name = it.______();
   System.out.println(name);
 }

A: hasNext, next

B: next, hasNext

C: The code does not compile with either A or B

D: The code throws an exception after being completed with A or B

The answers are posted here.