List Classes

Chapter 6 ■ Generics and Collections

This for-each is equivalent to the following code, which explicitly uses an Iterator:

for(Iterator<String> languageIter = languageList.iterator(); languageIter.hasNext();) { String language = languageIter.next();

System.out.println(language);

}

This code segment will also print the same output as the previous for-each loop code. Here is a step-by-step description of how this for loop works:

1.You use the iterator() method to get the iterator for that container. Since languageList is an ArrayList of type <String>, you should create Iterator with String. Name it languageIter.

2.Before entering the loop, you check if there are any elements to visit. You call the hasNext() method for checking that. If it returns true, there are more elements to visit; if it returns false, the iteration is over and you exit the loop.

3.Once you enter the body of the loop, the first thing you have to do is call next() and move the iterator. The next() method returns the iterated value. You capture that return value in the language variable.

4.You print the language value, and then the loop continues.

This iteration idiom—the way you call iterator(), hasNext(), and next() methods—is important to learn; we’ll be using either the for-each loop or this idiom extensively in our examples.

Note that you create ArrayList<String> and Iterator<String> instead of just using ArrayList or Iterator

(i.e., you provide type information along with these classes). The Collection classes are generic classes; therefore you need to specify the type parameters to use them. Here you are storing/iterating a list of strings, so you use <String>.

You can remove elements while traversing a container using iterators. Let’s create an object of ArrayList<Integer> type with ten elements. You’ll iterate over the elements and remove all of them (instead of using the removeAll() method in ArrayList). Listing 6-14 shows the code. Will it work?

Listing 6-14.  TestIterator.java

// This program shows the usage of Iterator

import java.util.*;

class TestIterator {

public static void main(String []args) { ArrayList<Integer> nums = new ArrayList<Integer>(); for(int i = 1; i < 10; i++)

nums.add(i); System.out.println("Original list " + nums); Iterator<Integer> numsIter = nums.iterator(); while(numsIter.hasNext()) {

numsIter.remove();

}

System.out.println("List after removing all elements" + nums);

}

}

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Chapter 6 ■ Generics and Collections

It prints the following:

Original list [1, 2, 3, 4, 5, 6, 7, 8, 9]

Exception in thread "main" java.lang.IllegalStateException

at java.util.AbstractList$Itr.remove(AbstractList.java:356) at TestIterator.main(Main.java:12)

Oops! What happened? The problem is that you haven’t called next() before calling remove(). Checking hasNext() in the while loop condition, moving to the element using next(), and calling remove() is the correct idiom for removing an element. If you don’t follow it correctly, you can get into trouble (i.e., you’ll get

IllegalStateException). Similarly, if you call remove() twice without sandwiching a next() between the statements, you’ll get this exception.

Let’s fix this program by calling next() before calling remove(). Here is the relevant part of the code:

Iterator<Integer> numsIter = nums.iterator(); while(numsIter.hasNext()) {

numsIter.next();

numsIter.remove();

}

System.out.println("List after removing all elements " + nums);

It prints the list with no elements, as expected:

List after removing all elements []

 Remember that next() needs to be called before calling remove() in an Iterator; otherwise, you’ll get an IllegalStateException. Similarly, calling remove() in subsequent statements without calling next() between these statements will also result in this exception. Basically, any modifications to the underlying container while an iterator is traversing through the container will result in this exception.

The ListIterator Interface

You should understand ListIterator first before you look at LinkedList. The ListIterator interface extends the Iterator interface, so it inherits the methods hasNext(), next(), and remove(). Additionally, ListIterator has many other methods (see Table 6-6). Using these methods, you can traverse in the reverse direction, get the previous or next index position, and set or add new elements to the underlying container.

Table 6-6.  Methods in the ListIterator Interface (in Addition to Iterator Methods)

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The LinkedList Class

The LinkedList class internally uses a doubly-linked list. So, insertion and deletion is very fast in LinkedList. However, accessing an element entails traversing the nodes one-by-one, so it is slow. When you want to add or remove elements frequently in a list of elements, it is better to use a LinkedList. You’ll see an example of LinkedList together with the ListIterator interface.

A palindrome is a word or phrase that reads the same forward and backward. A palindrome string “abcba” reads the same in both directions. Given a string, how can you determine whether the string is a palindrome or not?

Well, you can determine whether an input string is a palindrome or not by storing the input string in a String and using the charAt() method in a for loop to compare characters (one from the start and another from the end). To show you how to use ListIterator and LinkedList, see Listing 6-15 for a contrived solution that does the same thing.

Listing 6-15. ListIterator.java

// This program demonstrates the usage of ListIterator

import java.util.*;

class ListIteratorTest {

public static void main(String []args) { String palStr = "abcba";

List<Character> palindrome = new LinkedList<Character>();

for(char ch : palStr.toCharArray()) palindrome.add(ch);

System.out.println("Input string is: " + palStr); ListIterator<Character> iterator = palindrome.listIterator();

ListIterator<Character> revIterator = palindrome.listIterator (palindrome.size());

boolean result = true;

while(revIterator.hasPrevious() && iterator.hasNext()) { if(iterator.next() != revIterator.previous()){

result = false; break;

}

}

if (result)

System.out.print("Input string is a palindrome");

else

System.out.print("Input string is not a palindrome");

}

}

It prints the following:

Input string is: abcba

Input string is a palindrome

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