CSCI-UA 102 (Data Structures)

Lab 5: Iterating Over Things and the for-each Loop

Introduction

You might have used a for-each loop in some of your programs. You might have wondered how it works. Iterating over collections of items is something that we will do not only for lists and linear structures, but for all other data structures as well.

Let’s start with the following program:

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import java.util.ArrayList;

public class ForEach {
  public static void main (String [] args ) {
    ArrayList<String> list = new ArrayList<String>();


    list.add("Warsaw");
    list.add("Venice");
    list.add("Atlanta");
    list.add("New York");
    list.add("Rome");


    for ( String city : list )
      System.out.println(city);


    //add duplicates
    for (String city : list )
      list.add(city);


    //remove cities starting with "W"
    for ( String city : list )
      if ( city.startsWith("W") )
        list.remove (city) ;
  }
}

What do you think it does? Try to figure it out before attempting to compile it and run it.

The above code uses the for-each loop in three places. It is a handy tool, but (as you might have discovered when trying to run the above code) it does not always work.

These loops are based on iterators and you need to understand a bit more about iterators before you understand why only one of the above for loops works without causing a runtime error.

Part 1

For this part use the documentation as well as the source code for Java that you downloaded a couple of weeks ago.

  1. Look at the documentation page for ArrayList class. List all interfaces that this class implements.
  2. Look at the documentation of the Iterable<E> interface https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Iterable.html. List all the methods that are required by that interface. Indicate the ones that are provided by the ArrayList class directly (not the ones that may be inherited from other classes).

Part 2 Iterable<E> and Iterator<E>

One of the methods required by the Iterable<E> interface is called iterator() - it returns an instance of Iterator<E>. Look at the documentation for Iterator<E>, https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/Iterator.html and answer the following questions.

  1. Is Iterator<E> a class or an interface?
  2. List all the methods from Iterator<E>.
  3. Some of the methods have the default keyword listed next to their name. Research what that keyword does/means and briefly explain it.
  4. What is the purpose of the next() method? In what situations might it throw an exception?
  5. What is the purpose of the hasNext() method?

Part 3 Implementing the Iterable<E> Interface

Look at the source code for ArrayList class. Since it is implementing the Iterable<E> interface, it should have a method called iterator(). That method should be returning an instance of an Iterator<E>. Look for the class associated with that instance. Answer the following questions based on the source code that you are looking at.

  1. What is the name of the class that represents the object returned by the iterator() method of the ArrayList<E> class? HINT: you need to look at the source code to see the type of the object that is actually returned.
  2. Where is that class implemented (specify the file name and the line number)? What is the relationship between that class and the ArrayList<E> class?
  3. Look at the cursor data field in that class. In your own words, describe its purpose.
  4. Look at the next() method of that class. Ignoring the checkForComodification() function call, figure out what the function does and describe the steps.

  5. One of the lines in the next() method is

     Object[] elementData = ArrayList.this.elementData;

    What does this line do? What do you think ArrayList.this refers to? Could we replace it with this?

After reading the documentation and answering the questions in Parts 1-3, go back to the code example in the Introduction. Try to figure out why it does not work.

Part 4 Making a linked list iterable

Go to Ed Workspaces.

Part 5 What will these code fragments do?

Assume that a list is defined as follows

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    List<String> list = new ArrayList<String>();

    list.add("Warsaw");
    list.add("Venice");
    list.add("Atlanta");
    list.add("New York");
    list.add("Rome");
    list.add("Boston");
    list.add("Alberta");

Try to determine the answer to each question without executing the code. Confirm your results by running the code.

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  Iterator<String> itrA = list.iterator();

  while (itrA.hasNext() ) {
    System.out.println(itrA.next());
  }

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  Iterator<String> itrA = list.iterator();

  while (itrA.hasNext() ) {
    System.out.println(itrA.next() );
    System.out.println(itrA.next() );
  }

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  Iterator<String> itrA = list.iterator();
  Iterator<String> itrB = list.iterator();

  if (itrA.hasNext())   System.out.println(itrA.next());
  if (itrA.hasNext())   System.out.println(itrA.next());
  if (itrA.hasNext())   System.out.println(itrA.next());
  if (itrA.hasNext())   System.out.println(itrA.next());
  if (itrA.hasNext())   System.out.println(itrA.next());

  if (itrB.hasNext())   System.out.println(itrB.next());
  if (itrB.hasNext())   System.out.println(itrB.next());

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  Iterator<String> itrA = list.iterator();

  list.add(0, "Dublin");

  while (itrA.hasNext() ) {
    System.out.println(itrA.next());
  }

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  Iterator<String> itrA = list.iterator();

  Collections.sort(list);
  Iterator<String> itrB = list.iterator();

  if (itrA.hasNext())   System.out.println(itrA.next());
  if (itrA.hasNext())   System.out.println(itrA.next());

  if (itrB.hasNext())   System.out.println(itrB.next());
  if (itrB.hasNext())   System.out.println(itrB.next());