Category Archives: Java

Size of collection in a JSP/DSP page

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leaf pile on Flickr

(Photo: leaf pile by oeimah)

Sometimes in a JSP/DSP page you will want to get the size of a collection and unless you are within a Range, ForEach or similar droplet you won’t have access to this value.

Struts has a nice solution using the bean:size tag.  JSTL 1.1 has a nice solution using the fn:length function.

Here is an example of how to use Struts, DSPEL and JSTL to get the size of a collection.

  <dspel:getvalueof param="book.pages" var="pages"
                    vartype="java.util.Collection"/>
  <bean:size id="numPages" name="pages"/>
  Number of Pages: <c:out value="${numPages}"/>
  Number of Pages: <dspel:valueof value="${numPages}"/>

Here is an example of how to use JSTL 1.1 and DSPEL to get the size of a collection.

 

  <dspel:getvalueof param="book.pages" var="pages"
                    vartype="java.util.Collection"/>
  Number of Pages: <c:out value="${fn:length(pages)}"/>

Accessing RepositoryItems with JSTL

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Often we are accessing repository items in a JSP page like this.

  <dspel:droplet name="RQLQueryForEach" var="query">
    <dspel:param name="repository" bean="/betweengo/repository/Repository"/>
    <dspel:param name="itemDescriptor" value="Account"/>
    <dspel:param name="queryRQL" value="ALL"/>
    <dspel:setvalue param="account" paramvalue="element"/>
    <dspel:oparam name="output">
      <dspel:valueof param="account.name"/><br/>
    </dspel:oparam>
  </dspel:droplet>

With JSTL we could try to display the name like this.

      <c:out value="${query.account.name}"/>

If you are using JSP 2.0 you can display it even more simply.

      ${query.account.name}

However since account is a RepositoryItem object and there is no get method for the name property (i.e. it’s not a JavaBean object) the above will fail and produce an exception.

To get around this you can extend ATG’s RQLQueryForEach class by overriding the protected setElementParameter. In addition to setting the element parameter in the request you can set a new parameter which we will call “item”. This item is of class atg.beans.DynamicBeanMap and wraps a RepositoryItem.

public class RQLQueryForEachEL extends RQLQueryForEach {
  protected void setElementParameter(DynamoHttpServletRequest pRequest,
                                     String pElementName, Object pValue) {
    super.setElementParameter(pRequest, pElementName, pValue);
    DynamicBeanMap itemBean = new DynamicBeanMap(pValue, true);
    pRequest.setParameter("item", itemBean);
  }
}

Now we can access the repository item with JSTL like this.

      ${query.item.name}

Linked List Interview Questions

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  1. Implement Insert and Delete for a Singly Linked List. 
    public interface Link<E> {
	public Link<E> getNext();
	public void setNext(Link<E> link);

	public Link<E> getPrev();
	public void setPrev(Link<E> link);

	public E getValue();
	public void setValue(E value);
}

public class SingleLink<E> implements Link<E> {

	public SingleLink(E value) { this.value = value; }

	private Link<E> next;
	private E value;

	public Link<E> getNext() { return next; }
	public void setNext(Link<E> next) { this.next = next;}

	public Link<E> getPrev() { throw new UnsupportedOperationException(); }
	public void setPrev(Link<E> prev) { throw new UnsupportedOperationException(); }

	public E getValue() { return value; }
	public void setValue(E value) { this.value = value;}

	@Override
	public boolean equals(Object obj) { return this.value.equals(obj); }

	@Override
	public String toString() { return getValue().toString(); }
}

public abstract class LinkedList<E> {

	protected abstract Link<E> createHead(E value);
	protected abstract Link<E> removeHead();
	protected abstract Link<E> append(Link<E> tail, E value);
	protected abstract void chain(Link<E> from, Link<E> to);
	protected abstract void deleteAfterHead(E value);
	protected abstract boolean isInsert(Link<E> link);
	protected abstract boolean isEnd(Link<E> link);

	public void insert(E value) {

		if (head == null) {
			createHead(value);
			return;
		}

		Link<E> tmp = head;
		while (!isInsert(tmp))
			tmp = tmp.getNext();

		append(tmp, value);
	}

	public void delete(E value) {

		if (head == null)
			return;

		if (head.equals(value)) {
			removeHead();
			return;
		}

		deleteAfterHead(value);
	}

	Link<E> head;
}

public class SinglyLinkedList<E> extends LinkedList<E> {

	@Override
	protected Link<E> createHead(E value) {
		if (value != null) {
			head = new SingleLink<E>(value);
		} else {
			head = null;
		}
		return head;
	}

	@Override
	protected Link<E> removeHead() {
		if (isEnd(head.getNext())) {
			head = null;
			return null;
		}

		head = head.getNext();
		return head;
	}

	@Override
	protected Link<E> append(Link<E> tail, E value) {
		Link<E> link = new SingleLink<E>(value);
		tail.setNext(link);
		return link;
	}

	@Override
	protected void chain(Link<E> from, Link<E> to) {
		from.setNext(to);
	}

	@Override
	protected void deleteAfterHead(E value) {

		Link<E> tmp = head.getNext();
		Link<E> prev = head;
		while (!isEnd(tmp)) {
			if (tmp.equals(value)) {
				chain(prev, tmp.getNext());
				break;
			}
			prev = tmp;
			tmp = tmp.getNext();
		}
	}

	@Override
	protected boolean isInsert(Link<E> link) {
		return isEnd(link.getNext());
	}

	@Override
	protected boolean isEnd(Link<E> link) {
		return link == null;
	}
}
  2. Implement Insert and Delete for a Doubly Linked List. 
    public class DoubleLink<E> extends SingleLink<E> {

	public DoubleLink(E value) { super(value); }

	private Link<E> prev;

	@Override
	public Link<E> getPrev() { return prev; }

	@Override
	public void setPrev(Link<E> prev) { this.prev = prev; }
}

public class DoublyLinkedList<E> extends SinglyLinkedList<E> {

	@Override
	protected Link<E> createHead(E value) {
		if (value != null) {
			head = new DoubleLink<E>(value);
		} else {
			head = null;
		}
		return head;
	}

	@Override
	protected Link<E> removeHead() {
		head = super.removeHead();
		if (head != null) {
			head.setPrev(null);
		}

		return head;
	}

	@Override
	protected Link<E> append(Link<E> tail, E value) {
		Link<E> link = new DoubleLink<E>(value);
		tail.setNext(link);
		link.setPrev(tail);
		return link;
	}

	@Override
	protected void chain(Link<E> from, Link<E> to) {
		from.setNext(to);
		if (to != null) {
			to.setPrev(from);
		}
	}

	@Override
	protected void deleteAfterHead(E value) {

		Link<E> tmp = head.getNext();
		while (!isEnd(tmp)) {
			if (tmp.equals(value)) {
				chain(tmp.getPrev(), tmp.getNext());
				break;
			}
			tmp = tmp.getNext();
		}
	}
}
  3. Implement Insert and Delete for a Sorted Linked List. 
    public class SortedLinkedList<E> extends SinglyLinkedList<E> {

	@Override
	public void insert(E value) {

		if (head == null) {
			createHead(value);
			return;
		}

		Link<E> tmp = head;
		Link<E> prev = null;
		// XXX: yuck, dynamic casting to Comparable
		while (!isEnd(tmp) &&
	Comparable<E>) tmp.getValue()).compareTo(value) < 0) {
			prev = tmp;
			tmp = tmp.getNext();
		}

		Link<E> link = new SingleLink<E>(value);
		if (prev == null) {
			link.setNext(head);
			head = link;
		} else {
			prev.setNext(link);
			link.setNext(tmp);
		}
	}
}
  4. Implement Insert and Delete for a Circular Linked List. 
    public class CircularLinkedList<E> extends DoublyLinkedList<E> {

	@Override
	protected Link<E> createHead(E value) {
		head = super.createHead(value);
		if (head != null) {
			head.setNext(head);
			head.setPrev(head);
		}
		return head;
	}
	
	@Override
	protected Link<E> removeHead() {
		if (head.getNext() == head) {
			head = null;
			return null;
		}
		
		Link<E> prev = head.getPrev();
		head = head.getNext();
		head.setPrev(prev);
		prev.setNext(head);
		return head;
	}		

	@Override
	protected Link<E> append(Link<E> tail, E value) {
		Link<E> link = new DoubleLink<E>(value);
		tail.setNext(link);
		link.setPrev(tail);
		link.setNext(head);
		head.setPrev(link);
		return link;
	}
	
	@Override
	protected boolean isEnd(Link link) {
		return link == head;
	}

	@Override
	protected boolean isInsert(Link link) {
		return link.getNext() == head;
	}
}

Data Structures Interview Questions

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  1. Find the largest int value in an int array (try to not use native functionality like “max()”) 
    	public static int max(int [] array) {
		int max = Integer.MIN_VALUE;
		for (int ii = 0; ii < array.length; ii++) {
			if (array[ii] > max)
				max = array[ii];
			if (max == Integer.MAX_VALUE)
				break;
		}
		return max;
	}
  2. What’s the difference between a stack and a queue?
     
    Stacks are LIFO, i.e. last in, first out. You push things on top of the stack and pop them off the stack.
    Queues are FIFO, i.e. first in, first out. You push things in one side, pop them off the other.
     
  3. Write a function to reverse a string (try to not use native functionality like strrev() or text[::-1]) 
             char * strrev(char * str) {
           int ii = 0, jj = strlen(str) - 1;
           char tmp;
           while (ii < jj) {
             tmp = str[ii];
             str[ii++] = str[jj];
             str[jj--] = tmp;
           }
           return str;
         }
  4. Write a function that determines if two strings are anagrams. From wikipedia: an anagram is “the result of rearranging the letters of a word or phrase to produce a new word or phrase, using all the original letters exactly once.” (“abc” “bac” “cba” are anagrams, “abc” and “bba” are not)
    5. /**
 * Returns 1 if it's an anagram, 0 otherwise.
 */
int anagram(char * str1, char * str2) {

  // quick check
  if (strlen(str1) != strlen(str2)) {
    return 0;
  }

  // copy str2 to our tmp
  char * tmp = malloc(strlen(str2) + 1);
  strcpy(tmp, str2);

  int ii = 0;
  while (str1[ii] != 0) {
    char * ch = strchr (tmp, str1[ii]);

    // if not found return -1 to indicate not an anagram
    if (ch == 0)
      return 0;

    // remove this found letter from second string
    ch[0] = 0;
    if (ch == tmp) {
      tmp = ch + 1;
    } else if (*(ch+1) != 0) {
      tmp = strcat(tmp, ch+1);
	}

    // go to next character
    ii++;
  }

  return 1;
}

Computing Basics Interview Questions

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  1. Use recursion to write a function that determines the length of a string 
    int strlen(char * str) {
  if (str[0] == 0)
    return 0;
  return 1 + strlen(str + 1);
}
  2. What’s the number after F in hexadecimal?0x10
  3. Bitwise: what is 7 | 1? 7 & 1? ~7?

     7 (0111). 1 (0001). -8 (1000).

  4. Write a script that prints the numbers from 1 to 100. But for multiples of four print “Four” instead of the number and for the multiples of seven print “Seven”. For numbers which are multiples of both four and seven print “FourSeven”. 
    for (int ii = 1; ii <= 100; ii++) {
	boolean isMultipleOfFour = (ii % 4 == 0);
	boolean isMultipleOfSeven = (ii % 7 == 0);
	if (isMultipleOfFour && isMultipleOfSeven)
		System.out.println("FourSeven");
 	else if (isMultipleOfFour)
		System.out.println("Four");
 	else if (isMultipleOfSeven)
 		System.out.println("Seven");
	else
		System.out.println(ii);
}

ATG Consulting Interview

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Today I had the most detailed but at same time most interesting ATG consulting interview yet. Here are the questions I was asked with answers when appropriate in italics.

  1. Which versions of ATG have you worked with?
  2. What parts of ATG’s stack have you worked with?
  3. How do you compare ATG with Ruby on Rails?
  4. What is Nucleus?
  5. What is the ATG Repository?
  6. When creating form handlers typically what ATG base class do you extend? GenericFormHandler.java
  7. When creating droplets what ATG base class do you extend? DynamoServlet.java
  8. What form handlers and methods do you use during checkout? ShoppingCartFormHandler, numerous handlers like handleMoveToConfirm, etc.
  9. What does a user typically see during checkout? Add to shopping cart, login, billing and shipping address, payment, confirm, confirmation, email confirmation, shipped email.
  10. How do you compare strings in Java? If String a = “hello” and String b= “hello” what is a == b? True, a and b both reference the same constant string.

In another interview I was asked these questions.

  1. What is HTTP? How does it work?
  2. If HTTP is stateless then how does a web application maintain state.

Debugging a Category’s Bad Child Products

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Today I was using the CategoryLookup droplet to find a category’s child products. However when I accessed the child products I would get this JDBC error.

java.lang.NullPointerException
at java.lang.String.(String.java:166)
at oracle.sql.CharacterSet.AL32UTF8ToString(CharacterSet.java:1517)

I realized my product data was corrupted. However this category had over 70 child products. The stack trace wasn’t telling me which one was corrupt and going through each child product to find out which one was corrupt was too painful.

I first queried the database to find all the child product ID’s.

SQL> select child_prd_id from dcs_cat_chldprd where category_id='cat101' order by sequence_num;

I then created a simple JHTML page which would query each child product and output which ones were corrupted.

<java>
final String [] product_ids = { "prod101", "prod102", "prod103" };

for (int ii = 0; ii < product_ids.length; ii++) {
out.print(ii + ". [" + product_ids[ii] + "] ");
try {
</java>
<droplet name="/atg/commerce/catalog/ProductLookup">
<param name="id" value="`product_ids[ii]`">
<oparam name="output">
<valueof param="element.displayName"/><br/>
</oparam>
</droplet>
<java>
} catch (RuntimeException exc) {
out.println(exc + "<br>");
}
}
</java>

Once I knew which child products were bad I removed their mappings to the category in dcs_cat_chldprd.

SQL> delete from dcs_cat_childprd where child_prd_id = 'prod102' and category_id='cat101';

Then I updated the sequence numbers so that they are all consecutive by moving the ones at the end to fill the holes created by the previous deletes.

SQL> update dcs_cat_childprd set sequence_num = 1 where child_prd_id = 'prod103' and category_id='cat101';

Submitting a form with a radio button

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Submitting a form from a radio button is not common but it is a nice UI which is even better when done with AJAX.

A typical form with two radio buttons would look something like this.

<form action="test.html">
1 <input type="radio" name="test" value ="1"><br>
2 <input type="radio" name="test" value ="2">
<p><input type="submit" value="submit">
</form>
1
2

But with basic JavaScript you can make a simpler form like this.

<script>
function submitAction( form, absPath ) {
form.action = absPath;
form.submit();
}
</script>
<form action="test.html" id="test">
1 <input type="radio" name="test" value ="1" onchange="submitAction(document.getElementById('test'), 'test.html')"><br>
2 <input type="radio" name="test" value ="2" onchange="submitAction(document.getElementById('test'), 'test.html')">
</form>
1
2

If you are using Struts the above form’s JSP would be:

<html:form action="test.do" styleId="test">
1 <input type="radio" name="test" value ="1" onchange="submitAction(document.getElementById('test'), 'test.do')"><br>
2 <input type="radio" name="test" value ="2" onchange="submitAction(document.getElementById('test'), 'test.do')">
</html:form>

Printing out an array

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When printing out an array if you simply do a toString on the array you will get a “fairly useless result such as [C@16f0472”. Consider using Arrays.toString to convert the array into a readable String that gives the contents of the array.” I used to use Arrays.asList followed by toString but Arrays.toString is definitely more efficient and is available as of JDK 1.5.0.

This method is useful when you want to report on the values of an JDK 1.5 enum since the values method returns an array.

The quote is from a FindBugs report.

Accessing JavaBean’s Getters and Setters

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A JavaBeans is simply a Java class with a set of properties, each property which has a getter and/or a setter. For example if the Java class has a name property it could be defined like this.

public class Foo {
  private String mName;
  public String getName() { return mName; }
  public void setName(String pName) { mName = pName; }
}

Typically one accesses the property by directly calling its getter and/or setter. But sometimes you may not know the name of the property in advance but still might want to access the getter and/or setter. To do this one can use Java’s beans and reflection packages.

Here is an example of how one would read the value of a property of a JavaBean. Note that the property name should not be capitalized.

import java.beans.PropertyDescriptor;
import java.lang.reflect.Method;

public Object getProperty(MyBean myBean, String propertyName) {

  // get descriptor of property
  PropertyDescriptor descriptor = new PropertyDescriptor(propertyName, myBean.getClass());

  // invoke read method on property of myBean
  Method readMethod = descriptor.getReadMethod();
  return readMethod.invoke(memberSavingsProfile, (Object []) null);
}

Note that if your property only has a getter then you would create the PropertyDescriptor like this.

// get descriptor of property that only has a getter
String readMethodName = "get" + org.apache.commons.lang.StringUtils.capitalize(propertyName);
String writeMethodName = null;
PropertyDescriptor descriptor = new PropertyDescriptor(propertyName, myBean.getClass(), readMethodName, writeMethodName);