Lab 12: Text Prediction


In this lab, we will implement a trie for storing strings and use it to predict words as they are being typed.


  • Download the skeleton for this project.


Modern smartphones include keyboard assistants that will suggest words while you are typing. There are many data structures that can be used to assist in this interaction, to predict the remainder of words, and autocorrect typed words. One such data structure is a trie, which divides the words into their component characters, and then stores the words in such a way that words with a shared prefix share a part of the tree.

There are two main components to the model found in the skeleton project: a Trie to store the tree of strings, and a SortedArrayMap to map characters to child Tries. SortedArrayMap in turn is implemented in terms of a SortedArray class.

You will only have to write code in the Trie class, but you will need to also look at the other classes to see what methods are available. Methods that remain to be implemented in the Trie class have been marked with TODO for easy identification.

Step 1: size

Implement the size method. It should essentially count how many Trie nodes have isMember set to true. This can be done recursively:

  • If isMember is true, count 1.
  • Add up the result of calling size recursively on all children.

To iterate over all children, you can use a foreach loop, which depends on the iterator() method from SortedArrayMap.

Step 2: find

Implement find, a helper method you may find useful later. It follows an entire String through the trie, taking one step down per character. It returns a stack of Trie nodes that trace the path that corresponds to the prefix, pushing the Tries onto the stack as it descends. If the prefix is not present, the stack will contain Tries for all of the leading characters from the prefix that are present. The stack is implemented using an object of the ArrayDeque class. It does not matter whether isMember is set or not.

You might find the method getChildWith helpful as you write this method.

Step 3: contains, add, remove

The implementations of all three of these methods will be much simpler if find is employed as a helper method.

  • Implement contains, which tests whether a given string is contained in the trie (note “contained” means “isMember is set”).

  • Implement add, which adds a new word to the trie.

  • Implement a simple version of remove, which simply finds the end of the word to remove and then sets isMember to false. This will pass all the tests but leaves lots of useless Trie nodes lying around; you will fix this in Step 5.

Step 4: inorder, successorsTo

Implement inorder and successorsTo. See the comments in the code for descriptions of what they should do.

The implementation of successorsTo will be much simpler if find is employed as a helper method.

Step 5: better remove

Implement a better version of remove, which actually deletes any unneeded Trie nodes. The stack of nodes returned by find will be helpful in this task.

What to Hand In

Submit your implementation via Teams.


  • To Partially Complete this lab, complete Steps 1, 2, and 3.
  • To Complete this lab, do the above and complete Steps 4 and 5.