CSC 153 Grinnell College Spring, 2005
Computer Science Fundamentals
Laboratory Exercise

An Introduction to Trees in Java


This lab considers a binary search tree as a specific type of tree and provides practice with how such a tree structure might be implemented in Java.


  1. Review the readings for this lab regarding the basic defintions of general trees and binary search trees.

  2. Which of the following trees is(are) a binary search tree(s)?

Several trees

Implementing Binary Search Trees in Java

The implementation of a binary search tree in Java follows a similar approach to our implementation of lists.

This lab's readings discuss the approaches and implementations for each of these Java classes.

  1. Copy ~walker/java/examples/trees/ and ~walker/java/examples/trees/ to your account. Compile and run the programs to verify they produce the same results as the SchoolDirectory program described in the reading on generalization.

  2. What can you say about the order of the entries printed by the print procedure? Explain why this sequence is obtained.

The discussion of insertion into a binary search tree in the reading for this lab described the insertion of the number 153 into the following tree (which repeats the tree given above).

A BSTree Object
  1. Suppose a similar insert method was used to build the tree in the above example (with numbers 23, 37, 48, 96, 123, 185, 200, 285, and 309 rather than names and entries).

    1. What data do you think would have to be inserted first into the null tree?
    2. What item or items might have been inserted next?
    3. What flexibility might there be in the order of entering data to get the above tree, and what restrictions might apply?

    Explain your answers.

  2. Given the order of insertions in the main method of ~walker/java/examples/trees/, draw a picture of the binary search tree that is produced by that program.

  3. Add an update method to the BSTree class, analogous to the corresponding method for the SchoolDirectory program from the lab on generalization.

    Note: As with the similar problem for the SchoolDirectory, the body of this update method can be just two lines long!

Additional Practice

  1. Write an iterative version of the recursive lookup method.

  2. Use ideas from print to write a printLeaves method, which prints just the leaves within a tree. Here, one can still traverse the full tree -- but printing should occur only if a node has only null left and right subtrees.

  3. Ideas from print also can be used to count the number of (non null) nodes in a tree. Use this approach to write a countNodes method.

Extra Credit:

  1. The height of a tree is the maximum number of levels of nodes within the tree; by convention, the height of a tree with only one node is 0. Thus, the binary search tree shown between parts 4 and 5 above (with root 123) has height 3. Add to the BSTree class a method height which computes the height of a tree.

Work to be turned in:

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created April 18, 2000
last revised March 24, 2005
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