University of North Carolina at Asheville

CSCI 202: Introduction to Data Structures

Lab 02: Using Arrays in Java

[Introduction]   [Review of Arrays]   [Lab Project: Part 1]   [Lab Project: Part 2]   [Lab Project: Part 3]   [What To Submit]


In this lab you will use the NetBeans IDE to build and run short programs that demonstrate features of array processing in Java. In addition to a Main class partially written for you by NetBeans, you will be provided with an incomplete (and bug-ridden) source file for a class named ArrayTester. This is a so-called "container" class, which stores an array as its member data and contains methods that process the array in various ways. You must first revise and debug this file to make this class compile and run correctly. You will also be asked to add new functionality to the ArrayTester class definition, in the form of new methods.

Although the main theme of this lab project is array processing, you will also gain more experience working with Java control statements. These will include both conditional structures, such as if-else statements, and iteration structures such as for loops.

And if all that isn't enough, you will also be given a chance to work a bit with two-dimensional (2D) arrays.

Review of Arrays in Java

Note:This section contains review material only. If you know all about arrays in Java already, you can skip this section and go straight to Lab Project, Part I.

  1. Overview:

    Arrays are a fundamental component of most high-level languages because it is usually not practical to declare separate variables for each piece of data in programs which manage large amounts of information. An array may be defined as a programming language construct that allows us to organize data as a list of values. Arrays allow us to declare one variable that can hold multiple values or components, and access any desired component by its position or index in the array.

  2. Array Syntax:

    All values stored in an array must be of the same data type. In the examples below, the variable height is declared to be an array of integers:

      int [] height;       // Preferred Java syntax
      int height [];       // C/C++ style syntax... allowed, but not preferred

  3. Array Instantiation:

    A Java array is an object, and the name of an array is a reference to the object. Memory space for the actual array components must be reserved by instantiating the array, normally by using the new operator. Thus the statements above merely declare the reference height to an integer array, not the array itself. We may then instantiate the actual array as follows:

      height = new int [3];         // 3 components allocated
      height = { 4, 3, 7 };         // new not required here

    Of course we could also have created both the reference and the actual array in a single statement, as shown below:

      int [] height = new int [3];  // 3 components allocated
      int [] height = { 4, 3, 7 };  // new not required here

    Note that we must specify the size of the array so the computer can allocate the correct amount of memory space.

    Another important thing to remember is that the square brackets used to indicate the index of an array are treated like an operator in Java. It is called the index operator ([]), and in fact has the highest precedence of all Java operators. The index operator performs automatic bounds checking, which ensures the index is in range for the array being referenced. Whenever a reference to an array component is made, the index must be greater than or equal to zero and less than the size of the array. If the reference is in the valid range of indexes for the array (0 to 9 for the example below) the reference is carried out. If the index is not valid, an exception called ArrayIndexOutofBoundsException is thrown.

  4. Array Length:

    The size of an array is stored in a public instance field called length. For example, the size of the array height may be referenced as


    The length is set when the array is first created and cannot be changed. This means that once an array is declared, the values at each position may change but the number of values the array can hold is fixed.

  5. Array Indexing:

    The number corresponding to each position in the array is called the index. Array indexes may be integers, integer variables, integer expressions, or some combination thereof as long as the result evaluates to an integer. A Java array of size N is indexed from 0 to N-1. This means the index in Java always starts at the value 0 and if you want to store 10 elements in an array, they would be stored at index positions 0 through 9. Likewise the value stored at index 5 is actually the sixth value in the array. To access a value stored in an array, you use the name of the array followed by the index in square brackets. Be sure to never confuse the value of the index with the value stored in the array at that index.

      height [2] = 72;
      int last_height = height [10];
      average = (height[0]+height[1]+height[2])/3;
      System.out.println("The middle value is " + height[MAX/2]);
      pick = height[(int)Math.random()*11];

  6. Arrays as Parameters:

    A component of an array, as well as an entire array, can be passed as a parameter to a method.

    If the parameter being passed is a component - If the component is a primitive type, a copy of the value is passed. If the component is a reference to an object, a copy of the reference is passed.

    If the parameter being passed is an entire array - Because an array is an object, when an entire array is passed as a parameter, a copy of the reference to the original array is passed. The method can change a component of the array permanently because it is refering to the original component value. The method cannot permanently change the reference itself because a copy of the original reference is sent to the method. These rules are consistent with the rules which govern any object type.

      public static void main (String [] args) {
        int [] array = { 3, 1, 2 };
        printAll (array);
      public static void printAll (int [] array) {
        for (int i = 0; i < array.length; i++) {
          System.out.print (array[i] + " ");
    What this means is the impact of changes made to a parameter inside a method depends on the type of be careful!.
  7. Arrays and Control Statements

    Programmers commonly use for loops when handling arrays because, as discussed earlier, the number of components in an array is constant. In the example below, the for loop declares an an integer loop counter which is initialized and incremented in a step-wise fashion up to a value which is less than the size of the array. During each iteration the for loop executes the code between the braces which may be used to calculate, modify, or do whatever you like to the values of the array. In this example, the sum of all of the array components are added together:

      public int findSum () {
        int sum = 0;
        for (int i=0; i < array.length; i++) //don't confuse commas and semi-colons!
          sum += array [i];  // Same as: sum = sum + array [i];
        return sum;
  8. 2D and Higher-Dimensional Arrays

    Java treats two-dimensional (2D) arrays as arrays of arrays. Think of a 2D array as a 1D array, each of whose values is itself a 1D array of some component type (primitive or object). Thus the declaration

    int [][] matrix = new int [3][5];
    allocates a 1D array of length 3 (which you may think of as the number of rows in what follows here), each of whose components is itself a reference to an array of integers. Each of these arrays (a separate one for each row) has length 5, which you should regard as the number of columns). The variable name matrix is a reference to the entire array of arrays.

    Typically you will see nested loops used in conjunction with 2D array processing. For example, the following code prints the values of all components of matrix, using a table-like format:

        for (int row = 0; row < 3; row++) {
          for (int col = 0; col < 5; col++) {
            System.out.print (matrix[row][col] + "\t");

Lab Project: Part 1

The purpose of this section is to give you practice finding and fixing syntax errors, runtime errors, and logic errors in a Java console application involving a one-dimensional array.

  1. Login to your home directory. If you have not already done so in your previous lab, create a path csci/202/labs. If necessary, get help from your instructor for this step. The NetBeans project folder you will create in the next step should be a subfolder of csci/202/labs, so this path needs to be in your account before you proceed.
  2. Launch the NetBeans IDE and select the File/New Project... option in the toplevel menu bar. A dialog box will appear as shown below:

    In the Categories and Projects panes shown above, select Java and Java Application respectively and then click Next. This raises a second dialog, as shown below:

    In this dialog, enter Lab02 as the Project Name and set the Project Location to csci/202/labs. Otherwise just accept all the default settings and click Finish, so that NetBeans will automatically create a class named lab02.Main for you. In a later step you will add statements to the main() method of this class, but for now just leave it as is.

    Now go to the NetBeans explorer window and select the Files tab to view the directory structure that NetBeans has created to hold your new project. Note the file named Lab02/src/lab02/ As you should expect, this file contains the source code for lab02.Main.

  3. Use your browser to download the java source file and save it in the same directory that contains, namely Lab02/src/lab02.
  4. Attempt to compile in its original form. You should not expect to succeed...
  5. ...So by now you should be staring at a lot of error messages in the output window, which should look something like the following:

    You will need to decipher these messages and identify the syntax errors (aka compiler errors) in the file. One purpose of this exercise is to make you familiar with some of the messages that the Java compiler generates for syntax errors. As is usually the case, all the syntax errors you will find in this file can be identified and corrected without even knowing what the code is actually supposed to do (more on that later). But you will have to fix all the syntax errors before the compiler will generate a Java bytecode (.class) file for you.

  6. Proceed to correct each syntax error, starting with the first one reported by the compiler. Work with your lab instructor as necessary to learn some troubleshooting techniques. But for starters, note that if you click on a reported compilation error in the output window, NetBeans will take you to the line of code containing the error in the edit window.
  7. Once you have resolved all the syntax errors and successfully compiled the ArrayTester class, you should see a display like the following:

    Now switch back to the source file Copy the following lines of code into its (initially empty) main() method:

    	ArrayTester tester;
            if (args.length == 0) {
              tester = new ArrayTester();
            else {
              int [] inputArray = new int [args.length];
              for (int i = 0; i < args.length; i++) {
                inputArray[i] = Integer.parseInt (args[i]);
              tester = new ArrayTester(inputArray);
    	System.out.println ("Sum:     " + tester.sum());
    	System.out.println ("Average: " + tester.average());

    These lines will attempt to create an ArrayTester object and invoke its methods. After you have added these lines, try to Build the project.

  8. Before you attempt to run the project, look over the statements you added in You should be able to see that when this application runs, it should correctly calculate and display the following values in the output window of the NetBeans IDE:

    1. The number of components in the array.
    2. The components in the array.
    3. The sum of all the components in the array.
    4. The real-valued average of all the components in the array (NOTE: this should keep any fractional part).

    Now attempt to Run the project. Unless you have been very alert and fixed more than just the syntax errors, you may end up staring at the following results:

    This is because the source code may still contain one or more runtime errors. A program with only runtime errors compiles successfully, perhaps even without generating any warning messages. However, when you attempt to run it, at some point it "blows up", meaning that the Java interpreter (JVM)detects some problem in the code that it cannot abide and terminates your program abnormally. In such cases the JVM should display an error message indicating why (and where) it got so upset with your code.

    OK... so if you see a screen like the one above, fix the problem(s). Hopefully, you will eventually end up seeing results like the following:

    Unfortunately, you may still have errors in your program, of the type called logic errors. In this case your program runs to completion without upsetting the JVM, but it just does not produce all the results exactly as it should. These can be the most aggravating errors to track down, so of course you should expect to encounter at least one of them here...

  9. Proceed now to fix any logic errors in this program, so that it compiles and executes correctly. You may have to repeat the process of debugging, rewriting code, and recompiling several times to find and fix all of the errors.
  10. When you think everything is ok, demonstrate your working program to your lab instructor.

Lab Project: Part 2

  1. When you finally have a working version of this application, add the following new instance methods to the ArrayTester class:

    public int findMin () Returns the minimum value stored in the array
    public int findMax () Returns the maximum value stored in the array
    public int countOccurrences (int target) Returns the number of occurrences of the value target in the array (0 if none found)

    In each of these cases, it is your job to devise the algorithm for performing the stated task. Of course you should ask your lab instructor for help with implementation details, and even broad hints about the design, but you will have to develop the basic code for these methods yourself.

  2. Finally, in the main() method of class lab02.Main, add statements that invoke each of the methods you just added and allow you to check out their behavior. You should have no problems invoking findMin() and findMax(), but you may want to get help from your lab instructor to make a reasonable test for countOccurrences().
  3. Demonstrate your working program to your lab instructor.

Lab Project: Part 3

This last phase of your lab is intended to give you a little practice with 2D arrays.
  1. Download the source file and add it to your Lab02 project. The class defined by this file creates a 2D array of int values, called matrix. You can visualize this array as a table or "matrix", having 3 rows and 5 columns. The constructor initializes all the matrix components to random integers.
  2. You can compile and run this file as is, but it is not quite complete. In fact it has two stubbed methods, named rowSum() and colSum(). The first of these is supposed to return a 1D array whose length matches the number of rows in matrix. Each component of this array is supposed to contain the sum of all the column values for the corresponding row. Similarly, the second method is supposed to return a 1D array of the sums for each column.

    So, guess what... your job is to fill in each method so that both of them do what they are supposed to do.

  3. Demonstrate your working version of MatrixTester to your lab instructor.

What To Submit

When your project is complete and working correctly, be sure to demonstrate it to your lab instructor. Then, before you exit NetBeans, clean your Lab02 project. This step removes all the .class files, but leaves all your project sources (.java files) intact. Finally, before you logout, open a terminal window and use the cd command to enter your csci/202/labs directory. Then create a JAR file of your Lab02 project folder, using the command

jar cf Lab02Project.jar Lab02
Please leave Lab02Project.jar in your csci/202/labs directory for the remainder of the semester.

Last revised 2 September 2008, 9:50 am