CS1110       Lab 04. Writing functions    Fall 2010

Name _____________________                       NetId __________

The purpose of this lab is to give you practice with developing the bodies of methods. At the same time, this lab will give you practice with Strings. We also introduce you to the equality comparison operator == and its counterpart, function equals. After this lab, study Section 5.2 of the text, beginning on page 175.

A String object (instance, or folder) associates a number with each character in its list of characters. The number is called the index or position of the character. Type the following line into the Interactions pane of Dr. Java:

String s= "Java is fun.";

String object s now contains the list of characters "Java is fun.". The index of each character is shown below:

index   0 1 2 3 4 5 6 7 8 9 10 11
s       J a v a   i s   f u n  .

Note that the index of the first character is 0 (not 1), and that the period and each of the space characters between each of the words each have an index.

In the string "I will study every day.", what is the index of the character 'w'? How about the last space character?  Write down your answers:

A list of some functions that appear in each String object is given at the end of this handout. Refer to it when doing this lab. (There are more, which you can find in the specification of class String in the API package; we'll show you how to look at these later.) Note also that if a string s contains only digits (not even blanks), then the function call

Integer.parseInt(s)

yields the integer represented by s. For example, Integer.parseInt("345") is 345.

Important point about Equality

Symbol == is used for equality testing. You know that 2+3 == 5 has the value true. Importantly, when x and y are of the same class-type, the test x == y is made on the names (on the tabs) of the object. Therefore:

new C(args) == new C(args)   is always false because two objects, with different names, are created.

Evaluate the following expressions in the Interactions pane and write down their values. In the third one, for each occurrence of "ab", note that evaluation in the Interactions pane creates a new manilla folder of class String.

new String("ab") == new String("ab")      value:
new Integer(5) == new Integer(5)             value:
"ab" == "ab"                                                      value:

Method Object, the superest class of them all, has a boolean function equals(Object), which in class Object is defined to work exactly like ==. Because Object has function equals, each class can override function equals, and the convention is to define equals to test for the equality of all the fields in two objects. For example, classes String and Integer override this method. To see this, try the following in the Interactions pane and write down their values:

(new String("ab")).equals("ab")                          value:
(new Integer(5)).equals(new Integer(5))         value:
"ab".equals("ab")                                                      value:

You need to understand this distinction between == and function equals for the first prelim. Read about equality of strings on page 179 and equality testing on page 118.

Writing methods that deal with strings

File Methods.java contains specifications for a bunch of functions for you to write (you can also download them from the course webpage). Put the file in its own directory —always put separate projects in separate directories. The function bodies have "stub" return statements so that the class will compile. Write the bodies of as many of them as you can in this lab. You probably won't finish them. We hope that you will finish THREE of them during the lab —show them to your lab instructor at the end of the lab. How many of the others you do is up to you. The more you practice, the easier developing such programs will become.

The methods will, among other things, change a time in a String into a different format. The time comes in four formats:

What to do

First, open file Methods.java in DrJava.

Second, create a JUnit test class, as usual.

Third, for each function in class Methods, in turn, do the following:

1. Think about what test cases would be necessary for you to know that the method is correct. Create a testX method in class MethodTester and insert those test cases.

2. Write the body of the method.

3. Test the method.

When you are finished writing and testing (THREE of) the functions given below, show them to your lab instructor. We suggest that you save both .java files that you created on a USB storage key or else email them to yourself. If you do not have time to finish three in the allotted time, then show the completed lab to your instructor the next week.

Advice: If you need help, ask the TA or a consultant. Don't waste time! Some pondering is necessary, but don't overdo it.

Guidelines: Always keep your program indented properly, and don't let lines get too long. If horizontal scrolling is necessary, then split the line so that it is not necessary. Everything should be readable.

Functions available in class String