CS1110       Lab 04. Writing functions    Fall 2008

Name _____________________       NetId __________

Section time _______________        Section instructor ___________________

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. This lab will help you prepare for the prelim. We begin with some information on 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. 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  .

The index of the first character is 0 (not 1), and the space character between each of the words and the period 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 (there are more, which you can find in the specification of class String in the API package) is given at the end of this handout. Refer to it when doing this lab. 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 ==code> is used for equality testing. You know that 2+3 == 5 is true. However, when x and y are of the same class-type, the test x == y is made on the names of the folders. Therefore:

new C(args) == new C(args)   is always false because two folders, 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", 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 ==. Therefore, 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

Below, we specify a bunch of methods for you to write. Do 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 much 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

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.

First, create in DrJava a new class Methods and save it appropriately —remember, put all files for a new program in a new folder. To simplify your work, you can download file Methods.java from here or from the course website; it has all the methods in it already. The function bodies have return statements so that the class will compile.

Second, create a JUnit test class, as usual.

Third, for each function given below, in turn, do the following:

1. Write the body of the method. BEFORE YOU WRITE THE BODY, IF THE METHOD DOES NOT HAVE A SUITABLE SPECIFICATION IN A JAVADOC COMMENT, WRITE ONE.

2. 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.

3. Test the method.

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: You should check the javadoc comments (click button javadoc) to make sure they are right. 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.

/** s is a time in the 24-hour-string format; Return the same time in 24-hour-verbose format */
public static String toVerbose(String s) {
    return "";
}

/** s is a time in the 24-hour-string format; Return the same time in AM-PM format. */
public static String ToAMPM(String s) {
    return "";
}

/** s is a time in 24-hour-string format; Return the same time in 24-hour-correct time */
public static String timeToCorrect(String s) {
    return "";
}

/** s is a time in AM-PM-string format; Return the same time in 24-hour-string format.*/
public static String eliminateAMPM(String s) {
    return "";
}

/** s is a time in EITHER the 24-hour-string format OR the AM-PM-string format;
Return the same time in the 24-hour-string format*/
public static String removeAMPM(String s) {
    return "";
}

/** s is a time in the 24-hour-string format;
Return the time as the number of minutes. E.g. "14:35" is 14*60 + 35.*/
public static int timeInMinutes(String s) {
    return 0;
}

/** s is a time in the AM-PM-string format;
Return the time as the number of minutes. E.g. "14:35" is 14*60 + 35.
See if you can write the body as a single return statement. */
public static int AMPMtimeInMinutes(String s) {
    return 0;
}

/** = time s1 < time s2;
Precondition: s1 and s2 are in either 24-hour-string format or AM-PM-string format
See if you can write the body as a single return statement. */
public static boolean isLess(String s1, String s2)
    return false;
}