CS100J       Lab 04. Strings and wrapper classes    Spring 2005

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Section time _______________        Section instructor ___________________

An object of class String contains a list of characters.  The values "Java is fun.", "box", and "a%8ju& !" could each be stored in a String variable. Section 5.2 of the class text is a good reference on class String; refer to it often. You are expected to study it!!!!

Index of a character in a string

A String object (manila folder) associates a number, called its index, with each character in its list. 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:

s       J a v a   i s   f u n  .

index   0 1 2 3 4 5 6 7 8 9 10 11

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

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.

Open a web browser and look at the API specification for the class String.  The URL for the Java API is http://java.sun.com/j2se/1.4.2/docs/api/ and in the left lower pane you should scroll down to class "String" and follow the link.  Ask your TA or a consultant for help if you can't bring up the correct page.  As you do the exercise below, look at the API specification for the method you are using and read its description.

Type the following expressions into the interactions pane and write down their values.

Write down your answer to the following questions:

What value does method indexOf return if the input argument does not occur in the string?

What does method indexOf return if the input argument occurs more than once in the string?

Is method indexOf a function or a procedure?

Type the following expression into the interactions pane and note the result: s.length

Is length a method or a field?

Is the length of a string the same as the index of its last character?

If a string has length 10, what is the index of its first character?  What is the index of its last character?

Suppose that String variable sentence has been given a value (it contains some sequence of characters).  Write an expression in the interactions pane and then in the space below that would evaluate to true if sentence has more than one letter 'a' in it and would evaluate to false otherwise.  (Hint: use two of the methods in the table above.)

We look at three more string methods.  Keep your web browser handy and read the description of the following methods in the API for class String.

Type the following expressions in the interactions pane and write down their values.

What is the value of a call s1.compareTo(s2) when string s1 appears before argument s2 in a dictionary?  (positive or negative integer)  What about when s1 and s2 contain the same sequence of characters?

Finally, what do you think functions toLowerCase and toUpperCase do? Type these expressions into the interaction pane and copy their values onto this page:

"aBcDe.,$".toLowerCase()

"aBcDe.,$".toUpperCase()

The 'wrapper classes' Integer, Boolean, and Character

After the lab, STUDY SECTION 5.1 OF THE TEXT!

Primitive types and class-types String are different, and one can't use one in a place that requires the other. It would be nice to be able to use values of primitive types as if they were in objects (manilla folders). Java provides "wrapper classes" for this purpose. A manilla folder of class Integer has one field, of type int, which obviously contains an integer. Class Integer is called a "wrapper class", because a folder of that class "wraps around" the int value, much like you wrap a sandwich in saran wrap. So, if you want to use the value 52 as an object, then use this expression:

new Integer(52)

which creates a manilla folder of class Integer and puts the int value 52 in it.

In Java, each primitive type has a corresponding wrapper class. They are discussed in Chapter 5 of the text.

Look at the API for wrapper classes Integer, Boolean, and Character as we use their methods. Read the description of each method used below.

Type the following into the interactions pane and write down the result.  Write "error" if you get an error message.

Many classes have a method compareTo, and they all "work" the same way (that is, they all follow the same conventions of when to return a positive integer, negative integer, or zero).  Type the results of the following, or "Error".