Strings & StringList Methods

Today's agenda:

Overview of Java 's String class and some of its methods.
Introduction to StringList class ( StringList class contract)
Practice writing StringList methods.

Java class `String`

Please bring up the on-line Java documentation page on your browser, and find the String class, under "All Classes", in the lower-left frame. Part of our work today, is to start getting familiar with the Java documentation, and locating in there information we need.

Think of strings as letter beads on a cord (see picture above). Java strings are used to store words, sentences, and any other text. Strings consists of characters, such as letters (upper case and lower case letters are considered different characters), digits, punctuation marks (examples: .!,?), other symbols (+.-, etc.), spaces, tabs, and such.

To declare a string, one must use a class name String (as expected):
String myName;
String greeting;

String constants are written in double quotes. We can assigne a string literal to a String variable, as in:

    myName = "Elmo"; //"myName" has been declaired above
    greeting = "Hi there, it's great to see you!"; //"greeting" has been declaired above
    String myDateOfBirth = "4/1/88";
    String empty = ""; // this string has no characters

Take a look at the Java documentation for the String class. How many constructors do you see there?

Using the `\` (backslash)

A backslash in a string tells the Java compiler to interpret the symbol that follows it literally. What does that mean? Let's take an example. Note that string values are included in double quotes. What if we need to include a double quote in a string? Writing it directly will confuse the compiler: In the example below, the compiler interprets the double quote before the word Hi! as the closing quote for the string that begins with She said .

    String whatIsIt = "She said "Hi!""; //Compiler error!!! :-(

In order to define a string with the text She said "Hi!", you need to escape the embedded quotation marks by putting backslashes before them:

    String muchBetter = "She said \"Hi!\""; //Compiler is happy now! :-)

String Concatenation

You combine two strings with the + operator. The result is a string that puts the text of the second string right after the text of the first one (it's called string concatenation). For instance:

    String wish = "Happy " + "birthday!";

The string wish has the text Happy birthday! in it. Note the space after Happy in the first string.

Concatenation isn't only for constants:

    String word1 = "Happy ";
    String word2 = "birthday!";
    String wish = word1 + word2; //value of "wish" is: "Happy birthday!"

Some String Methods

Here are some of the String methods, relevant to today's lab. (For each one of them, make sure to check the Java on-line docmentation):

public String toUpperCase()
When applied to a string, toUpperCase() converts all of the characters in this String to upper case. See the following example:
```
      String str1 = "We are HeRe!";
      String str2 = str1.toUpperCase(); //str2 has the value of: WE ARE HERE!
      
```
There is also toLowerCase(), but we will not be using it today.

public boolean equals(String str)

This method compares this string (the receiver of the equals message) to the one passed as parameter. It returns true iff the two strings represent the same sequence of characters. Examples:

     String june = "June";
     String july = "July";
     boolean b1 = june.equals(july); // b1 is false
     boolean b2 = june.equals("June"); // b2 is true
     boolean b3 = july.equals("July"); // b3 is true
     boolean b4 = july.equals("july"); // b4 is false.  Why?

public int indexOf(String str)
If the string given as a parameter occurs within the string object that receives the indexOf() message, then the result is the index (i.e. the position) of the first occurrence of the parameter string. If the parameter does not occur in the given the string, the method returns -1. (Notice that the index of the first character in the string is 0.)
Here are some examples:
```
     String str1 = "coconut";
     String str2 = "nut";
     int n1 = str1.indexOf("co"); // n1 is 0
     int n2 = str1.indexOf(str2); // n2 is 4
     int n3 = str2.indexOf("co"); // n3 is -1
     
```
public int compareTo(String str)
It works as follows: Let s1 and s2 be two strings. Then s1.compareTo(s2)
- returns a value less than zero if s1 comes before s2 in alphabetical order;
- returns 0 if s1 and s2 are equal strings;
- returns a value more than zero if s1 comes after s2 in alphabetical order.
For example:
- "cat".compareTo("dog") returns a negative value;
- "cat".compareTo("cat") returns 0;
- "cat".compareTo("ant") returns a positive value.
Make sure to look at the on-line documentation for this method as well!

`StringList` class

In lecture, you were introduced to the class IntList, wich is an "in- house" developed Java class. Today, rather than using that class, we will be using an other "in-house" developed class, called StringList. (You will use it also in your next pset.) Here is the contarct to the StringList class. Notice how similar the contracts for those two classes are!

Lab exercises

Begin this assignment by downloading the folder lab9_programs from the cs111d account. Your task is to write several class methods that manipulate string lists. You should write the definitions of each of these class methods in the class LabOps within the file LabOps.java. To test your class methods, you should add testing statements to the main method in the LabOps class.

Helpful hints:

Write your methods one-by-one.
When you have written a method, write several test statements in main to make sure that your method works in all cases.
Don't forget to test your methods on empty lists (if applicable).

Here are the methods that you need to write (5 have red check marks):

mapPluralize ()
It takes a list of nouns, and returns a list with the plurals of each of the nouns.
The nouns are pluralized by adding the suffix "s". For instance, the plural of "cat" is "cats" and the plural of "mouse" is "mouses" (this simple-minded pluralizer doesn't know about special cases like "mice").
Examples:
mapPluralize(fromString("[dog,cat,mouse]")) returns the string list [dogs,cats,mouses].
mapPluralize(fromString("[]")) returns the string list [].
mapUpperCase ()
It takes a list of strings and returns a list where every string in the original list is in to upper case.
Examples:
mapUpperCase(fromString("[dog,cat,mouse]")) returns the string list [DOG,CAT,MOUSE].
mapUpperCase(fromString("[]")) returns the string list [].
Note: Use the toUpperCase() instance method from the String class to convert individual strings.
isMember (String s, StringList L)
It takes a string and a string list. It returns true if the given string is an element of the given list, false otherwise.
Examples:
isMember("cat", fromString("[dog,cat,goat]")) returns true.
isMember("tiger", fromString("[dog,cat,goat]")) returns false.
isMember("at", fromString("[dog,cat,goat]")) returns false.
isMember("dog", fromString("[]")) returns false.
isMember("", fromString("[dog,cat,goat]")) returns false.
Note: Make sure that the method handles the empty list!
explode (String s)
It takes a string and returns a list of single-character strings formed out of the characters of the given string s.
Examples:
explode("computer") returns the string list [c,o,m,p,u,t,e,r].
explode("") returns the string list [].
Notes: The LabOps class contains two helper methods that are useful for defining explode:
1. public static String first (String s)
  Returns a string consisting of the first character of s. For example, first("computer") returns the string "c".
2. public static String butFirst (String s)
  Returns a string consisting of all but the first character of s. For example, butFirst("computer") returns the string "omputer".
String implode (StringList L)
It takes a string list and returns a string that is the result of concatenating all the strings in the input list in order.
Examples:
implode(fromString("[dog,cat,mouse]")) returns the string "dogcatmouse".
implode(fromString("[lists, are, fun]")) returns the string "lists are fun".
implode(fromString("[]")) returns the string "".
filterMatches ()
It takes a string and a string list, and returns the list of strings in the input list that contain the input string as a substring.
Examples:
filterMatches("com", fromString("[computer,program,incomparable,intercom,Java]")) returns the string list [computer,incomparable,intercom]
filterMatches("com", fromString("[cat,dog,mouse]")) returns the string list []
filterMatches("com", fromString("[]")) returns the string list []
Note: To test if one string is a substring of another, you should use the indexOf() instance method from the String class.
Here are a few more excersises, if you have time:
public static StringList insert (String s, StringList L)
Assume that L is an n-element list of strings sorted in alphabetical order. Returns a new (n+1)-element list of alphabetically sorted strings that contains s in addition to all the elements of L.
Examples:
insert("dog", fromString("[cat,goat,lion]")) returns the string list [cat,dog,goat,lion].
insert("ant", fromString("[cat,goat,lion]")) returns the string list [ant,cat,goat,lion].
insert("tiger", fromString("[cat,goat,lion]")) returns the string list [cat,goat,lion,tiger].
insert("goat", fromString("[cat,goat,lion]")) returns the string list [cat,goat,goat,lion].
Note: The compareTo() instance method of the String class should be useful here.
public static StringList insertionSort (StringList L)
Returns a new list that contains all the elements of L in alphabetical order.
Examples:
insertionSort(fromString("[lion,dog,cat,ant,goat]")) returns the string list [ant,cat,dog,goat,lion].
insertionSort(fromString("[Time,flies,like,a,banana]")) returns the string list [Time,a,banana,flies,like].
insertionSort(fromString("[]")) returns the string list [].
Notes:
- Capitalized words (such as "Time") are considered to alphabetically precede uncapitalized words (such as "banana").
- Your insertionSort method should work by using the insert method you defined above to insert the head of the list into the result of recursively sorting the tail of the list.

Wedn, Oct 31