Welcome to Lesson 8! Learning Objectives By the end of today's class, you should know...
- How do you compare chars and use them in the test condition of an if statement?
- How do you compare Strings and use them in the test condition of an if statement?
Announcements- Midterm 1 - one week from today
- Study over your old quizzes, in-class activities and homework
- Material through today's lesson (Lesson 2 - Lesson 8)
- Quiz 3 after the break
- Lab 5 posted on CodeLab
Review Activity
With a partner, answer the following questions: - Write one line of code to declare a String variable (name of your choice) and assign it the value $.
- Write one line of code to declare a char variable (name of your choice) and assign it the value of $.
- Write a System.out.println statement to print the following message to the console: Sure! I'll do it. <tab> I "love" boring, repetitive tasks!
- Given the following string variable:
String statement = "Life is beautiful"; - Do the following to the String:
- Print out the String in all capitals
- Print out the length of the String
- Print out the first and last character of the String.
- Write an if statement to check if the value stored in the variable age is equal to 5. If so, print out the age
- With a partner, identify and correct the 2 mistakes in the code below:
int guess;
Scanner input = new Scanner(System.in);
System.out.print("Enter a guess: ");
guess = input.nextInt();
if (guess = 7) {
System.out.println(“Correct!”);
} else (guess != 7) {
System.out.println(“Incorrect!”);
}
Comparing Characters and Strings- Character data can be evaluated using relational operators as well.
- Because characters are primitive types, we can use the 6 relational operators to compare them.
- Comparing characters works because Java stores characters as numbers using their Unicode values.
- Note that letters nearer to the start of the alphabet have lower numerical Unicode values
- Thus a numerical comparison can decide the alphabetical order of characters
Example Program Comparing Characters1
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| public static void main(String[] args) {
System.out.println("'A' < 'B': " + ('A' < 'B'));
System.out.println("'A' > 'B': " + ('A' > 'B'));
System.out.println("'A' <= 'Z': " + ('A' <= 'Z'));
System.out.println("'X' >= 'Y': " + ('X' >= 'Y'));
System.out.println("'X' == 'X': " + ('X' == 'X'));
System.out.println("'X' != 'X': " + ('X' != 'X'));
}
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Comparing Strings - Equals and EqualsIgnoreCase Methods
- Strings are not primitive types.
- Therefore, we must use special String methods to compare them.
- We have already seen one String method for comparison: equals
System.out.println("car".equals("cat")); //prints false
- Another useful and related String method is equalsIgnoreCase
System.out.println("car".equalsIgnoreCase("CAR")); //prints true
Comparing Strings - CompareTo and CompareToIgnoreCase Methods - To compare Strings alphabetically, Java provides two methods: compareTo and compareToIgnoreCase
- These methods both return a numerical value to indicate which string came first alphabetically.
- the first string is comes first alphabetically -> negative number as result
- the strings are equal -> result of 0
- the second string comes first alphabetically -> positive number as result
- For example, imagine that we wish to compare "car" and "cat"
- Alphabetically, "car" comes before "cat":
- If we use the compareTo method on these two strings, we will get the following result:
//"car" comes BEFORE "cat":
System.out.println("car".compareTo("cat")); //negative number - Also, alphabetically, "car" comes before "card"
//"card" comes AFTER "car":System.out.println("card".compareTo("car")); //positive number
- CompareToIgnoreCase works the same way, except it ignores upper and lower case characters.
//"Car" comes BEFORE "car":
System.out.println("Car".compareTo("car")); // negative number
//Ignoring case, "Car" and "car" are equal
System.out.println("Car".compareToIgnoreCase("car")); // 0
Table Summarizing compareTo Method
Alphabetical Order
| Example | Result
| First String comes BEFORE second String
| "cat".compareTo("dog"); | Negative Number
| Two Strings are EQUAL
| "cat".compareTo("cat");
| 0 | First String comes AFTER second String
| "dog".compareTo("cat");
| Positive Number
|
Where Do the Numbers Come From? - According to Oracle's String API, the numerical value generated by compareTo is the difference in unicode values between the first differing characters in the two Strings.
Example Program Comparing Strings
| public static void main(String[] args) {
Scanner input = new Scanner(System.in);
String s1, s2;
System.out.print("Enter two strings: ");
s1 = input.next();
s2 = input.next();
if (s1.compareTo(s2) < 0)
System.out.println(s1 + " comes before " + s2);
else
System.out.println(s2 + " comes before " + s1 + " or is equals to " + s1);
}
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Activity 8.1: Let's Alphabetize! (10 pts) - Open up Eclipse and create a new Java project called Alphabetize, with a new class called Alphabetize.java
- Our program will take in two String inputs from the user, compare then and the output the two Strings in alphabetical order.
- At the top of your program, declare a string variables called word1.
String word1;
- Now declare a second String variable called word2.
- Next,
write a System.out.println statement welcoming your user to the program
and letting them know that this program will alphabetize two words.
System.out.println("Welcome! Give me two words...\nand I will return them to you in alphabetical order!\n");
- Run your program to make sure it is giving you the output you expected.
- Let's prompt the user for the first word and store the result as word1.
System.out.print("Please enter the first word: ");
word1 = input.next(); //assumes new Scanner declared at top of program
- Do the same for the second word.
- Now,
let's create an if-else statement to determine the ordering of the two
words. And, then output the result to our user. The if-else statement
will need to use String comparison as discussed above.
- The code that you have written could be a useful part of a larger program.
- Both partners should submit to Canvas when finished.
Wrap up- Answer the questions from today's learning objectives
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