Spring Semester 2005

Strings

How do we work with strings of characters?

The String class is described in the Java API.

A direct link to it is here.

It describes all that needs to be known about Strings of characters.

Here's a String:

"abc"

Here's another one:

"I am a longer string. You have a problem with that?"

The following are all examples of Strings:

" "	(a String of three spaces)
""	(the empty String)
"\"\""	(a String of 2 double quotes)
"\\\\\\\\\\"	(a String of 5 backslashes)
"\n\n\n"	(a String containing three newlines)
"123"	(a String of 3 digits. Looks like a number but isn't.)

Concatenating Strings

The + that we use for numbers acts as a concatenation operator for Strings.

So the expression

"blooming" + "ton"

evaluates to "bloomington".

There is no equivalent of -, * or / for Strings.

Reading Strings

To store a String value you need a String variable.

We could use a ConsoleReader to read Strings from the keyboard.

ConsoleReader c = new ConsoleReader(System.in); 
String a; 
a = c.readLine(); 

The fragment above:

• creates a new console reader and names it c
• defines a variable a of type String
• reads a line from the keyboard and stores the resulting String under the name of a

Next we describe a few operations on Strings.

(Strings are like Penguins in how you communicate to them.)

What Strings Can Do

Here are five operations to get us started:

• you can find out the number of characters in the string

  "automaton".length() returns 9

  There are 9 characters in "automaton" their indices are 0 through 8.

• you can extract a substring (one method is described here)

  "automaton".substring(2,8) has the value (returns the String) "tomato"

• you can extract a substring in more than one way (second method is described here)

  "automaton".substring(9) has the value "" (returns the empty String)

• you can extract a single character (you need to know the position of the character in the string)

  "automaton".charAt(0) returns the character 'a'

  To store it you would need a char variable (of type character).

  Notice that "a" + "b" evaluates to "ab" but 'a' + 'b' is a number. Why?

• you can determine if two strings read the same (are equal, so to speak).

  "automaton".equals("automa" + "ton") evaluates to true

  But "Automaton".equals("automaton") evaluates to false.

  To get true we would have to use equalsIgnoreCase (can you find it in the API?).

  To store true or false you need a boolean variable.

  Thus

  boolean a;
  a = 2 < 3; 
  System.out.println(a); 

  will print true.

Converting Strings into Numbers

To convert between Strings and numbers we need to use:

• Integer.parseInt(java.lang.String) (described here) converts well-formed Strings to integers.

  Integer.parseInt("23") evaluates to 23

• Double.parseDouble(java.lang.String) does the same type of conversion for Strings for representing floating-point numbers.

  Double.parseDouble("23.5") evaluates to 23.5

Converting Numbers into Strings

To convert numbers into Strings we concatenate the empty string to them:

• 1 + "" evaluates to "1"

• 1.23 + "" evaluates to "1.23"

When a plus (+) is in between two numbers it adds them up.

When it's in between two Strings it concatenates them.

When it's in between a String and a number:

• the number is turned into a String first,
• and then the two Strings are concatenated.

Thus 1 + "2" first becomes "1" + "2" and then evaluates to "2".

Strings are not Numbers

That's why

"1" + "1"

evaluates to "11" and NOT to 2.

Numbers are not Strings

That's why

"1024".substring(2, 3)

evaluates to "2" but 1024.substring(2,3) (without quotes) makes NO sense.

Likewise

"1024".length()

evaluates to 4, but 1024.length() (without quotes) makes NO sense.

So now you know how to convert between Strings and numeric types.

The Duplicity of +.

Here are three questions, to make sure you understand how + works:

• what does 1 + 2 + "3" evaluate to?
• what does (1 + 2) + "3" evaluate to?
• what does "1" + 2 + 3 evaluate to?
• what does Integer.parseInt("1") + 2 + 3 evaluate to?

The answers, in order are: "33", "33", "123" and 6.

Characters

Here's an extra question:

what does '1' + 2 evaluate to?

Answer (in two parts):

• if we know that characters have a numeric code
• then 'A' + 0 evaluates to the code of character A.

Also, (char)('A' + 3) is the code for 'D'.

A Complete Example

We finish with an example, let's solve this problem:

15.

Write a program that reads two times in military format (0900, 1730) and prints the number of hours and minutes between the two times. Here is a sample run. User input is in color. Please enter the first time: 0900 Please enter the second time: 1730 8 hours 30 minutes When the first time is later than the second time: Please enter the first time: 1730 Please enter the second time: 0900 15 hours 30 minutes

Here's a sample run with the program: frilled.cs.indiana.edu%java Fifteen Please enter the first time: 0920 Please enter the second time: 1025 1 hours 5 minutes frilled.cs.indiana.edu%java Fifteen Please enter the first time: 1025 Please enter the second time: 0920 22 hours 55 minutes frilled.cs.indiana.edu%

We'll write this program in class.

Here's a plan of attack:

1. Read String time1, and time2.

2. For time1 split it into

   String hour1 = time.substring(0, 2),
          mins1 = time.substring(2); 

3. Calculate time1 in minutes:

   int val1 = hour1 * 60 + mins1;

4. Similarly calculate val2

5. Then calculate the difference in minutes between the two:

   int diff = (val2 + 24 * 60 - val1) % (24 * 60); 

6. Transform diff in hours and minutes:

   int resH = diff / 60, 
       resM = diff % 60; 

Write this program (call it MilitaryTime.java) and verify that it works correctly.