Module 7: Characters and Strings

Consider this example:

• A string is a sequence of characters.

• Notice how a string is declared:

  

• Important: String is NOT a reserved word.
         =>
• String is a special kind of object in the Java language.

• The (zero or more) characters in a string are enclosed in double-quotes:

  

Java defines a special operator for concatenating strings:

• This is the same + used for number arithmetic.

• What's unusual is that one can combine numbers and strings to create longer strings:

  

• There is a shortcut operator += to combine assignment and concatenation to the same variable:

  

  The above is equivalent to: z = z + 2.718.

• The concatenation operator is particularly useful for System.out.println:

  

In-Class Exercise 3: Modify the code below so that only a single System.out.println is used.

public class ArrayPrint {

    public static void main (String[] argv)
    {
        int i = 5;
        int j = i * i;
        System.out.print ("The square of ");
        System.out.print (i);
        System.out.print (" is ");
        System.out.println (j);
    }

}

String methods:

• Because strings are objects, it is possible to define methods inside them.

• The Java String object has many such useful methods:

  

• Note that length is a method and that it is called using the dot operator with the variable:

  

• The method takes no parameters but returns an integer:

  

• One can extract a sub-string from a given string using the substring method:

  

  • Think of the characters in a string being indexed, as in an array, starting with 0.
  • The first parameter to substring is the index of the start of the substring.
  • The second parameter is one more than the index of the last char of the substring.

  public class OverloadingExample {
      public static void main(String[] args) {
          myIndexOf("hey");
          myIndexOf('a', 10);
      }
      public static int myIndexOf(int ch) {
          System.out.println("int parameter");
      }
      public static int myIndexOf(int ch, int fromIndex) {
          System.out.println("int parameter, with offset");
      }
      public static int myIndexOf(String str) {
          System.out.println("String parameter");
      }
      public static int myIndexOf(String str, int fromIndex) {
          System.out.println("String parameter, with offset");
      }
  }

Parsing is the act of making sense of a string of text. Java parses the code you write, and interprets it as code. Java is just reading in a large string, and trying to understand its syntax!

Where have you seen strings that your program will want to parse?

In-Class Exercise 6:

For the following sets of input, and output strings, which string methods could I use to transform the strings? Use the String javadoc to find methods to help you. Think of these as a puzzle.

• Input strings: "Hello World!", "Hello Class!", ...
  Desired output: "Goodbye World!", "Goodbye Class!", ...

• Input strings: "HEllO World!", "heLLo Class!", ...
  Desired output: "goodbye world!", "goodbye class!", ...

• Input strings: "      asdf", "     jkl;     ", ...
  Desired output: "asdf", "jkl;", ...

• Input strings: "Lucky number (10)!", "I like (40)!", ...
  Desired output: "10", "40", ...

In-Class Exercise 7: Write a simple program that will determine the winner in a simple card game of "war". Write a method that takes as input a string of the format "4s", which denotes the four of spaces. The suit is denoted by "s" = spades, "d" = diamonds, "h" = hearts, and "c" = clubs. In this exercise, you will write a war method to take two strings as input, and returns 0 if the first string contains a "higher value" card, 1 if the second string does, and -1 if the inputs are not correctly formatted.

  public class War {
      public static boolean formattedCorrectly(String s) {
          // Enter your logic here
      }
      public static int war(String a, String b)
      {
          // Enter your logic here
      }
      public static void main(String[] args)
      {
          assert (war("3c", "2c") == 0)   : "3c < 2c?";
          assert (war("3c", "3d") == 0)   : "3c < 3d?";
          assert (war("4s", "3c") == 0)   : "4s < 3c?";
          assert (war("4ss", "3c") == -1) : "improper format check";
          assert (war("4s", "3cc") == -1) : "improper format check";
          assert (war("s9", "3c") == -1)  : "improper format check";
	  // what other checks should we include?
      }
  }

In-Class Exercise 8: Use overloaded methods to also provide the following methods:

public static int war(char aNum, char aSuit, char bNum, char bSuit)
aNum and bNum are the numerical component of the cards, and aSuit and bSuit are the suit parts. You should not replicate your logic for the previous method. Instead call that method from this one!

When things go wrong