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Subgoals

Section 4.15 Worked Example: String methods

Subgoals for Calling a Method.

  1. Classify method as static method or instance method
    1. If static, use the class name
    2. If instance, must have or create an instance
  2. Write (instance / class) dot method name and ( )
  3. Determine whether parameter(s) are appropriate
    1. Number of parameters passed must match method declaration
    2. Data types of parameters passed must be compatible with method declaration
  4. Determine what the method will return and where it will be stored
  5. Evaluate right hand side of assignment. Value is dependent on method’s purpose

Subsection 4.15.1 Problem Statement

Given the following code, determine the output.
String alpha = "alphabet";
String beta = "bet";
String gamma = "The cat in the hat.";
System.out.println(alpha.length());
System.out.println(alpha.indexOf(beta));
System.out.println(alpha.charAt(2));
System.out.println(gamma.toUpperCase());
String delta = gamma.substring(0,5);
System.out.println(delta);
delta = gamma.substring(8);
System.out.println(delta);

Subsection 4.15.2 SG1: Classify method as static method or instance method

String is a very versatile class in Java and is used to store 0 or more characters as a single entity. There are many different methods within the String class and this worked example will introduce you to some of the most commonly used. All methods used in this example are instance methods.

Subsection 4.15.3 SG2: Write (instance / class) dot method name and ( )

The methods called in this example are:
alpha.length();
alpha.indexOf(beta);
alpha.charAt(2);
gamma.toUpperCase();
gamma.substring(0,5);
gamma.substring(8);
Each method is called on a specific String instance.

Subsection 4.15.4 SG3: Determine whether parameter(s) are appropriate

Some of the methods (length, toUpper) have no parameters. indexOf requires one String parameter, charAt requires an integer parameter, substring requires one or two integers.

Subsection 4.15.5 SG4: Determine what the method will return and where it will be stored

Let’s evaluate this code by looking at the lines.
String alpha = "alphabet";
String beta = "bet";
String gamma = "The cat in the hat.";
These first 3 lines of code create 3 String objects:
Figure 4.15.1. 
System.out.println(alpha.length());
The length method returns the number of characters stored in the String object. The string "alphabet" contains 8 characters, so the value 8 is printed.
System.out.println(alpha.indexOf(beta));
The indexOf method searches the String object for the first occurrence of the parameter. In this case, we are looking for the value of beta ("bet") within alpha ("alphabet"). String characters have positions and the first character is at position 0.
Table 4.15.2.
character a l p h a b e t
index 0 1 2 3 4 5 6 7
The string "bet" begins at position 5 within "alphabet" so 5 is printed. If the string contained in beta was not found within the string alpha, a -1 is returned.
System.out.println(alpha.charAt(2));
The charAt method returns the character at the position indicated as the parameter. So this code would print the value at index 2, or ’p’.
System.out.println(gamma.toUpperCase());
The toUpper method creates a new string that contains the upper case equivalent of all lower case characters. So calling the toUpper method on gamma ("The cat in the hat.") will create the string "THE CAT IN THE HAT." Any character that is not a lower case character is just copied directly into the new string. So this line would print "THE CAT IN THE HAT." (without the quotes). There is also a toLowerCase() method that creates a string with lower case characters.

Subsection 4.15.6 SG5: Evaluate right hand side (RHS) of assignment. Value is dependent on method’s purpose

Let’s evaluate the final code statements:
String delta = gamma.substring(0,5);
System.out.println(delta);
The substring method creates a new string containing the characters at the first parameter position up to (but not including) the second parameter. So this code would take the first 5 characters (positions 0, 1, 2, 3, 4) of the string gamma ("The ") and assign it to the delta variable. The value "The " would be printed.
Notice that the integer parameters must be valid (the integer values cannot be negative, must be within the range of possible values, and the first parameter < second parameter).
delta = gamma.substring(8);
System.out.println(delta);
The version of the substring method that takes one parameter, begins at that index and copies all the characters to the end of the string into a new string. So the characters beginning at position 8 "i" to the end will be assigned to delta. "in the hat." is printed.
Answer.
8
5
p
THE CAT IN THE HAT
The c
in the hat.
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