Lecture Summary for 211 on 7/07/03 -- O'Hare

Review Questions

1. Give three reasons why software engineers would want to write static methods, other than the main method.
   Answer: To avoid code duplication, for modularity, and to facilitate code reuse.

2. What does method invocation mean?
   Answer: It means to call a method.

3. How do you invoke a method with return type void?
   Answer: Place the method name with any arguments in parentheses on a line by itself.

4. What does the header of a method contain?
   Answer: It contains the visibility (public or private), whether the method is static, the return type, the name of the method, and any parameters.

5. What is the difference between a method argument and a method parameter?
   Answer: An argument is specified at method invocation, but a parameter is specified in the header of the method at definition.

6. How do you specify the return value of a non-void method in the method definition?
   Answer: You put the return type in the header.

7. How do you invoke a non-void method?
   Answer: Use the method call in an expression. The return value is used in the expression.

8. Write a Java application that inputs a list of items consisting of name, gender, and salary. Output the name of the woman with the largest salary.

9. True or False. A maximum of 20 objects can be created from a given class.
   Answer: False. Objects can be created as long as you have enough memory, usually at least thousands.

10. What does UML mean?
    Answer: Unified Modeling Language

11. What are the three sections of a UML class diagram?
    Answer: Class name, instance variables, methods.

The DecimalFormat Object

• A DecimalFormat object is used to format a floating point number.

• To create a DecimalFormat object, pass a format string to the DecimalFormat constructor.

• Here are some format strings and their meanings:

  Format String	Digits After Decimal Point?	Always Show Leading Digit?	Commas?
  "#0.0000"	4	Yes	No
  "#.000"	3	No	No
  "#,##0.00"	2	Yes	Yes
  "#,###.0"	1	No	Yes

• Here is Java code to create a DecimalFormat object df that formats a floating point number with two digits behind the decimal point, always includes a leading zero, and uses commas:
  DecimalFormat decimalFormatObject;
  decimalFormatObject = new DecimalFormat("#,##0.00");

• To actually format a floating point value, use the format method of the DecimalFormat class.

• Here is Java code to format the value 34,435.584743:
  double value = 34435.584743;
  String formattedValue;
  formattedValue = decimalFormatObject.format(34435.584743);
  System.out.println("The formatted value is " + formattedValue + ".");

• Look at the DecimalFormat Example in DecimalFormat.zip or DecimalFormat.txt.

UML

• UML means Unified Modeling Language.

• A UML class diagram shows the class name, instance variables, and public methods.

• The symbol + means a member is public.

• The symbol - means a member is private.

• Here are some UML class diagram examples:
  BasketballPlayer   Coin   VendingMachine   Kid   Student  

The Kid1 Example

• Look at the Kid1 Example in Classes1 (Classes1.zip or Classes1.txt).

Accessing Members of an Object

• A member of a class is an instance variable or a method.

• Use the . operator to access a member of a class or object.

• In the Kid1 Example,
  x.name
  accesses the name instance variable of the Kid object x.

• In the Kid1 Example,
  x.display();
  accesses the display method of the Kid object x.

• The toString method is supplied by the designed of a class to return the information in an object for display purposes.

• The toString method of an object is automatically invoked when an object is printed. For example, if x is an object,
  System.out.println(x);
  is short for
  System.out.println(x.toString());

• If an instance variable is left uninitialized, it assumes a default value according to the following table:

  Datatype	Default Value
  int	0
  double	0.0
  char	\0 (ascii code 0)
  boolean	false
  Object	null

• The Object datatype includes objects of all types including String objects.

The Kid2 Example

• Look at the Kid2 Example in Classes1 (Classes1.zip or Classes1.txt).

Constructors

• A constructor is called by the new operator to create a new object.

• No parameters are passed in to a noarg contstructor.
  Example: Kid x = new Kid();

• One or more parameters are passed in to a parameterized constructor.
  Example: Kid y = new Kid("Jane", 'F', 29);

Public vs. Private Members

• A public member of a class can be called from the Main class or from any other class.

• A private member of a class can only be accessed by methods within that same class.

• A instance variables tend to be declared private.

• Methods tend to be declared public.

• Accessor methods provide read only access to instance variables.

• Mutator methods can modify instance variables. Data validation or error checking can be performed.

Overloaded Methods

• The signature of a method is the ordered list of the types of its parameters.

• Two methods are different if their names differ, or if their names are the same but their signatures are different. For example f(5, "a"); and f("a", 5); are different methods because the signature (int, String) is different than (String, int).

• See the Overloaded Example in Overloaded.zip or Overloaded.txt.

Public vs. Private Members

^*Methods to manipulate instance variables are of two types:

1. Accessor methods provide read only access to instance variables.

2. Mutator methods can modify instance variables. Data validation or error checking can be performed.

The Student Example

• Look at the Student example in Classes1.

The LasVegas Project

The Die Example

• Look at the Die example in Die.zip or Die.txt.

The Trace3 Examples

• Work Problems 1 and 2