Lecture Summary for 211 on 1/14/04

Review Questions

1. How do you find the Java examples for this class?

2. When, where, and why was Java developed?

3. What is the difference between JDK1.3 and BlueJ?

4. Why is Java hard for beginners?

5. What is "boiler plate" code?

6. Write a line of Java code that will print the message "Java is fun."

7. What is an instance variable?

8. What is a method?

9. What is a constructor?

10. What is the difference between a class and an object?

11. Name three ways of specifying comments in Java?

12. What are the steps to creating a Java application in BlueJ?

Command Line vs. User Interface Applications

• Applications that output with System.out.println are called command line applications.

• Applications that create their own window and display output in labels, textfields, etc. are called user interface applications.

• Look at the Temperature Example in Intro (Intro.zip or Intro.txt).

Java Constants, Variables, and Datatypes

• Integer, floating point, character, and Boolean constants.

• Variables.

• int, double, char, and boolean datatypes.

• String objects.

• Go over the Variables Example in Primitive (Primitive.zip and Primitive.txt).

The Primitive Example

• Look at the Variables Example in Primitive (Primitive.zip and Primitive.txt).

Practice Problems

1. Create an integer variable and initialize it to 1.5 billion.

2. Create a variable and initialize it to 6 billion.

3. Create a character variable and initialize it to the upper case W character.

4. Create a boolean variable and initialize it to false.

Java Operators

• Arithmetic Operators:   +   -   *   /   %

• String Concatenation Operator:  +

• Comparison Operators:   ==   <   <=   >   >=   !=

• Logical Operators:   &&   ||   !

• Assignment Operators:   =   +=   -=   *=   /=   %=   ++   --

• Go over the Operators Example in Primitive (Primitive.zip and Primitive.txt).

• Go over the Trace1 Examples   (Trace1.txt).

The JOptionPane Class

• To use an input dialog in a class, use this statement at the top of the class definition:
  import javax.swing.JOptionPane;

• Look at the JOptionPane Example in Intro (Intro.zip and Intro.txt).

• Sometimes the input dialog will not appear when the program is run. You will have to find it behind other windows.

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

• The format method of DecimalFormat returns a string, not a double.

• 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 (DecimalFormat.zip or DecimalFormat.txt).

The parseInt Method

• Use the Integer.parseInt method call to convert a string representing an integer into an int variable.

• The following Java code converts the string "234" into the int value 234:
  String a = "234";
  int x;
  x = Integer.parseInt(a);

• If a nonnumeric string is passed to Integer.parseInt, the program will crash. A NumberFormatException will be produced.

Static vs. Nonstatic Methods

• A static method is a method that may be called using either the class directly or using an object of that class.

• For example, the method showInputDialog can either be called using the JOptionPane class as
  JOptionPane.showOptionPane(prompt);
  or by using the JOptionPane object jop as
  JOptionPane jop = new JOptionPane();
  jop.showOptionPane(prompt);

• A nonstatic method can only be called using an object.

• For example, the VendingMachine method depositQuarter is called using the VendingMachine object vm:
  VendingMachine vm = new VendingMachine();
  mv.depositQuarter();

Examples of Static and Nonstatic Methods

• println and print are NOT static methods because they are called from the special class System.out.

Practice Problems

1. Write a Java program that will input a temperature in Celsius, change the input string into an int, convert the temperature to Fahrenheit, and print the Fahrenheit temperature.

2. Write a Java program that will input an integer, then output a 0 if the input is even and a 1 if the input is odd.