Lecture Summary for 211 on 4/15/02 -- Loop Section

Review Questions

1. What do you do if a line is too long in Java?
   Answer: Just break the line wherever you need to. Don't break the line in the middle of a variable name or constant.

2. What do the following symbols mean in Java?
   +    *    /    .    %    !
   Answer: They are method selection, line terminator, begin block, string delimiter, start of comment, and contatenation, respectively.

3. Rank these operators in order of highest to lowest precedence:
   -    /    =    ( )    &&    .    ==
   Answer:   ( )    .    /    -    ==    &&    =

4. Write a line of java code to find the average of the four doubles a, b, c, d.   Answer:
   average = (a + b + c + d) / 2.0;

5. What is the difference between static and nonstatic methods?
   Answer: A nonstatic method requires an object to execute it. A static method can be executed by either a class or an object.

6. What import statement do you need to use an input dialog?   Answer:
   import javax.swing.JOptionPane;

7. What is the output?
   int x = 4, y = 7;
   x += y * 2;
   y -= x - 3;
   System.out.print(x + " ");
   x /= 3;
   y = x;
   y /= 2;
   System.out.println(y);

   Output:18 3
   Here is the variable trace table:

   x	y
   4	7
   18	7
   18	-8
   6	6
   3

The DecimalFormat Class

• 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

• In a format string
  
  0 indicates a digit that is always displayed,
  # indicates a digit that is only displayed if it is nonzero,
  . indicates where the decimal point is placed,
  , indicates that digits are grouped with commas.

• 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 + ".");

• To use a DecimalFormat object in a class, use this statement at the top of the class definition:
  import java.text.DecimalFormat;

• Look at the DecimalFormat Example in InputFormat (InputFormat.zip or InputFormat.txt).

• Practice Problem:   Write a line that creates a DecimalFormat object that formats a double with three digits after the decimal point. Do not display any nonzero digits to the left of the decimal point.

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

The String Class

• A reference variable refers to an object.

• To create an object, declare a reference variable for the object, then call a constructor to create the object. The constructor returns the address of the object which is assigned to the reference variable. This can be done by
  
  • declaration and assignment,
    
  • or by initialization.

• Example:   Write a line or lines of Java code to create a String object containing the letters in Illinois. There are four ways to do this.

  // Version 1. Use declaration and assignment,
  // with full String constructor call.
  String state;
  state = new String("Illinois");

  // Version 2. Use initialization
  // with full String constructor call.
  String state = new String("Illinois");

  // Version 3. Use declaration and assignment,
  // with short cut String creation.
  String state;
  state = "Illinois";

  // Version 4. Use initialization with
  // with short cut String creation.
  String state = "Illinois";

• A String object contains a sequence of chars.

• Some String methods: length, charAt, toUpperCase, toLowerCase.

• Example:   What is the output?
  String animal = "Rabbit";
  System.out.println(animal.length());
  System.out.println(animal.charAt(4));
  System.out.println(animal.toUpperCase());
  System.out.println(animal.toLowerCase());

  Answer:
  6
  i
  RABBIT
  rabbit

• Do Examples 7 and 8 of Trace1.

The Applet and Graphics Classes

• To create an applet, create your own class that extends applet using these import statements and class definition:
  import java.Applet.applet;
  import java.awt.*;
  class MyApplet extends Applet

• For MyApplet to draw a picture, it must include init and paint methods.

• To use all of the Applet, Color, and Graphics methods, MyApplet must extend Applet.

• Look at the Tree and Rainbow Examples in Applets (Applets.zip and Applets.txt).

• See the document The Applet, Color, and Graphics Classes.