Computer Science (CSCI) Department College of Engineering, Computer Science, & Technology (ECST)

JETT Workshops Java Basics Fall 2004

Control Structures and Relational Operators
(Example code and figures from Computing Concepts with Java Essentials, 3rd edition, by Cay Horstmann.
Visit www.wiley.com/college/horstmann for information on the current edition of this excellent text.)

• from the AP CS Java Subset
  • The control structures if, if/else, while, for, return are part of the AP Java subset.
  • The do/while, switch, plain and labeled break and continue statements are not in the subset.

• if statement

  if (amount <= balance) balance = balance - amount;

  
  

• if/else statement
  
  

  if (amount <= balance) balance = balance - amount; else balance = balance - OVERDRAFT_PENALTY;

• block statement
  

  if (amount <= balance) { double newBalance = balance - amount; balance = newBalance; }

• statement types

  • Simple statement

    balance = balance - amount;

    
    
  • Compound statement

    if (balance >= amount) balance = balance - amount;

    • while, for, etc., are also compund statements
      
      
  • Block statement

    { double newBalance = balance - amount; balance = newBalance; }

    
    
    
• Relational Operators
  
  

  Java	Description
  >	Greater than
  >=	Greater than or equal
  <	Less than
  <=	Less than or equal
  ==	Equal
  !=	Not equal

  
  
  

• Equality Testing vs. Assignment

• The == operator tests for equality

  if (x == 0) . . // if x equals zero

  
  
• The = operator assigns a value to a variable

  x = 0; // assign 0 to x

  
  
• Don't confuse them.
• Java == is the same as mathematical =
  
  
• Comparing Floating-Point Numbers
• Consider this code:

  double r = Math.sqrt(2); double d = r * r -2; if (d == 0) System.out.println( "sqrt(2)squared minus 2 is 0"); else System.out.println( "sqrt(2)squared minus 2 is not 0 but " + d);

  
  
• It prints:
   sqrt(2)squared minus 2 is not 0 but 4.440892098500626E-16
  
  
• Don't use == to compare floating-point numbers
  
• Two numbers are close to another if
  |x - y| <= ε
  
• ε is a small number such as 10^-14
  
• Not good enough if x, y very large or very small. Then use
  |x - y| / max(|x|, |y|) <= ε
  
• But if one of the numbers is zero, don't divide by max(|x |, |y|)
  
  
• String Comparison
• Don't use == for strings!

  if (input == "Y") // compares the references, not the strings

  
  
• Use the equals( ) method

  if (input.equals("Y"))

  
  
• == tests identity, equals tests equal contents
  
• Case insensitive test ("Y" or "y")

  if (input.equalsIgnoreCase("Y"))

  
  
• Lexicographic Comparison
• s.compareTo(t) < 0 means:
  s comes before t in the dictionary
• "car"comes before "cargo"
  "cargo" comes before "cathode"
  
• All uppercase letters come before lowercase:
  "Hello" comes before "car"
  
  
  

• Object Comparison
• == tests for identity, equals( ) for identical content
• given

  Rectangle cerealBox = new Rectangle(5, 10, 20, 30); Rectangle oatmealBox = new Rectangle(5, 10, 20, 30); Rectangle r = cerealBox;

  
  
• cerealBox != oatmealBox but cerealBox.equals(oatmealBox)
• cerealBox == r
  
• Caveat: the equals method must be defined for the class
  
  
  

• The null Reference
• null reference refers to no object at all
• Can be used in tests:
  if (account == null) . . .
• Use ==, not equals, to test for null
• showInputDialog returns null if the user hit the cancel button:
  String input = JOptionPane.showInputDialog("...");
  if (input != null) { ... }
• null is not the same as the empty string ""
  
  
• Multiple Alternatives

  if (condition1) statement1; else if (condition2) statement2; else if (condition3) statement3; else statement4;

  • The first matching condition is executed.
• Order matters!
  
  
• Earthquake Example
  • Earthquake.java
  • EarthquakeTest.java
    
    
• Multiple Alternatives
• Order matters:
  if (richter >= 0) // always true!
     r = "Generally not felt by people";
  else if (richter >= 3.5) // not tested
     r = "Felt by many people, no destruction";
  . . .
• Don't omit the else
  if (richter >= 8.0)
     r = "Most structures fall";
  if (richter >= 7.0) // omitted else--ERROR
     r = "Many buildings destroyed


• Nested Branches
• Branch inside another branch

  if (condition1) { if (condition1a) statement1a; else statement1b; } else statement2;

• Tax Return Example

If your filing status is Single

If the taxable income is over	But not over	The tax is	Of the amount over
$0	$21,450	15%	$0
$21,450	$51,900	$3,217.50 + 28%	$21,450
$51,900		$11,743.50 + 31%	$51,900

If your filing status is Married

If the taxable income is over	But not over	The tax is	Of the amount over
$0	$35,800	15%	$0
$35,800	$86,500	$5,370.00 + 28%	$35,800
$86,500		$19,566.00 + 31%	$86,500

• TaxReturn.java
• TaxReturnTest.java
  
  

• The boolean Type
• George Boole (1815-1864): pioneer in the study of logic
• value of expression amount < 1000 is true or false.
  
• boolean type: one of these 2 truth values
  
  
• Predicate Method
  
• return type is boolean
• Example:

  public boolean isOverdrawn() { return balance < 0; }

  
  
• Use in conditions for branching
  if (harrysChecking.isOverdrawn()) ...
  
  
• Useful predicate methods in Character class
  • isDigit
  • isLetter
  • isUpperCase
  • isLowerCase
    
    
• if (Character.isUpperCase(ch)) ...
  
  
• Boolean Operators
• &&  and
  
• ||  or
  
• !   not
  
• if (0 < amount && amount < 1000) ...
  
• if (input.equals("S") || input.equals("M")) ...
  
  
• short circuit evaluation for the && and || operators

• Truth Tables
  
  

A	B	A && B
true	true	true
true	false	false
false	Any	false




A	B	A || B
true	Any	true
false	true	true
false	false	false




A	! A
true	false
false	true



• Boolean Variables
• Set to truth value and use in conditions for branching

  boolean married; married = input.equals("M"); if (!married) ...

  
  
• Also called a flag
  
  
• Don't need to test boolean variables against truth values
  if (married == true) // don't do this
  if (married == false) // don't do this
  if (married != false) // don't do this