CS 5063  Day 8:  Fundamental Data Types

The Assignment Statement, Console Input and

Decisions

Objectives

• Write arithmetic expressions in Java
• Design and implement the Purse class
• Read input from the console

Assignments

Numeric data conversion

In Java, conversions can occur in three ways:

1. Assignment conversion (the value of the expression on the right has to be converted to agree with the type of variable on the left). For example:

      int i; 
      double d; 
      char ch; 
      i = 3; 
      d = i;   // stores a copy of i in d  (promotion in assignment) 
      i = 'A'; // stores the numeric value of A (65) in i 
   

2. Arithmetic promotion (the operands of an operation need to be in the same format before the operation can be performed). For example, in 8.0 * 5 the 5 is converted/promoted to a double before the multiplication, resulting in the value 40.0 rather than 40. Watch out for precedence: the expression (40.0 - 32.0)* (5/9) evaluates to 0.0.

3. Explicit casting for type conversion

      int n = 20; 
      int d = 8; 
      double ans; 
      ans = n/d;  // 2.0 is stored in ans. If both arguments of the / operator are integers, the result is an integer and the remainder is discarded.

Comparing Primitive Types and Objects

• Primitive variables hold values. Object variables hold references
• A copy of an object reference is another reference to the same object
  

The Math class

The Math class allows you to perform common math functions --- like taking the square root or raising to a power. Math doesn't store any data --- it just does calculation. Math is an example of a static class. You don't need to create a Math object to call Math's methods.  A static method does not operate on an object.  Example  ClassName. methodName(parameters)

   double z = Math.sqrt(y); // find the square root of y 
   double w = Math.min(y, z); // find the minimum of the values in y and z
   double p = Math.pow(3,4);  // calculate and return the value 3*3*3*3 = 81
   System.out.println(Math.abs(x)); // output the absolute value of x

Activity 1:  Arithmetic and Mathematical Functions

Activity 2:  The Purse Class

• A final variable is a constant.  Once its value has been set, it cannot be changed.
• Use named constants to make your programs easier to read and maintain
• Constants are declared with all caps usually located above the instance variables

• Project/Package name:  pursepkg
• New main class:  PurseTest
• New class (be sure to uncheck main method here):  Purse
  
• Save all
• Run ->  right click on the file PurseTest and chose run using defaults
  

• add 3 nickels to aPurse
• add 1 dime to aPurse
• add 2 quarters to aPurse
• get the total value of the money in the purse and store this value in a double variable totalValue and print this value with an appropriate label.

Activity 3  Enhancing the Purse Class

Reading Console Input and testing your enhancements       

• public String nextLine()  Returns as a character string all input remaining on the current line.
• public double nextDouble()
• public int nextInt()

Break

Boolean expressions with relational operators

A boolean expression is one that has a value that is either true or false. Use a boolean variable to hold the value of a boolean expression.

A relational operator is one that compares two values. (In contrast to an arithmetic operator that combines two values to produce a new arithmetic value.)

• The two operands of a relational operator should be the same data type, although Java does allow some mixing of types.
• Relational operators return a true or false answer
• Note difference between test for equality (==) and the assignment operator (=).
• When comparing floating-point numbers, don't test for equality.  Instead, check whether they are close enough

The if and if/else statements

One consequent:

   if (condition)
      truestatement;

Two alternatives:

   if (condition) 
      truestatement;
   else
      falsestatement;

Examples of using just an if statement:

   if ( numGrades != 0 ) 
      average = sum/numGrades; 


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

Examples of using the if/else statement:

   if ( value >= 0 )
      System.out.println("square root of " + value + " = " + Math.sqrt(value)); 
   else 
      System.out.println("nonpositive number " + value + " entered"); 


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

Example: What does the following do? (Trace with variables)

   if ( x > y ){ 
      temp = x; 
      x = y; 
      y = temp; 
   } 

Activity 4

• Assign theMin to be the smallest of x and y
• Write an if statement that assigns 100 to x when y is equal to 0.
• Write an if-else statement that assigns 0 to x when y is equal to 10.  Otherwise it should assign 1 to x.
• Write an if statement that sets the variable hours to 10 when the boolean variable minumum is equal to true
• Calculate wages as hoursWorked times hourlyRate with time and a half for overtime( hours over 40)

Activity 5  The QuadraticEquation  Class

• Project/Package name:  quadraticequationpkg
• New main class:  QuadraticEquationTest
• New class (be sure to uncheck main method here):  QuadraticEquation
  
• Save all
• Run ->  new -> browse (...) -> chose the file QuadraticEquationTest ->  ok