Copyright © 2013
Robert W. Hasker

Java Basics, continued

• Sections 3.11 through 3.23
• types, expressions

More on Types

• How did we declare a variable in the previous class?
• two basic types for numbers in computers: floating point and integer
  • Integers: counting numbers: -3, -2, -1, 0, 1, 2, 3, ...
  • For integers, use int
  • floating point: decimal values
  • When would you want to use decimal values?
  • =: assign initial (first) value
• In-class quiz:
  • Type for amount of cat food sold in a day, in tons.
  • Type for number of cats living in the city.
  • Type for number of people living on the planet.
  • Time until noon.

Example

• Write a program which computes number of minutes left in the quarter based on number of weeks, number of days, number of hours

Average Population

• Compute average population of three cities.
• Datatype?
• Solution: Average0.java
  • Run this solution
  • Clearly we need to more about types and expressions!

More on Expressions

• precedence: what to do when you mix operators
  • Example: what is 3 + 4 * 5 equal to?
  • precedence rules:

    	highest precedence:	()
    				* / %
    	lowest precedence:	+ -
    

  • parens change the order of evaluation:

    	3 + 4 * 5 => 23
    	(3 + 4) * 5 => 35
    

• grouping order or associativity: left to right
  • "x - y + z" means "(x - y) + z", not "x - (y + z)"
  • What is "2.0 / 4.0 * 5.0"? - 2.5, not 0.1!
• Mixing values in expressions:
  • if all values are integers, result is an integer
  • if any value is a double, result is a double
  • this is why it's sometimes important to add .0; what's the value of "3 * 7 / 2 * 2"?
  • How about "-5.5 + 1 / 2"?
• Assigning values to variables of a different type:

  	int    i;
  	double x;
  
  	x = 23;
  	i = 18.4; // warning: "possible loss of precision"
  

  • Ints are automatically converted into doubles and vice-versa.
  • When converting to an int, the decimal portion is truncated.
  • Tricky example:

    	someFloat = 1 / 2;	// what does this give?
    

• Forcing the conversion: type casting
  • Done by using (int) and (double).
  • Examples:

    	someFloat = (double)1 / 2;
    	someFloat = 1 / (double)someInt;
    	someInt   = (int)someFloat / 4;
    	someInt   = (int)(someFloat * 10) - 10;
    

  • How would you round a number?
• [How to fix computing population average?]

Text

• Focus to this point: numeric computation
• Real world: manipulate names, letters, words
• char
  • char first, middle, last;
  • first = 'r'; middle = 'w'; last = 'h';
  • System.out.println("Initials: " + first + middle + last);
• More common: groups of characters - String
  • String name = in.next();
  • System.out.println("Hello, " + name);
  • Can also assign to a string constant: name = "dr. janeway";
  • What if someone manages to put quotes in their name? name = "call me \"james\"";
    • Some other special characters: \', \t, \\
    • Note these are only in the code; can just type the characters to prompts

Introduction to Object-Orientation

• data such as int, double, char: indivisible, atomic
• Data such as a bank account: a combination of other data values
  • You can do lots of things to bank accounts: deposit money, take it out, add interest, check a PIN
  • operations: the things you can do to a piece of data
  • Supporting such operations is a key element of Java
• We've already started to work with objects
  • String: types of objects storing a sequence of characters
  • System: an object representing the computer
  • Scanner: types of objects which scan for information in strings