Understanding Data Types, Variables, and Strings in Java Programming

1. CSci 142 Data and Expressions

2. Data and Expressions • Topics • Strings • Primitive data types • Variables and constants • Expressions and operator precedence • Data conversions

3. Character Strings • A String consists of zero or more characters • Represent a String literal with double quotes "This is a string literal." "123 Main Street" "X" ""  this is called an empty String • Every string is an object in Java, defined by the String class

4. method name Printing Strings println ("Whatever you are, be a good one."); print ("Whatever you are, "); print ("be a good one."); information provided to the method (argument) • The ConsoleProgram class has two methods for printing: • print - no line break • println - inserts a line break • Both accept String arguments

5. String Concatenation • Concatenation is appending one string to the end of another • "Peanut butter " + "and jelly" • Concatentation can be used to break up a long line • println ("It was many and many a year ago, " +"in a kingdom by the sea, that a maiden there lived…");

6. The + Operator • Used for concatenation if either or both operandsare Strings • println(“Good ” + 4 + “U”); • Used for addition if both operands are numeric • println(3+6); • Evaluated left to right, but parenthesescan be used to force the order • println(“Hi ” + 2 + 3); • println(“Hi ” + (2 + 3));

7. Escape Sequences • How would we print the quote (") character? println ("I said "Hi" to you."); • An escape sequence is a series of characters that represents a special character • Begins with a backslash (\) println ("I said \"Hi\" to you.");

8. Escape Sequences

9. Primitive Data Types

10. Type byte short int long float double Storage 8 bits 16 bits 32 bits 64 bits 32 bits 64 bits Min Value -128 -32,768 -2,147,483,648 < -9 x 1018 +/- 3.4 x 1038 with 7 significant digits +/- 1.7 x 10308 with 15 significant digits Max Value 127 32,767 2,147,483,647 > 9 x 1018 Numeric Primitive Data • The difference between the various numeric primitive types is their size

11. char • A char variable stores a single character • Characters are delimited by single quotes: 'a' 'X' '7' '$' ',' '\n' • Example declarations: char topGrade = 'A'; char terminator = ';', separator = ' '; Note that a character variable can hold only one character, while a String can hold zero or more characters.

12. boolean • Abooleanvalue represents true or false • The reserved wordstrue andfalse are the only valid values for a boolean type boolean done = false;

13. Practice • What data type would you use to store: • A tax rate? • The current year? • Whether or not a customer should receive a discount? • Height?

14. data type variable name Variables • A variable is a named memory location • A variable must be declared by specifying the variable's name and the type of information that it will hold int count, temp, result; int total; Multiple variables can be declared in one statement

15. Variable names • May contain letters or numbers, but may not start with a number • you2 - valid • 2you - not valid • May not contain a space or any special characters, except the underscore (_) • why_not - valid • why not? - not valid • Should use camel case • gpa, numCredits, totalClassCount

16. Variable Initialization Declare the variable Initialize the variable Use the variable • A variable must be initialized before it can be used • intval; • val = 10; • println(val);

17. Variable Initialization Declare and initialize the variable Use the variable • A variable can be declared and initialized in one statement • intval = 10; • println(val);

18. Variable Assignment • An assignment statement changes the value of a variable • Read “=” as gets • The value that was in total is overwritten • Most current value of a variable is used • Values assigned to a variable must be consistent with the variable's declared type total = 55; int base = 32; println(base); base = 45; println(base);

19. String Variables Declare the variable Initialize the variable Use the variable • Strings can be stored in variables just like primitive data types • String name; • name = "Jose"; • println(name);

20. String Variables Declare and initialize the variable Use the variable • A variable can be declared and initialized in one statement • String name = "Jose"; • println(name);

21. Practice • Declare a variable to hold a total price • double price; • In two lines, declare and initialize a variable to hold your age • int age; • age = 21; • Declare and initialize a variable to hold your favorite pet • String pet = "alpaca"; • Declare and initialize a character variable to hold your middle initial • char initial = "J"; • Declare and initialize a boolean variable isMarriedto hold your marital status • booleanisMarried = true;

22. Constants • A constant is similar to a variable except that its value cannot change during program execution • As the name implies, it is constant, not variable • The compiler will issue an error if you try to changethe value of a constant public static final int MIN_HEIGHT = 62;

23. Why Constants are Cool • Give meaningto otherwise unclear literal values • MAX_LOAD means more than the literal 250 • They facilitate program maintenance • If a constant is used in multiple places, its value need only be updated once • They formally establish that a value should not change, avoiding inadvertent errors

24. Expressions • Arithmetic expressions use arithmetic operators: Addition + Subtraction - Multiplication * Division / Remainder % • If either or both operandsused by an arithmetic operator are floating point (decimal), then the result is a floating point

25. Operator Precedence • Operators can be combined into complex expressions result = total + count / max - offset; • Operations have a well-defined precedence • Parentheses • Multiplication (*), division (/), and remainder (%) • Addition and string concatenation (+), subtraction(-) • Assignment (=) • Arithmetic operators with the same precedence are evaluatedfrom left to right

26. Example answer = sum / 4 + MAX * lowest; 4 1 3 2 The expression is evaluated and the result is stored in the variable on the left hand side

27. Practice int a=3, b=5, c=2; int answer; • answer = a * b - c; • answer = b + a * c; • answer = b / a; • answer = a / b; • answer = b % a; • answer = a % b; • answer = b - a * b - c; • answer = b - a / c; • answer = (b - a) / c; • answer = a * (b + c);

28. Increment and Decrement • The increment operator (++) adds one to its operand • The decrement operator (--) subtracts one from its operand • The statement count++;is equivalent tocount = count + 1;

29. Assignment Operators • Often we perform an operation on a variable, and then store the result back into that variable • Example: • num = num + count; • This can be written using an assignment operator: • num += count;

30. Operator += -= *= /= %= Example x += y x -= y x *= y x /= y x %= y Equivalent To x = x + y x = x - y x = x * y x = x / y x = x % y Assignment Operators • There are many assignment operators in Java, including the following:

31. Data Conversion • Sometimes it is convenient to convert data from one type to another • These conversions do not change the type of a variable or the value that's stored in it • They only convert a value as part of a computation • Conversions must be handled carefully to avoid losinginformation

32. Practice int a=3, b=5, c=2; int answer; • answer = a += b; • answer = b -= c; • answer = a *= c; • answer = a++;

33. Data Conversion • widening conversions • Go from a small data type to a larger one • Example: short to an int • Safe • narrowing conversions • Go from a large data type to a smaller one • Example: int to a short • Can lose information • Types of conversion • Assignment conversion • Data conversion • Casting byte short int long float double narrowing conversions widening conversions

34. Assignment Conversion • Assignment conversion occurs when a value of one type is assigned to a variable of another double money; int dollars = 5; money = dollars; • Only widening conversions can happen via assignment • The value and type of dollars did not change converts the value in dollars to a float

35. Data Conversion • Promotion happens automatically in certain expressions • Example double sum = 5.0; int count = 3; double result = sum / count; countis Temporarilyconverted to a double

36. Casting • Casting is the most powerful, and dangerous, technique for conversion • May be used for both widening and narrowing conversions • To cast, the type is put in parentheses in front of the value being converted int total=3, count=2; double result = (double)total / count;

37. iResult = num1 / num4; dResult = num1 / num4; iResult = num3 / num4; dResult = num3 / num4; dResult = val1 / num4; Practice • Given the following declarations, what result is stored in each of the statements? intiResult, num1=25, num2=40, num3=17, num4=5; double dResult, val1=17.0, val2=12.78; • dResult = (double)num1 / num2; • dResult = num1 / (double)num2; • iResult = (int)(val1 / num4); • dResult = (int)((double)num1/num2); • iResult = num3%num4;