Java Fundamentals Expanded

1. Java Fundamentals Expanded SE1021 Dr. Mark L. Hornick

2. In Java, there are two basic categories of datatypes – primitives and classes Let's review primitive datatypes and cover them in more detail: • byte, short, int, long • float, double • char • boolean 1

3. The Integer Types • To access a data type's minimum and maximum values, use the MIN_VALUE and MAX_VALUE named constants that come with the datatype'swrapper class. • For example: System.out.println("Largest int = " + Integer.MAX_VALUE);

4. The default integer literal constant datatype is int That is, a literal value like 123 is an int To explicitly force an integer constant to be a long, use an l or L suffix • Thus, 123L is a long Examples: long ageOfPlanet = 4540000000; // compiler error! long ageOfPlanet = 4540000000L; // OK

5. The Floating-Point datatypes – float and double For doubles, the number of significant digits is approximately 15. For floats, the number of significant digits is approximately 6. Normally, the double type is preferred over the float type because the double type provides more accuracyas well as greater range

6. A floating-point data type's minimum and maximum values are defined via the MIN_NORMAL and MAX_VALUE named constants • Note that a floating-point MIN_NORMAL is qualitatively different from an integer MIN_VALUE. Instead of being the largest-magnitude negative value, it's the smallest-magnitude positive value. • For floating-point numbers, if you want the largest-magnitude negative value, use the negation operator (-) with the MAX_VALUE named constant. For example, here's how to print the largest-magnitude negative float value: System.out.println( "Largest-magnitude negative float = " + -Float.MAX_VALUE);

7. The default floating-point literal constant datatype is double That is, a literal value like 123.0 is a double If you declare a variable to be a float, you must append an f or F suffix to all floating-point constants that go into it, like this: float gpa1 = 3.22f; // OK float gpa2 = 2.75F; // OK float gpa3 = 4.0; // error!!

8. The chardatatype For most languages (including Java), character values have an underlying numeric value. • For example, the letter 'A' has the underlying value of 65. For most languages, the ASCII table specifies the underlying numeric values for characters.

9. The ASCII Table

10. What's the point of having an underlying numeric value? A: So characters can be ordered (e.g., 'A' comes before 'B' because A's 65 is less than B's 66). • Character ordering is necessary so characters (and strings) can be sorted.

11. You can concatenate a char value to a String using the + operator Example: char first = ‘J'; char last = 'D'; System.out.println("Hello, " + first + last + '!'); • When the JVM sees a String next to a + sign, it converts the operand on the other side of the + sign to a String, and then concatenates the Strings. • Q: What mechanism does the JVM use to convert the non-String operand to a String?

12. Be wary of applying the + operator to two chars The + operator does not perform concatenation; instead, it performs mathematical addition using the characters' underlying ASCII values char first = 'J'; char last = 'D'; System.out.println(first + last + ", What's up?"); // result is: 142, What's up? The intended output is: JD, What's up?Q: How can you fix the code?

13. Java is a strongly-typed language Each variable and each value within a Java program must be defined to have a particular datatypes, but statements within a program can often mix datatypes.Example: int x = 3; // intdouble y = 4.0 + x; // mixing double and int How? Through datatype conversions. • Conversions are only allowed for numeric types, not booleans.

14. Primitive datatype Conversions The ordering scheme for primitive datatype conversions: narrowerwider byte short int long float double char This diagram illustrates what datatypes “fit inside” other datatypes: A “narrower” datatype can “fit inside” a “wider” datatype.

15. There are two types of conversion: Promotion and Type casting Promotion: • A promotion = an implicitconversion • it's when an operand's type is automatically converted without having to use a typecast operator. • It occurs when there's an attempt to use a narrower type in a place that expects a wider type • it occurs when you’re going with the flow of the arrows in the previous diagram.

16. Examples of Promotions For each statement, what happens in terms of promotion? long x = 44; float y = x; double z = 3 + 4.5; int num = 'f' + 5; With a mixed expression, the narrower operand(s) is promoted to match the type of the wider operand. These are mixed expressions. Mixed expressions contain operands of different data types.

17. Promotions typically occur as part of assignment statements, mixed expressions, and method calls. Here's a method call example. public class MethodPromotion { public static void main(String[] args) { float x = 4.5f; printSquare(x); // x is a float! printSquare(3); // 3 is an int! } private static void printSquare(double num) { System.out.println(num * num); } } // end class MethodPromotion

18. Type casting = an explicit type conversion. It's when an expression's type is (forcibly) converted by using a cast operator. It's legal to use a cast operator to convert any numeric type to any other numeric type • the conversion can go in either direction in the previous "ordering scheme" diagram. Syntax: (<type>) <expression> double x = 12345.6; int y = (int) x; // result in y??

19. Example from p 444 import java.util.*; public class PrintCharFromAscii { public static void main(String[] args) { Scanner stdIn = new Scanner(System.in); int asciiValue; // user entered ASCII value char ch; // the asciiValue's associated character char nextCh; // the character after ch in the ASCII table System.out.print("Enter an integer between 0 and 127: "); asciiValue = stdIn.nextInt(); ch = (char) asciiValue; nextCh = (char) (asciiValue + 1); System.out.println("Entered number: " + asciiValue); System.out.println("Associated character: " + ch); System.out.println("Next character: " + nextCh); } // end main } // end class PrintCharFromAscii

20. There are two different modes for the ++ increment operator – prefix and postfix. Prefix mode: ++x  increment x beforex's value is used • Example: y = ++x;  x = x + 1; y = x; Postfix mode: x++  increment x afterx's value is used • Example: y = x++;  y = x; x = x + 1;

21. Trace this code fragment int x, y; x = 4; y = ++x; System.out.println(x + " " + y); x = 4; y = x++; System.out.println(x + " " + y);

22. Decrement operators work the same as increment operators …except that subtraction is performed instead of addition. Trace this code fragment: int a, b, c; a = 8; b = --a; c = b-- + --a; System.out.println(a + " " + b + " " + c);

23. Assignment expressions are sometimes embedded inside larger expressions int a, b = 8, c = 5; a = b = c; System.out.println(a + " " + b + " " + c); When that happens, remember that: • An assignment expression evaluates to the assigned value. • The assignment operator exhibits right-to-leftassociativity. In the interest of compactness, it's fairly common to embed an assignment expression inside a loop condition. For example: while ((score = stdIn.nextDouble()) != -1)

24. Conditional operator expressions implement if else logic using a more compact form Syntax: <condition> ? <expression1> : <expression2> Semantics: • If the condition is true, then the conditional operator expression evaluates to the value of expression1. • If the condition is false, then the conditional operator expression evaluates to the value of expression2. Assume x = 2 and y = 5. What does this expression evaluate to? (x>y) ? x+1 : y-1

25. A conditional operator expression cannot appear on a line by itself …because it is not considered to be a statement. Instead, it must be embedded inside of a statement. For example: int score = 88; booleanextraCredit = true; score += (extraCredit ? 2 : 0); Is the same as: int score = 88; booleanextraCredit = true; if( extraCredit ) score += 2; else score += 0;

26. Short-Circuit Evaluation What happens when the code runs with null input values? String input = JOptionPane.showInputDialog(“Enter a pos value”); int value; if( (input!=null) && value=Integer.parseInt(input)>0 ) { // do something with positive value } else { // print an error message }

27. The empty statement is a statement that does nothing. • It consists of a semicolon by itself. • Use the empty statement in places where the compiler requires a statement, but there is no need to do anything. • Example: • The below for loop can be used as a "quick and dirty" way to add a delay to your program: <print monster> for (longi=0; i<1000000000L; i++) ; <erase monster> Style requirement: Put the empty statement on a line by itself and indent it.

28. Empty Statements can also cause errors! Be aware that you can sometimes accidentally introduce the empty statement into a program • Such statements are usually the source of runtime errors. For example: System.out.print("Do you want to play a game (y/n)? "); while (stdIn.next().equals("y")); { // The code to play the game goes here ... System.out.print("Play another game (y/n)? "); } empty statement