Declaring Variables You must first declare a variable before you can use it! Declaring involves: – Establishing the variable’s spot in memory – Specifying what kind of data it will contain (its type) – Give it a name (its identifier) – Optionally give it an initial value Note: A program may declare as many variables as it needs Example of Java declarations: – Statement to: Declare an integer variable: int grade; – Statement to: Declare an integer variable with a value: int count = 5; – Statement to: Declare two integer variables: int grade, count = 5; Questions: What are the identifiers, initial value, and type in the above examples? Definition: A variable is a place in memory that can hold a value Important Note: Equal does NOT mean equal, it means assign

Primitive Variable types (not objects) There are many data types in Java There is a reserved word for each Examples: Integers, Fractions, String, boolean, and character Integer variables – byte i = 100; (a byte variable holds values from -128 to 127) – int x = 32768; (an int variable holds values to billions) – long total = ; (a long variable holds up to quintillians) Fraction variables – float percentage; (float variables are accurate to six digits) – double distance; (double variables are accurate to fifteen digits) Boolean variables: boolean isBig = true; or boolean isBig = false; String variables are a sequences of letters – String name = “bill”; (Enclose your strings with double quotes) Character variable: char c = ‘3’; // A single letter Definition: A type specifies a kind of data Question: Why all the types? Answer: Different variable types are stored differently in memory

Reference Data Types (objects) String (The most used non-primitive data type) Definition: A string is a sequence of characters Examples – A String literal: "abcdef" – Declare/initialize a string: String s = new String("abcdef"); – Convenient way: String s = "abcdef“; Reference variables contain methods and properties – s.length(); // returns the length of the string – s.charAt(0); // returns the first character of the string – s.toUpperCase(); // returns withall letters as upper case – s.substring(3,8); // returns substring from position 3 to (not including) 8 Important Note: Character positions count from zero, not from one

Literals Definition: A literal is a constant that we do not have to declare. Examples: – String literal: “abcdef” – Integer literal: -10 – Fraction literal: – Character literal: ‘t’ – Boolean literal: true

Expressions and Casting Expression: – A sequence of operations and operators – Example: x + y / 3 + (4 + z)/a Java is fussy about expressions with different types of data Casting Example: – z becomes 0: int x = 5, y = 8; float z = x/y; – z becomes 0.625: int x=5, y=8; float z = (float)x/y Definition: To cast means to access a variable as if it were a different type

Widening and Narrowing It is legal to store a primitive (non-object) variable type of a smaller range to one of a wider range (widening) – Example: ling x = 3; int y = 2; x = y; – Example: double z = 3; int y = 2; z = y; Attempting to store a primitive variable of a wider range into one of a smaller range causes a compile error (narrowing) – Example: long x = 3; int y = 2; y = x; – Example: double z = 3; int y = 2; y = z; – One must cast to eliminate the error: y = (int)z;

Identifier names Rules – All identifiers start with an alphabetic letter or underscore – Subsequent letters of an identifier can be alphabetic letters, underscores, or numeric digits (No Spaces) – Identifiers cannot be a Java reserved word Conventions followed by most programmers – Keep the identifiers relatively short – Identifiers should be easy to remember – Naming conventions Variables: start lower case, and first letter of subequent words should be upper case (ex: salesTotal) Classes: First letter of every word upper clase (ex: MilesPerGal) Symbolic constants: public final int MAX = 100; Definition: An identifier is the name we give to a variable Note: the final modifier means that the variable cannot be altered.

Programming conventions It is important to keep your programs readable Most programmers will – Indent blocks of instructions by several spaces – Add comments to the top with name, date, programmer name, source file name, purpose, modification purpose and date – Add comments to the top of each method to include its purpose and for automatic documentation generation Example: /** main method calculate miles per gallon args command line arguments (unused) */ public static void main(String[] args) { String data = “I’m indented”; System.out.println(data); }

Concatenation Definition: concatenation is gluing strings together. – String s = “abc” + “def” puts “abcdef” in s – Numbers and Strings are different yields 77 “33” + “44” yields “3344” “33” / “44” generates an error – String s = “abc ” + 33 puts “abc 33” in s because Java makes the 33 into a string and then does concatenation – What does the following print? x = 32.95; System.out.println(“you earned $” + x + “dollars”); Note: Assign means do what is on the right of the equal sign and store in the variable on the left

Escape Sequences ( or Characters) Definition: An escape sequence is two letters starting with a backslash (\) that has special meaning to Java Purpose: To represent characters that are not able to easily be displayed in a program Examples: – System.out.println(“don’t worry\tbe happy”); – System.out.println(“first line\nsecond line”); – System.out.println(“He said, \”hi\””); – System.out.println(“\fclear screen or new page”);

Numbering systems Binary: each digit is 2 times the previous Octal: each digit is 8 times the previous Decimal: each digit is 10 times the previous Hexadecimal: each digit is 16 times the previous Problems: – Convert 110 to decimal if it is binary, octal, or hexadecimal. – Convert: If FCA is a hexadecimal number, convert it to binary

Scope and Life Scope: – Where in a program can a variable can be accessed – A variable’s scope is from its declaration to the end of the block Life: – When memory for a variable is assigned for that variable – Variables live within the block where they are declared. Programming conventions: – limit scope as much as possible – This convention will result in less errors and make programs easier to maintain Example {// Outer block // Next instruction fails System.out.println(outer); int outer = 1; { // inner block int inner = 2; // The next is OK System.out.println (outer+inner); } // End of outer block // Next instruction fails System.out.println(inner); // Next instruction is OK. System.out.println(outer); }// End of outer block Block: the instructions within braces { }

Formatting Output import java.text.NumberFormat; import java.text.DecimalFormat; public class FormatTest { public static void main(String[] args) { System.out.print("Input args: " + args[0] + "/"); System.out.println(args[1]); System.out.println(); // Skip a line System.out.println("input args:" + args[0] + "/" + args[1] + "\n"); double v0 = Double.parseDouble(args[0]), v1 = Double.parseDouble(args[1]); System.out.printf("%-7s = %8.2f/%5.2f\n\n", "Printf", v0, v1); NumberFormat currency = NumberFormat.getCurrencyInstance(); System.out.println( "Currency: " + currency.format(v0) + "/" + currency.format(v1) + "\n"); DecimalFormat dec = new DecimalFormat("###0.0##"); DecimalFormat pct = new DecimalFormat("0.##%"); System.out.println( "Decimal: " + dec.format(v0) + "/" + pct.format(v1)); } }// End of FormatTest class

Formatting Strings for Output public class FormatExample { public static void main(String[] args) { String str = String.format("Cost: $%7.2f\n",35.23); str += String.format("Cost: $%-7.2f\n",35.23); str += String.format("Cost: $%.2f\n",35.23); str += String.format("Integer: $%4d\n",100); str += String.format("Left Justified:%-10s\n","Dan"); str += String.format("Right Justified:%10s\n","Dan"); str += String.format("All %.2f %d %-10s\n", 35.23, 100, "Dan"); JOptionPane.showMessageDialog(null, str); } }

Input from Keyboard // Import the correct package Import java.util.Scanner // Instantiate a an object from the Scanner class Scanner sc = new Scanner(System.in); // Input the appropriate kind of data int intData = sc.nextInt(); Long longData = sc.nextLong(); Double doubleData = sc.nextDouble(); String stringData = sc.nextLine(); From command line – Not for GUI point and click systems

Inputting Data: More GUI Friendly On the class web-site there is a java class called IO – Download by right clicking on the hyperlink – Store it in your lab folder so it can be used – Compile the.java file to make it useable We can use methods in this class to do input Examples: – int data = IO.readInt(“Enter a grade: ”); – String str = IO.readString(“Enter your name: ”); – double value = IO.readDouble(“Enter sales total:” );

Putting it all together import javax.swing.*; public class Average { public static final int NUM = 3; public static void main(String[] args) { int x = IO.readInt(“enter first: ”); int y = IO.readInt(“enter second: ”); int z = IO.readInt(“enter third: ”); double average = ((double)x + y + z)/NUM; JOptionPane.showMessageDialog (null, “The average is “ + average); } // End of main() method } // End of Average class

What prints? int x = 3; int y = x + 4; x = x + 2; x = x/2; int z = x + 2; String s = " abc " ; System.out.println(s+x + y + z); Remember: instructions execute one by one, in order

Review What is a data type? What is its purpose? Give some examples? What does it mean to cast? What is precision? Give some examples? What is the difference between a variable and an identifier? What is a symbolic constant? Why are they desirable? Can you declare a double and int in the same statement? Why or why not? How would you describe hexadecimal to someone without a clue? What is the difference between the scope and life of a variable? How are {} used in Java? How is ; used in Java? How is [] used? What does static mean? How about public and private? What is an escape sequence? What is concatenation? What are the programming conventions for naming variables, for indenting, and for commenting?