Object-Oriented Programming. Topic 2: Fundamental Programming Structures in Java

Transcription

1 Electrical and Computer Engineering Object-Oriented Topic 2: Fundamental Structures in Java Maj Joel Young 8-Sep-03 Maj Joel Young

2 Java Identifiers Identifiers Used to name local variables Names of attributes Names of classes Primitive Data Types Available in Java (size in bytes) byte (1), -128 to 127 short (2), to int (4), to long (8), to float (4), -3.4E38 to 3.4E38, 7 digit precision double (8), -1.7E308 to 1.7E308, 17 digits precision char (2), unicode characters boolean (, ), discrete values 2

3 Java Identifiers Naming Rules Must start with a letter After first letter, can consist of letters, digits (0,1,,9) The underscore _ and the dollar sign $ are considered letters Variables All variables must be declared in Java Can be declared almost anywhere (scope rules apply) Variables have default initialization values Integers: 0 Reals: 0.0 Boolean: False Variables can be initialized in the declaration 3

4 Java Identifiers Example Declarations int speed; // integer, defaults to 0 int speed = 100; // integer, init to 100 long distance = L; // L needed for a long float delta = 25.67f; // f needed for a float double delta = 25.67; // Defaults to double double bigdelta = 67.8E200d; // d is optional here boolean status; // defaults to boolean status = ; Potential Problems (for the C/C++ crew) long double delta = 3.67E204; // No long double in Java unsigned int = ; // No unsigned ints in Java 4

5 Java Types Arrays int[] numbers = new int[n] // Array of integers, size is n size can be computed at run time, but can't be changed allocated on heap (thus enabling run time size allocation) invalid array accesses detected at run time (e.g. numbers[6];) numbers.length; // read only variable specifying length of array reference semantics int[] winning_numbers; winning_numbers = numbers; // refer to same array numbers[0] = 13; // changes both 5

6 Java Types Strings String message = "Error " + errnum; strings are immutable can't be changed, although variables can be changed (and old string left for garbage collection) message = "Next error " + errnum2; use StringBuffer to edit strings StringBuffer buf = new StringBuffer(greeting); buf.setcharat( 4, '?'); greeting = buf.tostring(); 6

7 Java Types Strings String comparison if (greeting == "hello" ). // error, compares location only if ( greeting.equals("hello")). // OK string1.compareto(string2) // negative if string1 < string 2; // zero when equal, // positive if string1 > string2 string1.substring(2, 6); // return substring between position 2 and 5 7

8 Java Operators Operators/Order of Precedence Operator () or [ ] or. or ++ or or or ~ * or / or % + or - <<,>> & ^ Parens, array indices, object invocation, increment, decrement Unary plus, unary minus, complement Multiplication, division, modulus Addition, subtraction Bitwise shifts Bitwise AND Bitwise XOR Bitwise OR Operation Precedence

9 Java Operators Example Usage int anint1 = 2, anint2 = 1, anint3; double adbl1; anint3 = anint1 + anint2 * 4; // anint3 gets 6 anint3 = (anint1 + anint2) * 4; // anint3 gets 12 anint3 = ++anint2 * 2; // anint3 gets 4 anint2 = 1; anint3 = anint2++ * 2; // anint3 gets 2 anint2 = 1; anint3 = anint1 & anint2; // anint3 gets 0 anint3 = anint1 ^ anint2; // anint3 gets 3 anint3 = anint2 << 1; // anint3 gets 2 anint3 = 1 / 2; // anint3 gets 0 adbl1 = 1 / 2; // adbl1 gets 0.0 adbl1 = 1.0 / 2.0; // adbl1 gets 0.5 9

10 Java Statements Import Statements (discussed later) Class/Interface Declarations (discussed later) Assignments Conditionals Loops Miscellaneous (mixed in other discussions) 10

11 Java Statements Assignments General Format: variable = expression ; Where variable is a previously declared identifier and expression is a valid combo of identifiers, operators, and method (a.k.a. procedure or function) calls Shortcuts: var *= expr ; // Equivalent to var = var * (expr); var /= expr ; // Equivalent to var = var / (expr); var += expr ; // Equivalent to var = var + (expr); var -= expr ; // Equivalent to var = var (expr); var %= expr ; // Equivalent to var = var % (expr); var++; // Equivalent to var = var + 1; var--; // Equivalent to var = var - 1; 11

12 Java Conditional Constructs if Statements if with code block if (boolean_expr) if with single statement if (boolean_expr) statement; 12

13 Java Conditional Constructs if Statements (Continued) if-else if (boolean_expr) for else for 13

14 Java Conditional Constructs Boolean Expressions Boolean expressions use conditional operators such that they result in a value of or Conditional Operators (Not by order of precedence) Operator == or!= > or < >= or <=! & or && or Operation Equality, not equal Greater than, less than Greater than or equal, less than or equal Unary negation (NOT) Evaluation AND, short circuit AND Evaluation OR, short circuit OR 14

15 Java Conditional Constructs Boolean Expression Examples int i1 = 1, i2 = 2, i3 = 3, i4 = 0; boolean b1 =, b2 =, b3 = ; i2 > i1 // (i3 i2) > i1 // (i3 i2) >= i1 // (i2 > i1) & (i2 < i3) // (i2 < i1) (i2 > i1) // i2!= i1 // (i1 < i2) ((i1/i4) > 1) // Divide by 0 exception (i1 < i2) ((i1/i4) > 1) // (i1 < i2) (i1++ > 1) //, i1 contains 2 (i1 < i2) (i1++ > 1) //, i1 contains 1 b1 && b2 && b3 // (b1 b2) && b3 // b1 && (i1 == (i3 i2)) // 15

16 Java Conditional Constructs if-else Statement Example class Example static public void main(string args[]) // A very contrived example int i1 = 1, i2 = 2; System.out.print( Result: ); if (i1 > i2) System.out.println( i1 > i2 ); else System.out.println( i2 >= i1 ); 16

17 Java Conditional Constructs The Switch Statement switch (integer_expression) case int_value_1: break; case int_value_2: break; case int_value_n: break; default: 17

18 Don t forget the break switch (integer_expression) case int_value_1: // No break! case int_value_2: break; case int_value_n: break; default: 18

19 Java Conditional Constructs Example int n = 5; switch (n) case 1: n = n + 1; break; case 5: n = n + 2; break; default: n = n 1; 19

20 Java Conditional Constructs Example char c = b ; int n = 0; switch (c) case a : n = n + 1; break; case b : n = n + 2; break; default: n = n 1; 20

21 Java Looping Constructs while loop Exit condition evaluated at top do loop Exit condition evaluated at bottom for loop Exit condition evaluated at top Includes a initialization Includes a update for each iteration 21

22 Java Looping Constructs while loop while (boolean_expr) do loop do while (boolean_expr) 22

23 Java Looping Constructs for loop for (init_stmnt; bool_expr; update_stmnt) init init update update 23

24 Java Looping Constructs for loop for (init_stmnt; bool_expr; update_stmnt) init init update update 24

25 Java Looping Constructs class Example static public void main(string args[]) int i = 0; System.out.println("while loop"); while (i < 10) System.out.println(i); i++; System.out.println("do loop"); do System.out.println(i); i--; while (i > 0); System.out.println("for loop"); for (i = 0; i < 10; i++) System.out.println(i); // End main // End Example 25

26 Homework Go to: Explore the Java API Docs and Tutorials Go to: Explore: Read the docs for the String class Read the docs for the Integer class Read the docs for the System class Enter the code from the Example class (cut and paste from pdf or ppt) and compile and run it. 26