lecture02.pdf

Transcript

1. Session 2 ICS 211 Arrays
2. Memory Upload • Primitive data types & objects • Declare (make space for) arrays • Instantiate (create) arrays • Initialize (give values to) arrays • Out of Bounds Error (remember that computers start counting at zero) • jGRASP IDE and example programs
3. Java Language Basics • You should already know the Java language basics  Objects, primitive data types, conditional statements, loops, methods, arrays, exceptions, file I/O, classes, inheritance, and polymorphism
4. Please Review! • Don’t have to know every single detail, but you should have a general understanding • If your Java is rusty, please review the basics  For example, a seven year gap
5. Primitives vs. Objects • How do our programs store data? • The Java language uses both objects and primitive data types to store data  Primitive data types are the basic way of storing data (building blocks)  Objects are composed of primitive data types
6. Primitives & Java Language 1. Primitive data types are a part of the Java language  When folks at Sun Microsystems made the Java language, primitive data types were an essential component  Note that Java is now owned by Oracle
7. Objects & Java Language 1. The blueprint for objects are the classes from Java API (Application Programming Interface), so objects are not part of the Java language  See the link from the class web site to the Java API
8. Creating Primitives 2. Create a primitive data type by using name of the primitive data type int number = 1024;
9. Creating Objects 2. Instantiate (create) an object using constructors String name = new String("Nami");
10. Using Primitives 3. Use operators to manipulate data for primitive data types  For example, asterisk (*) is the multiplication operator number = number * 2;
11. Using Objects 3. Use methods to access and manipulate data for objects  For example, the substring method String name = new String("Nami"); String firstLetter; firstLetter = name.substring(0,1); //firstLetter is character "N"
12. Remember! firstLetter = name.substring(0,1); • REMEMBER: Computers are a bit funky! • They start counting at zero (0), NOT one (1) •Robot sports teams often shout “We’re number 0!”
13. Variables & Primitives 4. A variable for a primitive data type contains the primitive data type’s data  Visually represented by a box containing data int number = 1024; number 1024
14. Variables & Objects 4. A variable for an object contains an address that points to the object’s data  Visually represented by a box pointing to the data String name = new String("Nami"); name Nami
15. Wrapper Classes • Wrapper classes are the corresponding classes for the eight (8) primitive data types • Often wrapper classes are used to store data in a data structure
16. Primitives & Wrappers • Primitive data type Wrapper class 1. byte Byte 2. short Short 3. int Integer 4. long Long 5. float Float 6. double Double 7. char Character 8. boolean Boolean
17. Autoboxing • Autoboxing automatically converts from a primitive data type to a wrapper object int w = 1024; Integer x = w; //autoboxing w 1024 x 1024
18. Unboxing • Unboxing automatically converts from a wrapper object to a primitive data type Integer y = new Integer(108); int z = y; //unboxing y 108 x 108
19. Arrays • An array is an example of a very simple data structure  A data structure is a way to organize and store data
20. Arrays • An array is a numbered list of data of the same type  Numbers from 0, 1, 2, 3, … • Easy way to store lots of data
21. Terminology • Array index (subscript)  A number corresponding to the position of an element in an array • Numbered from 0, 1, 2, . . . N-1, where N is the size of the array
22. Terminology • Array element  The value that is stored in an array
23. Array of Integers examScores 0 80 1 77 2 92 3 55 4 80 5 95 index address element
24. Array of Doubles finalGrades 0 83.33 1 73.56 2 91.29 3 55.55 4 67.02 5 83.33 index address element
25. Declaring Arrays • Syntax for an array declaration  DataType [] arrayName; • Example Java code  Integer [] examScores;  Double [] finalGrades;
26. Declaring Arrays • A declaration creates a variable that is used to store the address to an array, but not the array itself  Since arrays are objects in Java, you must declare an array variable (which will later contain the address to the location of the array object)
27. Declaring Arrays • These declarations create a reference variable with a null value (does not point to an object)  Integer [] examScores; examScores null  Double [] finalGrades; finalGrades null
28. Instantiating Arrays • Syntax for array instantiation (creation)  arrayName = new DataType[SIZE]; • SIZE is the number of elements in the array
29. Instantiating Arrays • Example Java code  final Integer SIZE = 6;  examScores = new Integer[SIZE];  finalGrades = new Double[SIZE];
30. Instantiating Arrays • Instantiating an array creates an array object & stores the array object’s address in a variable • The array object itself will contain no addresses, because elements have not been assigned to the array yet
31. Example Array Instantiation • Array of integers examScores 0 null 1 null 2 null 3 null index 4 null address 5 null • Array of doubles finalGrades 0 null 1 null 2 null 3 null 4 null 5 null
32. Initializing Arrays • Syntax for initializing arrays  arrayName[i] = value; • Where i is the index of an element in the array • Example Java code  examScores[0] = 80;  examScores[1] = 77;  examScores[2] = 92;
33. Initializer List • You can also declare, instantiate, & initialize an array with one statement • Syntax  DataType [] arrayName = {value0, value1, value2, . . ., valueN-1}; • The size of the array is automatically calculated
34. Initializer List • Example Java code  Double [] finalGrades = {83.33, 73.56, 91.29, 55.55, 67.02,83.33}; • Double precision floating point is a format for decimal number by the IEEE (Institute of Electrical and Electronics Engineers)
35. Array Initialization examScores 0 80 1 77 2 92 3 55 4 80 5 95 index address element
36. Array Initialization finalGrades 0 83.33 1 73.56 2 91.29 3 55.55 4 67.02 5 83.33 index address element
37. Array Length • Size of an array is stored in a public variable called length  length is not a method!  Bug: array.length()  Useful in loops for(int i=0;i<examScores.length;i++){ System.out.print(examScores[i]+ ", "); }
38. Out of Bounds Error • What if an array index is greater or less than range of the array?  An exception will be thrown & the program will end
39. Out of Bounds Error • Example Java code for(int i=0;i<=examScores.length; i++){ System.out.print(examScores[i]); } • Program will end and display a message about an ArrayIndexOutOfBoundsException • This is bad
40. Out of Bounds Error • Since arrays start their index numbering at zero (0), a common bug is to be “off by one”  Range for arrays is 0 to SIZE-1 (length-1)
41. Example Code • See InitializingArrays.java  This program shows how to • Declare arrays • Instantiate arrays • Initialize arrays • Display arrays
42. Three Steps • How are we going to run this program!? 1.Edit a program with an editor  Write program in Java language & store on computer • Creates Program.java (text file)
43. Three Steps 2. Compile a program with a compiler  Check for errors & create Java bytecode • Creates Program.class (bytecode file)
44. Three Steps 3. Run (execute) a program with an interpreter  Translate each bytecode to machine code & implement machine code • Creates screen output
45. Software Installation • See the ICS 211 class webpage for links to sites to download the software  JDK (Java Development Kit) • Used to compile programs • Also includes JRE (Java Runtime Environment) which is the interpreter used to run programs
46. Software Installation • We also need an IDE (Integrated Development Environment) to edit, compile and run programs  jGRASP (Graphical Representations of Algorithms, Structures and Processes)
47. Software Installation  jGRASP (Graphical Representations of Algorithms, Structures and Processes) • Used in education & research • Made at Auburn University in Alabama! • Always use jGRASP for assignments in ICS 211
48. Java Coding Standard • In ICS 211, we have a Java Coding Standard that should be followed when writing programs  The main purpose is to make sure the programs are neat and easy to understand • See the “links” column of the class webpage
49. Java Coding Standard • Basic Coding Rules to Follow 1. Use descriptive and appropriate names for all identifiers (variables, method names, class names, constants, etc.) 2. Comment every 3-7 lines of code 3. Be neat
50. Example Code II • See ArraysLoopsModulus.java  This program shows how to • Initialize an array with a loop • Examples of integer division and modulus  Modulus is the remainder of a division • For example: 10/7 = 1 remainder 3 • 27/5 = 5 remainder 2
51. Memory Defragmenter • Primitive data types & objects • Declare (make space for) arrays • Instantiate (create) arrays • Initialize (give values to) arrays • Out of Bounds Error (remember that computers start counting at zero) • jGRASP IDE and example programs
52. Task Manager • Before the next class, you need to: 1.Do the assignment corresponding to this lecture 2.Download necessary software 3.Email me any questions you may have about the material 4.Turn in the assignment before the due date