Introduction to Programming (Java) 2/12

Transcription

1 Introduction to Programming (Java) 2/12 Michal Krátký Department of Computer Science Technical University of Ostrava Introduction to Programming (Java) 2008/2009 c Michal Krátký Introduction to Programming (Java) 1/34

2 James Gosling, Sun Microsystems, The original name Oak, Java in The first version was released in 1996 (JDK 1.0). Features: object-oriented language strong typed language it includes garbage collector it supports parallel threads it supports exceptions safe programming language c Michal Krátký Introduction to Programming (Java) 2/34

3 JDK and IDEs JDK Java Platform, the current version NetBeans, IDE. Eclipse, IDE. quick compiler. c Michal Krátký Introduction to Programming (Java) 3/34

4 References Slides for this course: R. Szturc: Introduction to Programming (Java). Department of Computer Science, VŠB Technical University of Ostrava, 2004, B. Eckel: Thinking in Java. 2002, Sun MicroSystems: Sun Developer Network. 2007, A lot of books. c Michal Krátký Introduction to Programming (Java) 4/34

5 Compilation vs Interpretation of Source Code Compiler a source code is compiled in an executable file. Interpreter statements are executed in the runtime. Java utilizes a combination of both issues. Code (*.java file) is compiled to the byte-code (*.class file). The class files are interpreted by the java virtual machine (javavm). The virtual machine is represented by the java executable file. Libraries (jar archives) must be defined in the CLASSPATH variable (the variable of the OS). c Michal Krátký Introduction to Programming (Java) 5/34

6 Java Platform Platform hardware and software environment for a program runtime. Java platform a software platform over a hardware platform. Java API Java Virtual Machine Operation System Hardware Java Platform Java API a set of classes input/output, networking, databases and so on. c Michal Krátký Introduction to Programming (Java) 6/34

7 Example 1.1 public class Example0101 { public s t a t i c void main ( S t r i n g [ ] args ) { System. out. p r i n t l n ( " Hello World! \ n " ) ; } } c Michal Krátký Introduction to Programming (Java) 7/34

8 Compilation and Start-up of a Program Compilation: javac Example0101.java Start-up: java Example0101 The start-up with the redirection of the standard output into the file file.txt: java Example0101 > file.txt c Michal Krátký Introduction to Programming (Java) 8/34

9 Compiling and Running Program in an IDE We must create a project in an IDE (Integrated Development Environment). Source files are added into this project. One file is labeled as the main file. It means that the main() method of this file starts a program. c Michal Krátký Introduction to Programming (Java) 9/34

10 HelloWorld in NetBeans c Michal Krátký Introduction to Programming (Java) 10/34

11 Debugging Efficient Finding of Bugs 1/2 Debugging is the sole efficient tool for finding of bugs in a source code. We can utilize a command-line debugger or an IDE s debugger (NetBeans, Eclipse and so on). We add breakpoints on lines, run-time of a program is stopped on these lines. c Michal Krátký Introduction to Programming (Java) 11/34

12 Debugging Efficient Finding of Bugs 2/2 Functions: Step Into - a step into a method Step Over - a step on the next line without a step into a method Run - program is run until the next breakpoint is reached Step Out - a step out from the current method We can look: Stack Trace - a stack of called methods values of local and global variables c Michal Krátký Introduction to Programming (Java) 12/34

13 Syntax vs Semantics Syntax it defines constructs to be possible in the language. Which program is or is not correct. Semantics it defines the meaning of constructs. E.g., what is executed after a statement. Two types of errors: An error during a compilation it is reported by a compiler. A syntactic error e.g. missing ; A semantic error e.g. incompatible data type is assigned. An error in run-time it is reported by the virtual machine during the run-time. We catch these errors by exceptions. c Michal Krátký Introduction to Programming (Java) 13/34

14 A program consists of classes. Each class includes data (attributes) and methods. Methods consists of the header (name, parameters, and return type) and body with statements. Statements work with variables. Performance of statements includes the statement evaluation. Each variable, value or expression has a data type. The program is a sequence of lexical symbols at the lowest level. c Michal Krátký Introduction to Programming (Java) 14/34

15 Lexical Symbols White characters nad remarks: White space: space (SP), horizontal tab (HT), form feed (FF), newline (LF), carriage return (CR). Remarks: /* remark */ // remark Basic types of lexical symbols: identifiers: i j System9 number_of_elements keywords: while float int public class literals: 124 true d "hello" delimiters: ( ) { } [ ] ; :,. operators: + / && = = < >>= c Michal Krátký Introduction to Programming (Java) 15/34

16 Example 2.1 public class Example0201 { public s t a t i c void main ( S t r i n g [ ] args ) { i n t i = 5 ; i n t j = i + 3 ; System. out. p r i n t l n ( " i = " + i + ", j = " + j + " \ n " ) ; } } i n t d=0xff ; / / hexadecimal value i n t e =077; / / o c t a n t value System. out. p r i n t l n ( " d= " + d + ", e= " + e + " \ n " ) ; / / d=255, e=63 c Michal Krátký Introduction to Programming (Java) 16/34

17 Types, values, and variables Data is stored in variables. Each variable is specified by the name and data type (so called declaration). We distinguish two types of data types: Primitive data types. A variable of primitive data type contains the one value of the defined size: boolean b = true; // boolean type int i = 456; // integer double f = ; // real number References Address of an object (the instance of a class) or array in memory: Hashtable h = new Hashtable(); int[] a = new int[20]; c Michal Krátký Introduction to Programming (Java) 17/34

18 Integer Data Types 1/2 Type Range Size [b] byte short int long char c Michal Krátký Introduction to Programming (Java) 18/34

19 Integer Data Types 2/2 Operators: Comparison operators: <, <=, >, >=, ==,! = Unary plus and minus: +, Binary arithmetic operators: +,,, /, % Prefix and postfix operators for +1 and -1 operations, respectively: ++, Signed and unsigned bit shifts: <<, >>, >>> A bit complement: ~ Bit operators: & (AND), (OR), ^ (XOR) c Michal Krátký Introduction to Programming (Java) 19/34

20 Assignment Operator An expression: expr1 = expr2. An evaluation: Evaluation of the left side (expr1). Evaluation of the right side (expr2). The value of the right side is stored into the variable of the legth side. The value of the whole expression is the value of the right side. int b = c + 1; a[i++] = x + y; c Michal Krátký Introduction to Programming (Java) 20/34

21 Example 2.3 public class Example0203 { public s t a t i c void main ( S t r i n g [ ] args ) { i n t i = 5 ; i + + ; / / i = 6 i n t j = i + 3; / / i = 5, j = 9 j = i + 3 ; / / i = 4, j = 7 j + = 6 ; / / j = 1 3 boolean b = ( j = = 1 ) ; / / b = f a l s e ; System. out. p r i n t l n ( " i = " + i + ", j = " + j + ", b= " + b + " \ n " ) ; } } c Michal Krátký Introduction to Programming (Java) 21/34

22 Example 2.4 public class Example0204 { public s t a t i c void main ( S t r i n g [ ] args ) { i n t i = 1 ; / / i = 0001 i = i < < 1 ; / / i = 2 ( ) i n t j = 1 ; i n t k = i j ; / / k = 3 ( ) k = i & j ; / / k = 0 System. out. p r i n t l n ( " i = " + i + ", j = " + j + ", k= " + k + " \ n " ) ; } } c Michal Krátký Introduction to Programming (Java) 22/34

23 Example 2.5 1/2 public class Example0205 { public s t a t i c void main ( S t r i n g args [ ] ) { i n t a = ~ 0 ; / / a = 1, a = a = a < < 1 ; / / a = a = 1; i n t aa = a > > 1 ; / / aa = 1 i n t ab = a > > > 1; / / ab = / / ab = c Michal Krátký Introduction to Programming (Java) 23/34

24 Example 2.5 2/2 a = 3 < < 3 0 ; / / a = aa = a > > 1 ; / / aa = } } i n t b = 2 0, c = 3 ; i n t d = b / c ; / / d = 6 i n t e = b % c ; / / e = 2 ; c Michal Krátký Introduction to Programming (Java) 24/34

25 Real Data Types 1/2 Real number form: s m 2 exp. Type s m exp Size [b] float {-1,1} double {-1,1} Type Min Value Max Value float e-45f e+38f double e e+308 c Michal Krátký Introduction to Programming (Java) 25/34

26 Real Data Types 2/2 Operators: Comparison operators: <, <=, >, >=, ==,! = Unary plus and minus: +, Binary arithmetic operators: +,,, / Prefix and postfix operators for +1 and -1 operations, respectively: ++, c Michal Krátký Introduction to Programming (Java) 26/34

27 Example 2.6 public class Example0206 { public s t a t i c void main ( S t r i n g args [ ] ) { double f = 1. 0 ; f = f ; / / f = f = f / 5. 0 ; / / f = 4. 0 f ; / / f = 3. 0 } } c Michal Krátký Introduction to Programming (Java) 27/34

28 Bool Data Type Bool data type has two possible values: true and false. Operators: Comparison operators: ==,! = A logical complement:! Binary logical operators: &,, ˆ Conditional AND and OR operators: &&, Ternary conditional operator:?: Boolean expressions controls these constructs: if while do for c Michal Krátký Introduction to Programming (Java) 28/34

29 Example 2.7 1/2 public class Example0207 { public s t a t i c void main ( S t r i n g [ ] args ) { boolean f l a g = true ; f l a g =! f l a g ; / / f l a g = f a l s e boolean bvar = true ; boolean r1 = f l a g bvar ; / / r1 = t r u e boolean r2 = f l a g & bvar ; / / r2 = f a l s e i f ( r1 r2 ) / / t r u e or f a l s e = t r u e { System. out. p r i n t l n ( " r1 or r2 = t r u e " ) ; } c Michal Krátký Introduction to Programming (Java) 29/34

30 Example 2.7 2/2 double f V a l = r1? 1. 0 : 2. 0 ; / / f V a l = 1. 0 } } System. out. p r i n t l n ( " f l a g = " + f l a g + ", bvar= " + bvar + ", f V a l = " + f V a l + " \ n " ) ; c Michal Krátký Introduction to Programming (Java) 30/34

31 Compound Assignment Operators =, / =, % =, + =, =, <<=, >>=, & =, = ˆ = The meaning of the expr1 op= expr2 is the same as expr1 = expr1 op expr2. But then expr1 is evaluated only one times. x *= 6; x = x * 6; a[i++] += 3; is not equivalent with a[i++] = a[i++] + 3; c Michal Krátký Introduction to Programming (Java) 31/34

32 Casting Default casting: byte b ; i n t i ;... i = b ; b = i; is wrong. We must use the expression: (type)expr1, which transforms the type of the expression value at the type. b = (byte)i; c Michal Krátký Introduction to Programming (Java) 32/34

33 Priority of Operators from the highest priority 1. () 2. [], postfix ++ and 3. unary + and, ~,!, casting, prefix ++ and 4., /, % 5. +, 6. <<, >>, >>> 7. <, >, <=, >=, instanceof 8. ==,! = 9. & 10. ˆ && ? : 15. =, =, / =, % =, + =, =, <<=, >>=, >>>=, & =, ˆ =, = c Michal Krátký Introduction to Programming (Java) 33/34

34 Associativity of Operators The majority of the binary operators is associative from the left. Therefore: a + b + c has the same meaning as: (a + b) + c Some operators are associative form the right. a = b = c has the same meaning as: a = (b = c) c Michal Krátký Introduction to Programming (Java) 34/34