17 Hello world 18 Type: String: literal 19 Standard API: System: out.println() 20 Hello world 21 Statement 22 Statement: simple statements are ended w

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1 List of Slides 1 Title 2 Chapter 2: Sequential execution and program errors 3 Chapter aims 4 Section 2: Example:Hello world 5 Aim 6 Class: programs are divided into classes 7 Class: public class 8 Class: definition 9 Hello world 10 Method: main method: programs contain a main method 11 Method: main method: is public 12 Method: main method: is static 13 Method: main method: is void 14 Method: main method: is the program starting point 15 Command line arguments: program arguments are passed to main 16 Method: main method: always has the same heading 0-0

2 17 Hello world 18 Type: String: literal 19 Standard API: System: out.println() 20 Hello world 21 Statement 22 Statement: simple statements are ended with a semi-colon 23 Hello world 24 The full HelloWorld code 25 Trying it 26 Coursework: HelloWorld in French 27 Section 3: Example:Hello world with a syntactic error 28 Aim 29 Error 30 Error: syntactic error 33 Hello world with a syntactic error 34 Trying it 35 Type: String: literal: must be ended on the same line 36 Trying it 0-1

3 37 Coursework: Fortune syntactic errors 38 Section 4: Example:Hello world with a semantic error 39 Aim 40 Error: semantic error 42 Error: compile time error 43 Hello world with a semantic error 44 Trying it 45 Coursework: ManchesterWeather semantic errors 46 Section 5: Example:Hello solar system 47 Aim 48 Execution: sequential execution 49 Hello solar system 50 Trying it 51 Coursework: HelloFamily 52 Section 6: Example:Hello solar system with a run time error 53 Aim 54 Error: run time error 55 Hello solar system with a run time error 0-2

4 56 Trying it 57 Trying it 58 Trying it 59 Coursework: Quote run time errors 60 Section 7: Example:Hello anyone 61 Aim 62 Command line arguments: program arguments are accessed by inde 63 Type: String: concatenation 64 Hello anyone 65 Trying it 66 Trying it 67 Trying it 68 Trying it 69 Trying it 70 Coursework: FlatterMe 71 Section 8: Example:Hello anyone with a logical error 72 Aim 73 Error: logical error 0-3

5 74 Hello anyone with a logical error 75 Trying it 76 Coursework: Birthday logical errors 77 Section 9: Hello solar system, looking at the layout 78 Aim 79 Code clarity: layout 81 Hello solar system, looking at the layout 82 Code clarity: layout: indentation 84 Coursework: Limerick layout 85 Concepts covered in this chapter 0-4

6 Title Java Just in Time John Latham October 25, 2017 October 25, 2017 Java Just in Time - John Latham Page 1(0/0)

7 Chapter 2 Sequential execution and program errors October 25, 2017 Java Just in Time - John Latham Page 2(0/0)

8 Chapter aims Introduce some very basic Java concepts. Especially sequential execution. Look at kinds of errors we can have in programs. Because you will make errors! You don t need to be afraid of them they are part of the programming experience! October 25, 2017 Java Just in Time - John Latham Page 3(0/0)

9 Section 2 Example: Hello world October 25, 2017 Java Just in Time - John Latham Page 4(0/0)

10 Aim AIM: To introduce some very basic Java concepts, including the main method and System.out.println(). October 25, 2017 Java Just in Time - John Latham Page 5(0/0)

11 Class: programs are divided into classes Program source text separated into pieces called classes. Each piece (usually) stored in separate file. File name is name of class, with.java appended. E.g. HelloWorld in HelloWorld.java. One reason for dividing makes management easier program maybe thousands of lines. Another reason: make sharing between programs easier software reuse helps productivity. Every program has at least one class. Its name reflects intention of the program. Convention: class names start with upper case letter. October 25, 2017 Java Just in Time - John Latham Page 6(0/0)

12 Class: public class A class declared public can be accessed from anywhere in the running Java environment; in particular the virtual machine can access it. Source text starts with reserved word public. A reserved word is part of the Java language e.g. cannot have a program called public. October 25, 2017 Java Just in Time - John Latham Page 7(0/0)

13 Class: definition After public we write reserved word class, then name, then left brace ({), body of text and finally closing right brace (}). public class MyFabulousProgram {... Lots of stuff here. } October 25, 2017 Java Just in Time - John Latham Page 8(0/0)

14 Hello world The heading for our HelloWorld class. 001: public class HelloWorld Then the opening bracket. 002: { October 25, 2017 Java Just in Time - John Latham Page 9(0/0)

15 Method: main method: programs contain a main method All Java programs contain a section of code called main. This is where the computer will start to execute the program. Sections of code are called methods contain instructions how to do something. The main method always starts with following heading. public static void main(string[] args) October 25, 2017 Java Just in Time - John Latham Page 10(0/0)

16 Method: main method: is public The main method starts with reserved word public so virtual machine has access to it. public October 25, 2017 Java Just in Time - John Latham Page 11(0/0)

17 Method: main method: is static The main method has reserved word static. Thus is allowed to be used in the static context. A context is an allocation of computer memory for the program and data, etc.. The virtual machine creates the static context when program is loaded. A dynamic context is a kind of allocation of memory made during run of the program. Main method must be able to run in the static context else program could not be started! public static October 25, 2017 Java Just in Time - John Latham Page 12(0/0)

18 Method: main method: is void A method might calculate and return some result if so we state this in its heading. E.g. method might calculate square root of a number, and return the answer as a number. If it does not we write reserved word void. Void means without contents. The main method does not return a value. public static void October 25, 2017 Java Just in Time - John Latham Page 13(0/0)

19 Method: main method: is the program starting point The program starting part main method is always called main it is main part of program. public static void main October 25, 2017 Java Just in Time - John Latham Page 14(0/0)

20 Command line arguments: program arguments are passed to main Programs can be given command line arguments. So can Java programs. Program arguments are list of text strings. In Java, String[] means list of strings. Must give a name for this list, usually args so we can refer to given data from within program if needed. public static void main(string[] args) October 25, 2017 Java Just in Time - John Latham Page 15(0/0)

21 Method: main method: always has the same heading Java program main methods always have this heading: public static void main(string[] args) Even if we do not intend to use command line arguments. Typical single class program looks like: public class MyFabulousProgram { public static void main(string[] args) {... Stuff here to perform the task. } } October 25, 2017 Java Just in Time - John Latham Page 16(0/0)

22 Hello world Back to HelloWorld : public static void main(string[] args) 004: { October 25, 2017 Java Just in Time - John Latham Page 17(0/0)

23 Type: String: literal A string literal is a fixed piece of text to be used as data. We enclose text in double quotes: "This is a fixed piece of text data -- a string literal" Might be used as a message to the user. October 25, 2017 Java Just in Time - John Latham Page 18(0/0)

24 Standard API: System: out.println() Simplest way to print a message on standard output: System.out.println("This text will appear on standard output"); System is a class in Java s application programming interface (API). Inside System there is a thing called out. This has a method called println. Overall is called System.out.println. It takes a string in its brackets displays it on the standard output. October 25, 2017 Java Just in Time - John Latham Page 19(0/0)

25 Hello world Back to HelloWorld : System.out.println("Hello world!"); Observe semi-colon... October 25, 2017 Java Just in Time - John Latham Page 20(0/0)

26 Statement A command that makes computer perform a task is a statement. E.g. System.out.println("I will output whatever I am told to") October 25, 2017 Java Just in Time - John Latham Page 21(0/0)

27 Statement: simple statements are ended with a semi-colon All simple Java statements must end with semi-colon. a rule of the Java language syntax. Coffee time: Can you think of a reason why Java insists on the programmer putting a semi-colon at the end of statements? October 25, 2017 Java Just in Time - John Latham Page 22(0/0)

28 Hello world Back to HelloWorld : } 007: } October 25, 2017 Java Just in Time - John Latham Page 23(0/0)

29 The full HelloWorld code 001: public class HelloWorld 002: { 003: public static void main(string[] args) 004: { 005: System.out.println("Hello world!"); 006: } 007: } October 25, 2017 Java Just in Time - John Latham Page 24(0/0)

30 Trying it We create source code and compile it. $ ls -l HelloWorld.java Console Input / Output -rw jtl jtl 117 Mar 26 20:09 HelloWorld.java $ javac HelloWorld.java $ ls -l HelloWorld.* -rw jtl jtl 426 Mar 26 20:09 HelloWorld.class -rw jtl jtl 117 Mar 26 20:09 HelloWorld.java $ _ Run We run program to get message on standard output. Console Input / Output $ java HelloWorld Hello world! $ _ Run October 25, 2017 Java Just in Time - John Latham Page 25(0/0)

31 Coursework: HelloWorld in French (Summary only) Write a program to greet the whole world, in French! October 25, 2017 Java Just in Time - John Latham Page 26(0/0)

32 Section 3 Example: Hello world with a syntactic error October 25, 2017 Java Just in Time - John Latham Page 27(0/0)

33 Aim AIM: To introduce the principle of program errors, in particularsyntacticerrors. Wealsoseethatastringliteralmust be ended on the same line its starts on. October 25, 2017 Java Just in Time - John Latham Page 28(0/0)

34 Error To err is Human... when you write source code you will get some things wrong. Lots of rules of Java to obey for a valid program. Being new to it you will break these rules. Even seasoned Java programmers make errors. October 25, 2017 Java Just in Time - John Latham Page 29(0/0)

35 Error: syntactic error When we break syntax rules of Java we have a syntactic error. E.g. omitting closing bracket, semi-colon... Similar to grammatical error in natural language. E.g. sign strapped to back of a poodle... October 25, 2017 Java Just in Time - John Latham Page 30(0/0)

36 Error: syntactic error My other dog an Alsatian. October 25, 2017 Java Just in Time - John Latham Page 31(0/0)

37 Error: syntactic error The compiler gives error messages for syntactic errors. Watch out: compiler can get confused... October 25, 2017 Java Just in Time - John Latham Page 32(0/0)

38 Hello world with a syntactic error 001: public class HelloWorld 002: { 003: public static void main(string[] args) 004: { 005: System.out.println("Hello world!); 006: } 007: } Coffee time: Can you spot the syntactic error? October 25, 2017 Java Just in Time - John Latham Page 33(0/0)

39 Trying it Console Input / Output $ javac HelloWorld.java HelloWorld.java:5: unclosed string literal System.out.println("Hello world!); ˆ HelloWorld.java:5: ; expected System.out.println("Hello world!); ˆ HelloWorld.java:7: reached end of file while parsing } ˆ 3 errors $ _ Run Error messages from compiler can look very scary. Read carefully observe the parts... October 25, 2017 Java Just in Time - John Latham Page 34(0/0)

40 Type: String: literal: must be ended on the same line In Java string literals must end on same line they start on. October 25, 2017 Java Just in Time - John Latham Page 35(0/0)

41 Trying it Coffee time: What has caused the other error message(s)? October 25, 2017 Java Just in Time - John Latham Page 36(0/0)

42 Coursework: Fortune syntactic errors (Summary only) Take a given program that has syntactic errors in it, and get it working. October 25, 2017 Java Just in Time - John Latham Page 37(0/0)

43 Section 4 Example: Hello world with a semantic error October 25, 2017 Java Just in Time - John Latham Page 38(0/0)

44 Aim AIM: To introduce semantic errors and note that these and syntactic errors are compile time errors. October 25, 2017 Java Just in Time - John Latham Page 39(0/0)

45 Error: semantic error A semantic error we obey syntax rules but write something with no meaning (semantics). E.g. another sign, another poodle... October 25, 2017 Java Just in Time - John Latham Page 40(0/0)

46 Error: semantic error My other dog is a Porsche. October 25, 2017 Java Just in Time - John Latham Page 41(0/0)

47 Error: compile time error Java syntactic errors and semantic errors are detected by compiler. Collectively called compile time errors. October 25, 2017 Java Just in Time - John Latham Page 42(0/0)

48 Hello world with a semantic error 001: public class HelloWorld 002: { 003: public static void main(text[] args) 004: { 005: System.out.println("Hello world!"); 006: } 007: } Coffee time: Can you spot the semantic error? October 25, 2017 Java Just in Time - John Latham Page 43(0/0)

49 Trying it $ javac HelloWorld.java Console Input / Output HelloWorld.java:3: cannot find symbol symbol : class Text location: class HelloWorld public static void main(text[] args) ˆ 1 error $ _ Run A little cryptic? Read carefully. You ll get used to it. October 25, 2017 Java Just in Time - John Latham Page 44(0/0)

50 Coursework: ManchesterWeather semantic errors (Summary only) Take a given program that has semantic errors in it, and get it working. October 25, 2017 Java Just in Time - John Latham Page 45(0/0)

51 Section 5 Example: Hello solar system October 25, 2017 Java Just in Time - John Latham Page 46(0/0)

52 Aim AIM: To introduce the principle of sequential execution. October 25, 2017 Java Just in Time - John Latham Page 47(0/0)

53 Execution: sequential execution Programs have many statements in a list. Usually placed on separate lines enhance human readability. Java doesn t care about layout we should. Statements in a list are executed one after the other. Actually compiler turns each into byte codes. The virtual machine executes each collection of byte codes in turn. Known as sequential execution. October 25, 2017 Java Just in Time - John Latham Page 48(0/0)

54 Hello solar system 001: public class HelloSolarSystem 002: { 003: public static void main(string[] args) 004: { 005: System.out.println("Hello Mercury!"); 006: System.out.println("Hello Venus!"); 007: System.out.println("Hello Earth!"); 008: System.out.println("Hello Mars!"); 009: System.out.println("Hello Jupiter!"); 010: System.out.println("Hello Saturn!"); 011: System.out.println("Hello Uranus!"); 012: System.out.println("Hello Neptune!"); 013: System.out.println("Goodbye Pluto!"); 014: } 015: } October 25, 2017 Java Just in Time - John Latham Page 49(0/0)

55 Trying it Console Input / Output $ javac HelloSolarSystem.java $ java HelloSolarSystem Hello Mercury! Hello Venus! Hello Earth! Hello Mars! Hello Jupiter! Hello Saturn! Hello Uranus! Hello Neptune! Goodbye Pluto! $ _ Run October 25, 2017 Java Just in Time - John Latham Page 50(0/0)

56 Coursework: HelloFamily (Summary only) Write a program to greet some of your family. October 25, 2017 Java Just in Time - John Latham Page 51(0/0)

57 Section 6 Example: Hello solar system with a run time error October 25, 2017 Java Just in Time - John Latham Page 52(0/0)

58 Aim AIM: To introduce the principle of run time errors. October 25, 2017 Java Just in Time - John Latham Page 53(0/0)

59 Error: run time error Errors detected when the program is run are run time errors. Java calls them exceptions. Messages can look very cryptic? Read carefully, get used to them. E.g. Exception in thread "main" java.lang.nosuchmethoderror: main Best clue: look either side of the colon (:). October 25, 2017 Java Just in Time - John Latham Page 54(0/0)

60 Hello solar system with a run time error 001: public class HelloSolarSystem 002: { 003: public static void Main(String[] args) 004: { 005: System.out.println("Hello Mercury!"); 006: System.out.println("Hello Venus!"); 007: System.out.println("Hello Earth!"); 008: System.out.println("Hello Mars!"); 009: System.out.println("Hello Jupiter!"); 010: System.out.println("Hello Saturn!"); 011: System.out.println("Hello Uranus!"); 012: System.out.println("Hello Neptune!"); 013: System.out.println("Goodbye Pluto!"); 014: } 015: } Coffee time: What will cause a run time error? October 25, 2017 Java Just in Time - John Latham Page 55(0/0)

61 Trying it It compiles okay. Console Input / Output $ javac HelloSolarSystem.java $ _ Run But when we run it... Console Input / Output $ java HelloSolarSystem Exception in thread "main" java.lang.nosuchmethoderror: main $ _ Run The virtual machine says our program has no main method. Called it Main instead of main! October 25, 2017 Java Just in Time - John Latham Page 56(0/0)

62 Trying it Another example run time error. Console Input / Output $ java HelloMum Exception in thread "main" java.lang.noclassdeffounderror: HelloMum $ _ Run October 25, 2017 Java Just in Time - John Latham Page 57(0/0)

63 Trying it Coffee time: Imagine a version of HelloSolarSystem, called HelloSolarSystemNoArgs, with a lower case m on main, but String[] args has been omitted. Explain the following. Console Input / Output $ javac HelloSolarSystemNoArgs.java $ java HelloSolarSystemNoArgs Exception in thread "main" java.lang.nosuchmethoderror: main $ _ Run October 25, 2017 Java Just in Time - John Latham Page 58(0/0)

64 Coursework: Quote run time errors (Summary only) Take a given program that has run time errors in it, and get it working. October 25, 2017 Java Just in Time - John Latham Page 59(0/0)

65 Section 7 Example: Hello anyone October 25, 2017 Java Just in Time - John Latham Page 60(0/0)

66 Aim AIM: To introduce the principle of making Java programs perform a variation of their task based on command line arguments, which can be accessed via an index. We also meet string concatenation. October 25, 2017 Java Just in Time - John Latham Page 61(0/0)

67 Command line arguments: program arguments are accessed by index The command line arguments given to main method a list of strings from the command line. Each has integer index, starting from zero. To access one, use its index in square brackets. E.g. args[0] is first command line argument. October 25, 2017 Java Just in Time - John Latham Page 62(0/0)

68 Type: String: concatenation The + operator gives concatenation of two strings. E.g. "Hello " + "world" has same value as "Hello world". (Note where space came from.) Most useful with one or more variable values. E.g. "Hello " + args[0] E.g. System.out.println("Hello " + args[0]) Coffee time: When might we concatenate two string literals? October 25, 2017 Java Just in Time - John Latham Page 63(0/0)

69 Hello anyone 001: public class HelloAnyone 002: { 003: public static void main(string[] args) 004: { 005: System.out.println("Hello " + args[0]); 006: } 007: } October 25, 2017 Java Just in Time - John Latham Page 64(0/0)

70 Trying it Console Input / Output $ javac HelloAnyone.java $ java HelloAnyone John Hello John $ java HelloAnyone Lizzy Hello Lizzy $ _ Run What if no argument? Console Input / Output $ java HelloAnyone Exception in thread "main" java.lang.arrayindexoutofboundsexception: 0 at HelloAnyone.main(HelloAnyone.java:5) $ _ Run Observe source name and line number. October 25, 2017 Java Just in Time - John Latham Page 65(0/0)

71 Trying it What if name contains space? Console Input / Output $ java HelloAnyone "John Latham" Hello John Latham $ java HelloAnyone John Latham Hello John $ _ Run Empty string? Console Input / Output $ java HelloAnyone "" Hello $ _ Run October 25, 2017 Java Just in Time - John Latham Page 66(0/0)

72 Trying it D:\JJIT\Example 2.7>dir HelloAnyone.java Volume in drive D is DATA Volume Serial Number is 5C90 0C33 Directory of D:\JJIT\Example /03/ : HelloAnyone.java 1 File(s) 130 bytes 0 Dirs(s) 8,389,459,968 bytes free D:\JJIT\Example 2.7>javac HelloAnyone.java D:\JJIT\Example 2.7>dir HelloAnyone.* Volume in drive D is DATA Volume Serial Number is 5C90 0C33 Directory of D:\JJIT\Example /03/ : HelloAnyone.java 26/03/ : HelloAnyone.class 2 File(s) 716 bytes 0 Dirs(s) 8,389,459,968 bytes free D:\JJIT\Example 2.7>java HelloAnyone "John Latham" Hello John Latham D:\JJIT\Example 2.7>_ October 25, 2017 Java Just in Time - John Latham Page 67(0/0)

73 Trying it October 25, 2017 Java Just in Time - John Latham Page 68(0/0)

74 Trying it October 25, 2017 Java Just in Time - John Latham Page 69(0/0)

75 Coursework: FlatterMe (Summary only) Write a program to say how wonderful the user is. October 25, 2017 Java Just in Time - John Latham Page 70(0/0)

76 Section 8 Example: Hello anyone with a logical error October 25, 2017 Java Just in Time - John Latham Page 71(0/0)

77 Aim AIM: To introduce the principle of logical errors. October 25, 2017 Java Just in Time - John Latham Page 72(0/0)

78 Error: logical error Most tricky kind of error logical error. No help from compiler, nor virtual machine. Code is meaningful to Java. But program does not do what we want! Java is too stupid to know that. Subtle ones slip through our testing. i.e. bugs. October 25, 2017 Java Just in Time - John Latham Page 73(0/0)

79 Hello anyone with a logical error 001: public class HelloAnyone 002: { 003: public static void main(string[] args) 004: { 005: System.out.println("Hello + args[0]"); 006: } 007: } Coffee time: Can you spot the logical error? October 25, 2017 Java Just in Time - John Latham Page 74(0/0)

80 Trying it Compiles and runs without error. Console Input / Output $ javac HelloAnyone.java $ java HelloAnyone John Hello + args[0] $ _ Run October 25, 2017 Java Just in Time - John Latham Page 75(0/0)

81 Coursework: Birthday logical errors (Summary only) Take a given program that has logical errors in it, and get it working. October 25, 2017 Java Just in Time - John Latham Page 76(0/0)

82 Section 9 Hello solar system, looking at the layout October 25, 2017 Java Just in Time - John Latham Page 77(0/0)

83 Aim AIM: To begin to explore the decisions behind the way we lay out the source code for a program. October 25, 2017 Java Just in Time - John Latham Page 78(0/0)

84 Code clarity: layout Java doesn t care about layout white space must separate symbols that would be one symbol otherwise. E.g. public void would be publicvoid. Could put program on one line, minimum space. public class HelloSolarSystem{public static void main(string[]args){system.out.println("hello Mercu Or split just to fit on page. public class HelloSolarSystem{public static void main(string[]args){ System.out.println("Hello Mercury!");System.out.println( "Hello Venus!");System.out.println("Hello Earth!");System.out.println ("Hello Mars!");System.out.println("Hello Jupiter!");System.out. println("hello Saturn!");System.out.println("Hello Uranus!");System. out.println("hello Neptune!");System.out.println("Goodbye Pluto!");}} October 25, 2017 Java Just in Time - John Latham Page 79(0/0)

85 Code clarity: layout Layout important for human reader. Take pride in making your work most readable. Split lines in good places. Use indentation to show structure. October 25, 2017 Java Just in Time - John Latham Page 80(0/0)

86 Hello solar system, looking at the layout 001: public class HelloSolarSystem 002: { 003: public static void main(string[] args) 004: { 005: System.out.println("Hello Mercury!"); 006: System.out.println("Hello Venus!"); 007: System.out.println("Hello Earth!"); 008: System.out.println("Hello Mars!"); 009: System.out.println("Hello Jupiter!"); 010: System.out.println("Hello Saturn!"); 011: System.out.println("Hello Uranus!"); 012: System.out.println("Hello Neptune!"); 013: System.out.println("Goodbye Pluto!"); 014: } 015: } New line after class heading. New line plus indentation 2 or3spaces formainmethod. New line, same indentation. More indentation, each statement on own line. Line up with opening braces. October 25, 2017 Java Just in Time - John Latham Page 81(0/0)

87 Code clarity: layout: indentation A class contains nested structures: class has heading and body body has main method main method has heading and body body has statements. Use indentation to show structure the more nested, the more space. Be consistent: always same number of spaces per nesting two or three is good. don t use tabs! Opening and closing braces have same indentation. October 25, 2017 Java Just in Time - John Latham Page 82(0/0)

88 Code clarity: layout: indentation Some people prefer this style subjectively less clear? public class HelloWorld { } public static void main(string[] args) { } System.out.println("Hello world!"); October 25, 2017 Java Just in Time - John Latham Page 83(0/0)

89 Coursework: Limerick layout (Summary only) Take a given program and lay it out properly. October 25, 2017 Java Just in Time - John Latham Page 84(0/0)

90 Concepts covered in this chapter Each book chapter ends with a list of concepts covered in it. Each concept has with it a self-test question, and a page reference to where it was covered. Please use these to check your understanding before we start the next chapter. October 25, 2017 Java Just in Time - John Latham Page 85(0/0)