Programming Fundamentals. Java Boot Camp Module 1

Transcription

2 Objectives At the end of the lesson, the student should be able to: Identify the basic parts of a Java program Differentiate among Java literals, primitive data types, variable types,identifiers and operators Develop a simple valid Java program using the concepts learned in this chapter

3 Dissecting my First Java Program

4 Dissecting my First Java Program indicates the name of the class which is SampleOne In Java, all code should be placed inside a class declaration The class uses an access specifier public,, which indicates that our class in accessible to other classes from other packages (packages are a collection of classes). We will be covering packages and access specifiers later.

5 Dissecting my First Java Program The curly brace { indicates the start of a block. There should be a corresponding closing curly brace }to end the block.

6 Dissecting my First Java Program The next three lines are a Java comment.

7 Dissecting my First Java Program The main method is the starting point of a Java program. Make sure to follow the exact signature and note that it has a closing curly brace.

8 Dissecting my First Java Program The next line is another Java comment, a single- line comment.

9 Dissecting my First Java Program The command System.out.println(),, prints the text enclosed by quotation on the screen.

10 Best Practices Your Java programs should always end with the.java extension. Filenames should match the name of your public class. So for example, if the name of your public class is Hello,, you should save it in a file called Hello.java. You should write comments in your code explaining what a certain class does, or what a certain method do.

11 Java Comments Comments These are notes written to a code for documentation purposes. These notes are not part of the program and do not affect the flow of the program. 3 Types of comments in Java C++ Style Comments C Style Comments Javadoc Comments

12 Java Comments C++-Style Comments C++ Style comments start with // All the text after // is treated as a comment For example:

13 Java Comments C-Style Comments C-style comments, also called multiline comments start with a /* and end with a */. All text in between the two delimiters is treated as a comment. Unlike C++ style comments, it can span multiple lines. For example:

14 Java Comments Javadoc Comments Javadoc comments are used for generating HTML documentation for your Java programs. You can create javadoc comments by starting the line with /** and ending it with */.

15 Java Statements Statement one or more lines of code terminated by a semicolon.

16 Java Blocks Block is one or more statements bounded by opening and closing curly braces that group the statements as one unit. Block statements can be nested indefinitely. Any amount of whitespace is allowed.

17 Java Statements and Blocks Best Practice You should indent block-enclosed statements four spaces after the start of a block. For example:

18 Java Identifiers Identifiers are tokens that represent names of variables, methods, classes, and other user-defined program elements. Examples of identifiers are: Hello, main, System, out. Java identifiers are case-sensitive. This means that the identifier Hello is not the same as hello.

19 Java Identifiers Identifiers must begin with either a letter, an underscore _, or a dollar sign $. Letters may be lower or upper case. Subsequent characters may use numbers 0 to 9. Identifiers cannot use Java keywords like class, public, void,, etc. We will discuss more about Java keywords later.

20 Java Identifiers Best Practices For names of classes,, capitalize the first letter of the identifier. For example: ThisIsAnExampleOfClassName For names of methods and variables,, the identifier should start with a lower-case letter. For example: thisisanexampleofmethodname When creating multi-word identifiers, use capital letters to indicate the start of each word except the first word. For example, chararray, filenumber, ClassName.

21 Java Identifiers Best Practices For constants use all capitals and separate words with underscores. For example: EXAMPLE_OF_A_CONSTANT Avoid using underscores at the start of the identifier such as _read or _write.

22 Java Keywords Keywords are predefined identifiers reserved by Java for specific purposes. You cannot use keywords as identifiers for any user-defined elements such as variables, methods, classes, and constants. The next slide contains the list of the Java Keywords.

23 Java Keywords

24 Java Literals Literals are tokens that do not change - they are constant. The different types of literals in Java are: Integer Literals Floating-Point Literals Boolean Literals Character Literals String Literals

25 Primitive Data Types The Java programming language defines eight primitive data types. Logical boolean Textual char Numerical byte short int long (integral) double float (floating point).

26 Primitive Data Types: Logical-boolean A boolean data type represents one of two states: true and false. An example is The example declares a boolean type variable named result and assigns it a value of true.

27 Primitive Data Types: Textual-char A character data type (char( char), represents a single Unicode character. It must have its literal enclosed in single quotes( ). To represent special characters like ' (single quotes) or " (double quotes), use the escape character \.

28 Primitive Data Types: Textual-char The following literals all refer to the same character: Hexadecimal

29 Primitive Data Types: Textual-char String is not a primitive data type, but due to its very common use we will introduce String in this section. A String is a data type composed of multiple characters. As such it is not a primitive data type, it is a class. It has its literals enclosed in double quotes( ).

30 Primitive Data Types: Integral byte, short, int & long Integral data types have the following ranges:

31 Primitive Data Types: Integral byte, short, int & long Integral data types in Java can take values in three forms decimal, octal or hexadecimal. Integral types have int as default data type. You can define a long value by appending the letter l or L (upper or lowercase 'L')

32 Primitive Data Types: Floating Point float and double Floating-point data types have the following specs: type bits accuracy range represented (approximate only) float 32 6 to 7 significant digits ±1.40e-45 to ±3.40e+38 double to15 significant digits ±4.94e-324 to ±1.79e+308 Though floating-point data types can represent the exact value of some integers, as a general practice we never regard floating-point representations as exact.

33 Primitive Data Types: Floating- Point float and double Floating-point types have double as default data type. Floating-point literals/constants include either a decimal point or one of the following, E or e //(add exponential value) F or f //(float) D or d //(double) Examples:

34 Casting Among integral types, casting is implicit if from narrow to wide

35 Casting Among integral types, casting is implicit if from narrow to wide

36 Casting Casting to a narrower datatype requires an explicit cast. Initial bits will be dropped.

37 Casting Similar rules apply to floating-point primitives:

38 Casting Integral types can be implicitly cast to floating-point types.

39 Casting If a floating-point is cast to an integral, digits after the decimal-point are dropped: value of i: 4

40 Casting char can be implicitly cast to int, long or floating-point types

41 Casting booleans cannot be cast to any other data type

42 Variables A variable is an item of data used to store the state of objects. A variable has a: data type The data type determines the type of value that the variable can hold. name The variable name must follow rules for identifiers.

43 Declaring and Initializing Variables Declare a variable as follows: <data type> <name> [=initial value]; Note: Values enclosed in < > are required values, while those values in [ ] are optional.

44 Declaring and Initializing Variables: Sample Program

45 Declaring and Initializing Variables: Best Practices It always good to initialize your variables as you declare them. Use descriptive names for your variables. For example, if you want a variable that contains a student's grade, give it a name like grade or englishgrade,, and not just random letters and/or numbers like g1.

46 Declaring and Initializing Variables: Best Practice Declare one variable per line of code. For example, the variable declarations are preferred over the declaration

47 Scope of a Variable The scope determines where in the program a variable is accessible. determines the lifetime of a variable or how long the variable can exist in memory. The scope is determined by where the variable declaration is placed in the program. To simplify, think of the scope as anything between the curly braces {...}. Areas outside the curly braces are called the outer blocks,, and blocks enclosed by the curly braces are called inner blocks.

48 Scope of a Variable A variable's scope is Inside the block where it is declared, starting from the point where it is declared, and in the inner blocks.

49 Example 1 A B C D E

50 Example 1 The code we have here represents five scopes indicated by the boxes and the letters representing the scope. Given the variables i, j, k, m and n, and the five scopes A, B, C, D and E, we have the following scopes for each variable: The scope of variable i is A. The scope of variable j is B. The scope of variable k is C. The scope of variable m is D. The scope of variable n is E.

51 Example 2 A B C D E

52 Example 2 In the main method, the scopes of the variables are, ages[] scope A i in B scope B i in C scope C In the test method, the scopes of the variables are, arr[] scope D i in E scope E

53 Scope of a Variable When declaring variables, only one variable with a given identifier or name can be declared in a scope. That means that if you have the following declaration, your compiler will generate an error since you should have unique names for each variable within a single block.

54 Scope of a Variable However you can have two variables of the same name if they are not declared in the same block. For example,

55 Outputting Variable Data In order to output the value of a certain variable to console, we can use the following commands:

56 Outputting Variable Data: Sample Program The program will output the following text on screen:

57 System.out.println() vs. System.out.print() System.out.println() Appends a newline at the end of the data output System.out.print() Does not append newline at the end of the data output

58 System.out.println() vs. System.out.print() Program 1: Output: Program 2: Output:

59 Reference Variables vs. Primitive Variables Two types of variables in Java: Primitive Variables Reference Variables Primitive Variables variables with primitive data types such as int or long. stores data in the actual memory location of where the variable is

60 Reference Variables vs. Primitive Variables Reference Variable a variable that stores the address in its memory location points to another memory location where the actual data is stored. When you declare a variable of a certain class, you are actually declaring only a reference to a possible instance of that class (which might not exist yet), and are not creating the object itself.

61 Example Suppose we have two variables with data types int and String.

62 Example The picture shown below is the actual memory of your computer, wherein you have the address of the memory cells, the variable name and the data they hold.

63 Operators Different types of operators: arithmetic operators relational operators logical operators conditional operators These operators follow a precedence that tells the compiler which operation to evaluate first when multiple operators are used in one statement.

64 Arithmetic Operators

65 Arithmetic Operators: Sample Program

68 Arithmetic Operators: Sample Program Output

69 Pop Quiz What is the result of writing this code:

70 Pop Quiz What is the result of writing this code: Will not compile!

71 Arithmetic Operators byte, short and char get automatically promoted to int before any arithmetic operation.

72 Arithmetic Operators In mixed expressions, all participants get promoted to datatype of widest participant Floating point numbers are always considered wider than integrals. all promoted to float

73 Increment and Decrement Operators unary increment operator (++) unary decrement operator (--) Increment and decrement operators increase and decrease a value stored in a number variable by 1. For example, the expression, is equivalent to

74 Increment and Decrement Operators

75 Increment and Decrement Operators The increment and decrement operators can be placed before or after an operand. When used before an operand, it causes the variable to be incremented or decremented by 1, and then the new value is used in the expression in which it appears.

76 Increment and Decrement Operators When the increment and decrement operators are placed after the operand, the old value of the variable will be used in the expression where it appears.

77 Increment and Decrement Operators: Best Practice Always keep expressions containing increment and decrement operators simple and easy to understand.

78 Relational Operators Relational operators compare two values and determines the relationship between those values. The output of evaluation are the boolean values true or false.

79 Relational Operators: Sample Program

82 Relational Operators: Sample Program Output

83 Logical Operators Logical operators have one or two boolean operands that yield a boolean result. There are six logical operators: && (short-circuit logical AND, binary) & (logical AND, binary) (short-circuit logical OR, binary) (logical inclusive OR, binary) ^ (logical exclusive OR, binary)! (logical NOT, unary)

84 Logical Operators The basic expression for a logical operation is, x1 op x2 where, x1, x2 - can be boolean expressions, variables or constants op - is either &&, &,, or ^ operator. The truth tables that will be shown next, summarize the result of each operation for all possible combinations of x1 and x2.

85 Logical Operators: &&(logical) and &(boolean logical) AND Here is the truth table for && and &

86 Logical Operators: &&(logical) and &(boolean logical) AND The basic difference between && and & operators : && supports short-circuit evaluations (or partial evaluations), while & doesn't. Given an expression: exp1 && exp2 && will evaluate the expression exp1, and immediately return a false value is exp1 is false. If exp1 is false, the operator never evaluates exp2 because the result of the operator will be false regardless of the value of exp2. In contrast, the & operator always evaluates both exp1 and exp2 before returning an answer.

87 Logical Operators: &&(logical) and &(boolean logical) AND

88 Logical Operators: &&(logical) and &(boolean logical) AND The output of the program is Note, that the j++ on the line containing the && operator is not evaluated since the first expression (i>10( i>10) ) is already equal to false.

89 Logical Operators: (logical) and (boolean logical) inclusive OR Here is the truth table for and,

90 Logical Operators: (logical) and (boolean logical) inclusive OR The basic difference between and I operators : supports short-circuit evaluations (or partial evaluations), while doesn't. Given an expression: exp1 exp2 will evaluate the expression exp1,, and immediately return a true value is exp1 is true If exp1 is true, the operator never evaluates exp2 because the result of the operator will be true regardless of the value of exp2. In contrast, the operator always evaluates both exp1 and exp2 before returning an answer.

91 Logical Operators: (logical) and (boolean logical) inclusive OR

92 Logical Operators: (logical) and (boolean logical) inclusive OR The output of the program is, Note, that the j++ on the line containing the operator is not evaluated since the first expression (i<10( i<10) ) is already equal to true.

93 Logical Operators: ^ (boolean logical exclusive OR) Here is the truth table for ^, The result of an exclusive OR operation is TRUE, if and only if one operand is true and the other is false. Note that both operands must always be evaluated in order to evaluate the result of an exclusive OR.

94 Logical Operators: ^ (boolean logical exclusive OR)

95 Logical Operators: ^ (boolean logical exclusive OR) The output of the program is

96 Logical Operators:! (logical NOT) The logical NOT takes in one argument, wherein that argument can be an expression, variable or constant. Here is the truth table for!,

97 Logical Operators:! (logical NOT) The output of the program is

98 Logical Operators: Conditional Operator (?:) The conditional operator?: is a ternary operator. This means that it takes in three arguments that together form a conditional expression. The structure of an expression using a conditional operator is exp1?exp2:exp3 Result: exp1 - is a boolean expression whose result must either be true or false If exp1 is true, exp2 is the value returned. If it is false, then exp3 is returned.

99 Logical Operators: Conditional Operator (?:) The output of this program will be

100 Logical Operators: Conditional Operator (?:)

101 Operator Precedence

102 Operator Precedence Given a compound expression, 6%2*5+4/ we can re-write the expression and place some parentheses based on operator precedence, ((6%2)*5)+(4/2)+88-10

103 Operator Precedence: Best Practice To avoid confusion in evaluating mathematical operations, keep your expressions simple and use parentheses.

104 Summary Java Comments (C++-Style Comments, C- Style Comments, Special Javadoc Comments) Java statements, blocks, identifiers, keywords Java Literals (integer, floating point, boolean, character, String) Primitive data types( boolean, char, byte, short, int, long, float, double) Casting

105 Summary Variables (declare, initialize, output) Scope of a variable System.out.println() vs. System.out.print() Reference Variables vs. Primitive Variables Operators (Arithmetic operators, Increment and Decrement operators, Relational operators, Logical operators, Conditional Operator (?:), Operator Precedence)