Writing a Java Program

Transcription

1 Writing a Java Program Chapter 2 Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 1

2 Variables Variables hold data A variable has a declared type A simple type specifies the range of values that a variable can assume A variable s type does not change through out its lifetime Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 2

3 Structure of Declaration Statements Description General Form Example Declare one variable and do not give it an initial value Declare one variable and give it an initial value Declare multiple variables of the same type and give none of them initial values Declare multiple variables of the same type and give some or all of them initial values type variable; int x; type variable = value; int x = 5; type variable 1, variable 2,..., variable n ; type variable 1 = value 1, variable 2 = value 2,..., variable n = value n ; int x, sum, height; int x = 5, sum, height = 0; Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 3

4 Assignment Statement The = operator means assignment Think is assigned the value of The lefthand side of the assignment statement must be a variable x = 2; Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 4

5 Arithmetic Operators Operator Meaning + addition - subtraction * multiplication / division x = ; Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 5

6 Identifiers A variable name is an example of an identifier Identifiers must contain at least one character The first character must be an alphabetic letter (upper or lower case), the underscore, or the dollar sign ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_$ The remaining characters (if any) may be alphabetic characters (upper or lower case), the underscore, the dollar sign, or a digit ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_$ No other characters (including spaces) are permitted in identifiers A reserved word cannot be used as an identifier Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 6

7 Sometimes called keywords Reserved Words abstract else interface super boolean extends long switch break false native synchronized byte final new this case finally null throw catch float package throws char for private transient class goto protected true const if public try continue implements return void default import short volatile do instanceof static while double int strictfp Cannot be used as identifiers Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 7

8 Variable Declarations All variables in Java must be declared before their use The compiler can catch typographical errors: int ZERO = 0; ZER0 = 1; When variables must be declared, the compiler can catch invalid operations Requiring the programmer to declare variables forces the programmer to plan ahead and think more carefully about the variables a program might require Statically-typed languages are generally more efficient than dynamicallytyped languages Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 8

9 Comments Java supports three kinds of comments: single line comments // My comment... block comments /* My comment extended over several lines of code */ documentation comments /** A document comment */ Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 9

10 Use of Comments Comments are ignored by the compiler Comments are for humans Comments should not document the obvious Comments should be used to enhance the understanding of a human reader Example of a poor comment: result = 0; // Assign the value zero to the variable named result Example of a much better comment: result = 0; // Ensures result has a well-defined minimum value Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 10

11 Code Formatting The physical arrangement of the source code aids its readability by humans Some initial formatting hints: Put each statement on its own line Do not unnecessarily split a statement between two lines of text Every close curly brace aligns vertically with the line that ends with the corresponding open curly brace Contained elements like the main() method within the class and the statements within the main() method are indented several spaces Use spaces (like around + and =) to improve readability See for the full scoop Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 11

12 Errors Syntax errors Runtime errors Logic errors Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 12

13 Syntax Errors A syntax error results from the misuse of the language For example, in English one can say using correct syntax The sentence The boy walks quickly. The boy walk quickly. is not correct syntactically Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 13

14 Java Syntax Errors The following assignment statement is correct x = ; The following assignment statement is incorrect = x; Syntax errors are caught by the compiler There are other kinds of compile-time errors that are not syntax errors Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 14

15 Runtime Errors Some errors cannot be detected by the compiler Consider the statement z = x / y; What if y is zero? Runtime errors are caught by the JRE (Java Runtime Environment) Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 15

16 Logic Errors Some errors cannot be detected by the compiler and cannot be detected by the JRE Compare the statement z = x / y; to z = y / x; Which is correct? Logic errors are the most difficult to diagnose and correct Fundamentals of Programming Writing a Java Program 2001 Richard L. Halterman 16