🗊Презентация Elementary programming. Motivations

ELEMENTARY PROGRAMMING

Motivations In the preceding lesson, you learned how to create, compile, and run a Java program. Starting from this chapter, you will learn how to solve practical problems programmatically. Through these problems, you will learn Java primitive data types and related subjects, such as variables, constants, data types, operators, expressions, and input and output.

Introducing Programming with an Example Computing the Area of a Circle

Trace a Program Execution public class ComputeArea { /** Main method */ public static void main(String[] args) { double radius; double area; // Assign a radius radius = 20; // Compute area area = radius * radius * 3.14159; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } }

Trace a Program Execution public class ComputeArea { /** Main method */ public static void main(String[] args) { double radius; double area; // Assign a radius radius = 20; // Compute area area = radius * radius * 3.14159; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } }

Trace a Program Execution public class ComputeArea { /** Main method */ public static void main(String[] args) { double radius; double area; // Assign a radius radius = 20; // Compute area area = radius * radius * 3.14159; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } }

Trace a Program Execution public class ComputeArea { /** Main method */ public static void main(String[] args) { double radius; double area; // Assign a radius radius = 20; // Compute area area = radius * radius * 3.14159; // Display results System.out.println( "The area for the circle of radius " + radius + " is " + area); } }

Trace a Program Execution public class ComputeArea { /** Main method */ public static void main(String[] args) { double radius; double area; // Assign a radius radius = 20; // Compute area area = radius * radius * 3.14159; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } }

Identifiers An identifier is a sequence of characters that consist of letters, digits, underscores (_), and dollar signs ($). An identifier must start with a letter, an underscore (_), or a dollar sign ($). It cannot start with a digit. An identifier cannot be a reserved word. (See Appendix A, “Java Keywords,” for a list of reserved words). An identifier cannot be keyword: true, false, or null. An identifier can be of any length.

Variables // Compute the first area radius = 1.0; area = radius * radius * 3.14159; System.out.println("The area is “ + area + " for radius "+radius); // Compute the second area radius = 2.0; area = radius * radius * 3.14159; System.out.println("The area is “ + area + " for radius "+radius);

Declaring Variables int x; // Declare x to be an // integer variable; double radius; // Declare radius to // be a double variable; char a; // Declare a to be a // character variable;

Assignment Statements x = 1; // Assign 1 to x; radius = 1.0; // Assign 1.0 to radius; a = 'A'; // Assign 'A' to a;

Declaring and Initializing in One Step int x = 1; double d = 1.4;

Constants final datatype CONSTANTNAME = VALUE; final double PI = 3.14159; final int SIZE = 3;

Numerical Data Types

Numeric Operators

Integer Division +, -, *, /, and % 5 / 2 yields an integer 2. 5.0 / 2 yields a double value 2.5 5 % 2 yields 1 (the remainder of the division)

Remainder Operator Remainder is very useful in programming. For example, an even number % 2 is always 0 and an odd number % 2 is always 1. So you can use this property to determine whether a number is even or odd. Suppose today is Saturday and you and your friends are going to meet in 10 days. What day is in 10 days? You can find that day is Tuesday using the following expression:

NOTE Calculations involving floating-point numbers are approximated because these numbers are not stored with complete accuracy. For example, System.out.println(1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1); displays 0.5000000000000001, not 0.5, and System.out.println(1.0 - 0.9); displays 0.09999999999999998, not 0.1. Integers are stored precisely. Therefore, calculations with integers yield a precise integer result.

Number Literals A literal is a constant value that appears directly in the program. For example, 34, 1,000,000, and 5.0 are literals in the following statements:   int i = 34; long x = 1000000; double d = 5.0;

Integer Literals An integer literal can be assigned to an integer variable as long as it can fit into the variable. A compilation error would occur if the literal were too large for the variable to hold. For example, the statement byte b = 1000 would cause a compilation error, because 1000 cannot be stored in a variable of the byte type. An integer literal is assumed to be of the int type, whose value is between -231 (-2147483648) to 231–1 (2147483647). To denote an integer literal of the long type, append it with the letter L or l. L is preferred because l (lowercase L) can easily be confused with 1 (the digit one).

Floating-Point Literals Floating-point literals are written with a decimal point. By default, a floating-point literal is treated as a double type value. For example, 5.0 is considered a double value, not a float value. You can make a number a float by appending the letter f or F, and make a number a double by appending the letter d or D. For example, you can use 100.2f or 100.2F for a float number, and 100.2d or 100.2D for a double number.

Scientific Notation Floating-point literals can also be specified in scientific notation, for example, 1.23456e+2, same as 1.23456e2, is equivalent to 123.456, and 1.23456e-2 is equivalent to 0.0123456. E (or e) represents an exponent and it can be either in lowercase or uppercase.

Arithmetic Expressions

How to Evaluate an Expression

Problem: Converting Temperatures Write a program that converts a Fahrenheit degree to Celsius using the formula:

Problem: Displaying Current Time

Shortcut Assignment Operators

Increment and Decrement Operators

Increment and Decrement Operators, cont.

Increment and Decrement Operators, cont.

Assignment Expressions and Assignment Statements Prior to Java 2, all the expressions can be used as statements. Since Java 2, only the following types of expressions can be statements: variable op= expression; // Where op is +, -, *, /, or % ++variable; variable++; --variable; variable--;

Numeric Type Conversion Consider the following statements: byte i = 100; long k = i * 3 + 4; double d = i * 3.1 + k / 2;

Conversion Rules When performing a binary operation involving two operands of different types, Java automatically converts the operand based on the following rules:   1.    If one of the operands is double, the other is converted into double. 2.    Otherwise, if one of the operands is float, the other is converted into float. 3.    Otherwise, if one of the operands is long, the other is converted into long. 4.    Otherwise, both operands are converted into int.

Type Casting Implicit casting double d = 3; (type widening) Explicit casting int i = (int)3.0; (type narrowing) int i = (int)3.9; (Fraction part is truncated) What is wrong? int x = 5 / 2.0;

Problem: Computing Loan Payments

Character Data Type char letter = 'A'; (ASCII) char numChar = '4'; (ASCII) char letter = '\u0041'; (Unicode) char numChar = '\u0034'; (Unicode)

Unicode Format

Problem: Displaying Unicodes Write a program that displays two Chinese characters and three Greek letters.

Escape Sequences for Special Characters

Appendix B: ASCII Character Set

ASCII Character Set, cont.

Casting between char and Numeric Types

The String Type The char type only represents one character. To represent a string of characters, use the data type called String. For example,  String message = "Welcome to Java";   String is actually a predefined class in the Java library just like the System class and JOptionPane class. The String type is not a primitive type. It is known as a reference type. Any Java class can be used as a reference type for a variable. Reference data types will be thoroughly discussed in Chapter 7, “Objects and Classes.” For the time being, you just need to know how to declare a String variable, how to assign a string to the variable, and how to concatenate strings.

String Concatenation // Three strings are concatenated String message = "Welcome " + "to " + "Java";   // String Chapter is concatenated with number 2 String s = "Chapter" + 2; // s becomes Chapter2   // String Supplement is concatenated with character B String s1 = "Supplement" + 'B'; // s1 becomes SupplementB

Programming Style and Documentation Appropriate Comments Naming Conventions Proper Indentation and Spacing Lines Block Styles

Appropriate Comments Include a summary at the beginning of the program to explain what the program does, its key features, its supporting data structures, and any unique techniques it uses. Include your name, class section, instructor, date, and a brief description at the beginning of the program.

Naming Conventions Choose meaningful and descriptive names. Variables and method names: Use lowercase. If the name consists of several words, concatenate all in one, use lowercase for the first word, and capitalize the first letter of each subsequent word in the name. For example, the variables radius and area, and the method computeArea.

Naming Conventions, cont. Class names: Capitalize the first letter of each word in the name. For example: ComputeArea. Constants: Capitalize all letters in constants, and use underscores to connect words. For example: PI and MAX_VALUE

Proper Indentation and Spacing Indentation Indent two spaces. Spacing Use blank line to separate segments of the code.

Block Styles Use end-of-line style for braces.

Programming Errors Syntax Errors Detected by the compiler Runtime Errors Causes the program to abort Logic Errors Produces incorrect result

Syntax Errors public class ShowSyntaxErrors { public static void main(String[] args) { i = 30; System.out.println(i + 4); } }

Runtime Errors public class ShowRuntimeErrors { public static void main(String[] args){ int i = 1 / 0; } }

Logic Errors public class ShowLogicErrors { // Determine if a number is between 1 and 100 inclusively public static void main(String[] args) { // Prompt the user to enter a number String input = JOptionPane.showInputDialog(null, "Please enter an integer:", "ShowLogicErrors", JOptionPane.QUESTION_MESSAGE); int number = Integer.parseInt(input);   // Display the result System.out.println("The number is between 1 and 100, " + "inclusively? " + ((1 < number) && (number < 100)) );   System.exit(0); } }

Debugging Logic errors are called bugs. The process of finding and correcting errors is called debugging. A common approach to debugging is to use a combination of methods to narrow down to the part of the program where the bug is located. You can hand-trace the program (i.e., catch errors by reading the program), or you can insert print statements in order to show the values of the variables or the execution flow of the program. This approach might work for a short, simple program. For a large, complex program, the most effective approach for debugging is to use a debugger utility.

Debugger Debugger is a program that facilitates debugging. You can use a debugger to: Execute a single statement at a time. Trace into or stepping over a method. Set breakpoints. Display variables. Display call stack. Modify variables.

JOptionPane Input Two ways of obtaining input. Using the Scanner class (console input) Using JOptionPane input dialogs

Getting Input from Input Dialog Boxes String input = JOptionPane.showInputDialog ("Enter an input");

Getting Input from Input Dialog Boxes String string = JOptionPane.showInputDialog(null, “Prompting Message”, “Dialog Title”, JOptionPane.QUESTION_MESSAGE);

Two Ways to Invoke the Method There are several ways to use the showInputDialog method. For the time being, you only need to know two ways to invoke it. One is to use a statement as shown in the example: String string = JOptionPane.showInputDialog(null, x, y, JOptionPane.QUESTION_MESSAGE); where x is a string for the prompting message, and y is a string for the title of the input dialog box. The other is to use a statement like this: JOptionPane.showInputDialog(x); where x is a string for the prompting message.

Converting Strings to Integers The input returned from the input dialog box is a string. If you enter a numeric value such as 123, it returns “123”. To obtain the input as a number, you have to convert a string into a number.   To convert a string into an int value, use the static parseInt method of Integer class as follows:  int intValue = Integer.parseInt(intString);  where intString is a numeric string such as “123”.

Converting Strings to Doubles To convert a string into a double value, you can use the static parseDouble method in the Double class as follows:   double doubleValue =Double.parseDouble(doubleString);   where doubleString is a numeric string such as “123.45”.

Problem: Computing Loan Payments Using Input Dialogs