基本数据类型与表达式.ppt

Java程序基本结构解析,程序解析： 1、注释 2、类声明 3、标识符 4、关键字 5、字符串 6、标准输出对象 7、语句 8、方法与参数,第二章 基本数据类型与表达式,数据类型 常量与变量 输入与输出 表达式,1. 数据类型,基本数据类型,数据类型转换,自动数据类型转换 低级别向高级别转换：byte-short-int-long-float-double 例：3*a+6.85 强制数据类型转换 格式：(数据类型名称）表达式 可能导致精度损失,2.常量与变量,标识符 标识符只能由字母、数字、美元符和下划线组成，如root-1,my=you,100%等不符合要求 第一个字符不能是数字，如3abc 标识符可以是任意长的字符，但只有前32个字符有效 关键字：Java保留使用的标识符 判断标识符定义对错： char 0ax_li fLu1 Cy%ty $123 aa 命名习惯： 变量与方法：radius area showInputDialog 类名：ComputeArea 常量：PI MAX_VALUE,Java标识符是大小写敏感的,保留字,常量,定义：程序运行过程中不能改变其值的量 整型常量 不允许包含逗号、小数点和特殊符号（包括美元符号） 默认是int类型 浮点型常量 具有小数点的数字 默认是double类型 字符型常量（Unicode编码集） 布尔型常量: true, false,常量,final datatype CONSTANTNAME= VALUE; final double PI = 3.14159; final int SIZE = 3;,变量,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;,Trace a Program Execution,public class ComputeArea /* Main method */ public static void main(String args) final double PI=3.14159; double radius; double area; / Assign a radius radius = 20; / Compute area area = radius * radius * PI; / Display results System.out.println(“The area for the circle of radius “ + radius + “ is “ + area); ,20,radius,memory,1256.636,area,print a message to the console,char letter = A; (ASCII) char numChar = 4; (ASCII) char letter = u0041; (Unicode) char numChar = u0034; (Unicode),Four hexadecimal digits.,NOTE: The increment and decrement operators can also be used on char variables to get the next or preceding Unicode character. For example, the following statements display character b. char ch = a; System.out.println(+ch);,字符类型,Unicode Format,Java characters use Unicode, a 16-bit encoding scheme established by the Unicode Consortium to support the interchange, processing, and display of written texts in the worlds diverse languages. Unicode takes two bytes, preceded by u, expressed in four hexadecimal numbers that run from u0000 to uFFFF. So, Unicode can represent 65535 + 1 characters.,Unicode u03b1 u03b2 u03b3 for three Greek letters,Example: Displaying Unicodes,Write a program that displays two Chinese characters and three Greek letters.,import javax.swing.JOptionPane; public class DisplayUnicode public static void main(String args) JOptionPane.showMessageDialog(null, “u6B22u8FCE u03b1 u03b2 u03b3“, “u6B22u8FCE Welcome“, JOptionPane.INFORMATION_MESSAGE); ,Description Escape Sequence Unicode Backspace b u0008 Tab t u0009 Linefeed n u000A Carriage return r u000D Backslash u005C Single Quote u0027 Double Quote “ u0022,Appendix B: ASCII Character Set,ASCII Character Set is a subset of the Unicode from u0000 to u007f,ASCII Character Set, cont.,ASCII Character Set is a subset of the Unicode from u0000 to u007f,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.,字符串类型,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; / s becomes SupplementB,Obtaining Input,Using JOptionPane input dialogs Using the JDK 1.5 Scanner class,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, you can use the static parseInt method in the 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”.,import javax.swing.JOptionPane; /import java.lang.String; public class Addition public static void main(String args) String inputX=JOptionPane.showInputDialog(“请输入第一个整数：“); int x = Integer.parseInt(inputX); String inputY=JOptionPane.showInputDialog(“请输入第二个整数：“); int y = Integer.parseInt(inputY); int sum; sum = x + y; char ch = A; System.out.printf(“sum=%d“, sum); JOptionPane.showMessageDialog(null,“sum“+ sum); ,Example:,Getting Input Using Scanner,1. Create a Scanner object Scanner scanner = new Scanner(System.in); 2. Use the methods next(), nextByte(), nextShort(), nextInt(), nextLong(), nextFloat(), nextDouble(), or nextBoolean() to obtain to a string, byte, short, int, long, float, double, or boolean value. For example, System.out.print(“Enter a double value: “); Scanner scanner = new Scanner(System.in); double d = scanner.nextDouble();,Example:,import java.util.Scanner; /import java.lang.String; public class Addition public static void main(String args) Scanner sc = new Scanner(System.in); System.out.print(“请输入第一个整数：“); int x = sc.nextInt(); System.out.print(“请输入第二个整数：“); int y = sc.nextInt(); int sum; sum = x + y; System.out.printf(“sum=%d“, sum) ,Output,JOptionPane.showMessageDialog(null,x); System.out. print方法输出后光标不换行 println方法输出后光标换行 printf 格式化输出(占位符 %s） 注意：printf方法在jdk1.5之后,3.表达式,赋值表达式 算术表达式 关系表达式 逻辑表达式 条件表达式 位运算表达式,赋值表达式,赋值运算符：= 赋值表达式：变量=表达式 数据类型转换 数据类型兼容：自动类型转换 不兼容：必须进行数据类型的强制转换 例：x=x+1 x=y=3,算术表达式,单目：+,-,+,- 双目：+,-,*,/,% 优先级：先乘除（取余），后加减 算术赋值运算符：+=,-=,*=,/=,%= 字符类型的数据以该字符的Unicode码值参加运算,关系表达式,关系运算符：,=,y-z f=+x*2=y,逻辑表达式,逻辑运算符： m-3n&(n-8)!=0|y*2x,设年份为year，当year能被4整除但不能被100整除，或者year能被400整除时，year为闰年。写出满足上述逻辑的表达式。 ( year%4=0&year%100!=0)|(year%400=0),import javax.swing.JOptionPane; public class LeapYear public static void main(String args) / Prompt t