計算圓面積時，半徑小於0時，該如何處理？
public class EX00_CircleArea {
public static void main(String[] args) {
int radius = -3;
if (radius < 0) {
System.out.println("圓的半徑不能小於0");
} else {
double area = radius * radius * Math.PI;
System.out.printf("面積＝%.2f", area);
}
}
}
p.96 Table 3.1 Comparison Operations
參考 p.97 程式 Listing 3.1 AdditionQuiz.java
public class EX01_AdditionQuiz {
public static void main(String[] args) {
int number1 = (int) (System.currentTimeMillis() % 10);
int number2 = (int) (System.currentTimeMillis() * 7 % 10);
// Create a Scanner
Scanner input = new Scanner(System.in);
System.out.print("What is " + number1 + " + " + number2 + "? ");
int answer = input.nextInt();
System.out.println(number1 + " + " + number2 + " = " + answer + " is "
+ (number1 + number2 == answer));
}
}
3-6 有 Two-Way if Statements
if (boolean-expression) { // 可以計算（評估）的boolean運算式，可以是true, false
statement(s); // 若為true, 則執行這些敘述(statement)
}
參考 p.99 程式 Listing 3.2 SimpleIfDemo.java
public class EX02_SimpleIfDemo {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.println("Enter an integer: ");
int number = input.nextInt();
// test input 12, 15, 11, 30
if (number % 5 == 0)
System.out.println("HiFive");
if (number % 2 == 0)
System.out.println("HiEven");
}
}
參考 p.100 程式 Listing 3.3 GuessBirthday.java
if (boolean-expression) { // 可以計算（評估）的boolean運算式，可以是true, false
statement(s)-for-true; // 若為true, 則執行這些敘述(statement)
} else {
statement(s)-for-false; // 若為false, 則執行這些敘述(statement)
}
public class EX03A_EvenOdd {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("請數入一個整數：");
int num = input.nextInt();
if (num % 2 == 0) {
System.out.printf("%d 是偶數\n", num);
} else {
System.out.printf("%d 是偶數\n", num);
}
}
}
p.104 Figure 3.4 A preferred format for multiple alternative if statements is shown in (b)
參考 p.105 Error 1, 2, 3, 4
參考 p.107 程式 Listing 3.4 SubtractionQuiz.java
參考 p.108 程式 Listing 3.5 ComputeBMI.java
public class EX05_ComputeBMI {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
// Prompt the user to enter weight in pounds
System.out.print("Enter weight in pounds: ");
double weight = input.nextDouble();
// Prompt the user to enter height in inches
System.out.print("Enter height in inches: ");
double height = input.nextDouble();
final double KILOGRAMS_PER_POUND = 0.45359237; // Constant
final double METERS_PER_INCH = 0.0254; // Constant
// Compute BMI
double weightInKilogram = weight * KILOGRAMS_PER_POUND;
double heightInMeters = height * METERS_PER_INCH;
double bmi = weightInKilogram / (heightInMeters * heightInMeters);
// Display result
System.out.printf("Your BMI is %5.2f\n", bmi);
if (bmi < 16)
System.out.println("You are seriously underweight");
else if (bmi < 18)
System.out.println("You are underweight");
else if (bmi < 24)
System.out.println("You are normal weight");
else if (bmi < 29)
System.out.println("You are overweight");
else if (bmi < 35)
System.out.println("You are seriously overweight");
else
System.out.println("You are gravely overweight");
}
}
參考 p.110 程式 Listing 3.6 ComputeTax.java
p.112 Table 3.3 Boolean Operators
p.112 Table 3.4 Truth Table for Operator !
p.112 Table 3.5 Truth Table for Operator &&
p.113 Table 3.6 Truth Table for Operator ||
p.113 Table 3.7 Truth Table for Operator ^
參考 p.113 程式 Listing 3.7 TestBooleanOperators.java
public class EX07_TestBooleanOperators {
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Receive an input
System.out.print("Enter an integer: ");
int number = input.nextInt();
System.out.println("Is " + number + " divisible by 2 and 3? "
+ ((number % 2 == 0) && (number % 3 == 0)));
System.out.println("Is " + number + " divisible by 2 or 3? "
+ ((number % 2 == 0) || (number % 3 == 0)));
System.out.println("Is " + number
+ " divisible by 2 or 3, but not both? "
+ ((number % 2 == 0) ^ (number % 3 == 0)));
}
}
public class E07A_LogicOP {
public static void main(String[] args){
// short-circuit 與 side effect
int score = 53, miss=3;
boolean pass;
pass = score >= 60 && miss++ < 5; // short-cut
System.out.printf("&& pass=%b, miss=%d\n", pass, miss);
score=53; miss=3;
pass = score >= 60 & miss++ < 5;
System.out.printf("& pass=%b, miss=%d\n", pass, miss);
// And
int money = 30000; //現金
int day = 7; //假期天數
System.out.print("澳洲團成行：");
System.out.println(money>=40000 && day>=7);
// Or
boolean isDiesel = true;
int cc = 1800; //排氣量
System.out.print("購買汽車：");
System.out.println(isDiesel || cc<=1500);
// Xor
boolean MassageChair = true; //送按摩椅
boolean skylight = true; //送天窗
System.out.print("成交：");
System.out.println(MassageChair ^ skylight);
// Not
int age = 16; //年齡
System.out.print("未滿18歲：");
System.out.println(!(age>=18));
}
}
p.114 Caution and Note
// error 1 <= Num <= 31 // ok (1 <= Num) && (Num <= 31)
參考 p.115 程式 Listing 3.8 LeapYear.java
public class E08_LeapYear {
public static void main(String args[]) {
// Create a Scanner
Scanner input = new Scanner(System.in);
System.out.print("Enter a year: ");
int year = input.nextInt();
// Check if the year is a leap year
boolean isLeapYear = (year % 4 == 0 && year % 100 != 0)
|| (year % 400 == 0);
// Display the result in a message dialog box
System.out.println(year + " is a leap year? " + isLeapYear);
}
}
參考 p.115 程式 Listing 3.8 Lottery.java
import java.util.Scanner;
public class EX09_Lottery {
public static void main(String[] args) {
// Generate a lottery
int lottery = (int) (Math.random() * 100);
// Prompt the user to enter a guess
Scanner input = new Scanner(System.in);
System.out.printf("magic number = %d\n", lottery);
System.out.print("Enter your lottery pick (two digits): ");
int guess = input.nextInt();
// Get digits from lottery
int lotteryDigit1 = lottery / 10;
int lotteryDigit2 = lottery % 10;
// Get digits from guess
int guessDigit1 = guess / 10;
int guessDigit2 = guess % 10;
System.out.println("The lottery number is " + lottery);
// Check the guess
if (guess == lottery)
System.out.println("Exact match: you win $10,000");
else if (guessDigit2 == lotteryDigit1 && guessDigit1 == lotteryDigit2)
System.out.println("Match all digits: you win $3,000");
else if (guessDigit1 == lotteryDigit1 || guessDigit1 == lotteryDigit2
|| guessDigit2 == lotteryDigit1 || guessDigit2 == lotteryDigit2)
System.out.println("Match one digit: you win $1,000");
else
System.out.println("Sorry, no match");
}
}
switch (switch-expression) { // char, byte, short, int
case value1:
statement(s)1;
break;
case value2:
statement(s)2;
break;
...
case valueN:
statement(s)N;
break;
...
default:
statement(s)-for-default;
}
public class EX09A_Switch {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("請輸入月份： ");
int month = input.nextInt();
int days;
switch (month) {
case 1:
days = 31;
System.out.printf("month=%d, days=%d\n", month, days);
break;
case 4:
days = 30;
System.out.printf("month=%d, days=%d\n", month, days);
break;
default:
days = 0;
System.out.printf("month=%d, days=%d\n", month, days);
}
}
}
int max, num1=1, num2=2;
// following statement
if (num1 > num2) {
max = num1;
} else {
max = num2;
}
// equal
max = (num1 > num2) ? num1 : num2;
Dispaly only two digits after the decimal point in a floating-point value
double x = 2.0 / 3;
System.out.println("x=" + (int) (x * 100) / 100.0);
// or
System.out.printf("x=%.2f\n",x);
p.120 Table 3.8 Frequently Used Specifiers
p.120 Table 3.9 Examples of Specifying Width and Precision
p.121 Caution and Tips and 圖形解說
p.122 Table 3.10 Operator Precedence Chart 或課本 p 736, 附錄C
參考 p.123 程式 Listing 3.10 GuessBirthdayUsingConfirmationDialog.java