double salary;
int vacationDays;
long earthPopulation;
boolean done;

int vacationDays;
vacationDays = 12;

int vacationDays = 12;
double salary = 65000.0;

public class Constants
{
    public static void main(String[] args)
    {
        final double CM_PRE_INCH = 2.54;
        double paperWidth = 8.5;
        double paperHeight = 11;
        System.out.println("Paper size in centimeters: " + paperWidth * CM_PRE_INCH + " by " + paperHeight * CM_PRE_INCH);
    }
}

public class Constants2
{
    public static final double CM_PRE_INCH = 2.54;
    public static void main(String[] args)
    {
        double paperWidth = 8.5;
        double paperHeight = 11;
        System.out.println("Paper size in centimeters: " + paperWidth * CM_PRE_INCH + " by " + paperHeight * CM_PRE_INCH);
    }
}

// 自定義枚舉類型
enum Size { SMALL, MEDIUM, LARGE, EXTRA_LARGE };

// 聲明枚舉變量, 只能為Size中的某個值，或者null
Size s = Size.MEDIUM;

// 計算平方根
double x = 4;
double y = Math.sqrt(x);
System.out.println(y);  // prints 2.0

// 數學常量
Math.PI
Math.E

// 捨去浮點位
double x = 9.997;
int nx = (int) x; // prints 9

// 使用四捨五入
double x = 9.997;
int nx = (int) Math.round(x); // prints 10;

x += 4; 
// 等價於
x = x + 4;

int m = 7;
int n = 7;
int a = 2 * ++m; // a is 16, m is 8
int b = 2 * n++; // b is 14, n is 8

a && b || c  等價於 (a && b) || c
a += b += c  等價於 a += (b += c)

String e = "";  // empty string
String greeting = "Hello";

String greeting = "Hello";
String s = greeting.substring(0, 3);   // s is "Hel"

String expletive = "Expletive";
String PG13 = "deleted";
String message = expletive + PG13;  // message is "Expletivedeleted"

int age = 13;
String rating = "PG" + age;  // rating is "PG13"

String all = String.join(" / ", "S", "M", "L", "XL");  // all is "S / M / L / XL"
String repeated = "java".repeat(3);  // repeated is "JavaJavaJava"

// 通過拼接替換字符串
String greeting = "Hello";
greeting = greeting.substring(0,3) + "p!";  // greeting is "Help!"

String s = "s";
String t = "t";
s.equals(t);  // false

String greeting = "Hello"; 
"Hello".equals(greeting);  // true;

// 不區分大小寫進行比較
"Hello".equalsIgnoreCase("hello");  // true;

StringBuilder builder = new StringBuilder();
builder.append(ch); // appends a single character
builder.append(str) // appends a string
String completedString = builder.toString(); // 構建完成時，調用toString方法得到字符串對象

// 構造與標準輸入流System.in 關聯的Scanner對象
Scanner in = new Scanner(System.in);

System.out.print("What is your name? ");
// 讀取一行輸入
String name = in.nextLine();
// 讀取一個單詞
String firstName = in.next();

System.out.print("How old are you? ");
// 讀取一個整數
int age = in.nextInt();

// 使用Console類讀取密碼
Console cons = System.console();
String username = cons.readLine("User name: ");
char[] passwd = cons.readPassword("Password: ");

double x = 10000.0 / 3.0;
System.out.print(x);   // 3333.3333333333335
System.out.printf("%8.2f", x) // 3333.33
// 提供多個參數
System.out.printf("Hello, %s. Next year, you'll be %d", name, age);
// 打印時間
System.out.printf("%tc", new Date());

// 讀入文件
Scanner in = new Scanner(Path.of("myfile.txt"), StandardCharsets.UFT_8);
// 寫入文件
PrintWriter out = new PrintWriter("myfile.txt", StandardCharsets.UFT_8);

Scanner in = new Scanner(System.in);
System.out.print("Select an option (1, 2, 3, 4) ");
int choice = in.nextInt();
switch (choice) {
  case 1:
    ...
    break;
  case 2:
    ...
    break;
  case 3:
    ...
    break;
  default:
    // bad input
    ...
    break;
}

while (year <= 100) {
  balance += payment;
  double interest = balance * interestRate / 100;
  balance += interest;
  if (balance >= goal) break;
  years++;
}

Scanner in = new Scanner(System.in);
for (count = 1; count <= 100; count++)
{
    System.out.print("Enter a number, -1 to quit:");
    n = in.netInt();
    if(n < 0) continue;
    sum += n;
}

BigInteger a = BigInteger.valueOf(100);

BigInteger reallyBig = new BigInteger("2222322343454545656565676767676700")

// 使用add 方法進行求和運算
BigInteger c = a.add(b); // c = a + b;
// 使用multiply 方法進行乘法運算
BigInteger d = c.multiply(b.add(BigInteger.valueOf(2))); // d = c * (b+2)

// 聲明整型數組 a
int[] a;

// 聲明並初始化一個可以存儲100個整數的數組
int[] a = new int[100]; 
var a = new int[100];

// 創建數組並同時提供初始值
int[] smallPrimes = {2, 3, 5, 7, 11, 13};
String[] authors = {
  "James Gosling",
  "Bill Joy",
  "Guy Steele",
};

//使用匿名數組重新初始化變量
smallPrimes = new int[] {17, 19, 23, 29, 31, 37};

// 創建長度為0的數組
int[] a = new int[0];
int[] b = new int[] {};

int[] a = new int[100];
for (int i = 0; i < 100; i++){
    a[i] = i;  // fill the array with numbers 0 to 99
}

// 將字符串數組初始化未空串
String[] names = new String[10];
for (int i = 0; i < 10; i++) names[i] = "";

// 使用array.length 獲得數組中元素個數
int[] a = new int[100];
for (int i = 0; i < a.length; i++) {
  System.out.println(a[i]);
}

int[] a = {1, 2, 3, 4, 5};
for (int element : a) {
  System.out.println(element);
}

int[] smallPrimes = {17, 19, 23, 29, 31, 37};
int[] luckyNumbers = smallPrimes;
luckyNumbers[5] = 12;  // now smallPrimes[5] also 12

int[] luckyNumbers = {17, 19, 23, 29, 31, 37};
// 實現數組值的全部複製
int[] copiedLuckyNumbers = Arrays.copyOf(luckyNumbers, luckyNumbers.length);
// 擴容數組
luckyNumbers = Arrays.copyOf(luckyNumbers,  2 * luckyNumbers.length);
// 裁切數組
luckyNumbers = Arrays.copyOf(luckyNumbers,  luckyNumbers.length-2);

public class Message 
{
  public static void main(String[] args)
  {
    if (args.length == 0 || args[0].equals("-h"))
    {
      System.out.print("Hello,")
    }
    else if (args[0].equals("-g"))
    {
      System.out.print("Goodbye,")
    }
    // print other command line arguments
    for (int i = 1; i < args.length; i++)
    {
      System.out.print(" " + args[i]);
    }
    System.out.println("!");
  }
}
=> java Message -g cruel world   // print: Goodbye, cruel world!

double[][] balances;
balances = new double[10][10];

int[][] magicSquare = {
  {16, 3, 2, 13},
  {5, 10, 11, 8},
  {9, 6, 7, 12},
  {4, 15, 14, 1}
};

// 通過下標訪問
int a = magicSquare[1][2];

// for each 循環
for (double[] row : a) {
  for (double value : row){
    // do something with value
  }
}
// 打印二維數組內容
System.out.println(Arrays.deepToString(a));