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public class Main {
  public static void main(String[] args) {
    System.out.println("Hello World");
  }
}

Java每一行程式碼都屬於一個class，class都是大寫開頭，例如Main。

main()

每個Java程式都必須要有main()，且一定會被執行。

Java Variables

type variable = value;

例如：
String name = "John"

先宣告變項，再assign數值：

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int myNum;
myNum = 15;

Final Variables

變項的數值不能被覆寫(overwrite)
final int myNum = 15;

多個同類型變數同時宣告

int x = 5, y = 6, z = 7;

資料類型

https://www.w3schools.com/java/java_data_types.asp

Casting

primitive資料類型轉為另一種。

  • Widening Casting (automatically) - converting a smaller type to a larger type size
    byte -> short -> char -> int -> long -> float -> double

  • Narrowing Casting (manually) - converting a larger type to a smaller size type
    double -> float -> long -> int -> char -> short -> byte

String method

  • length()
  • toUpperCase(), toLowerCase()
  • indexOf()
  • concat()

算數方法

  • Math.max(x, y), Math.min(x, y)
  • Math.sqrt(x)
  • Math.abs(x)
  • Math.random()

If…Else條件式簡寫

variable = (condiction) ? expressionTrue : expressionFalse;

Switch

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switch(expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}

while & do/while

差別：do/while至少會運行一次，才檢查condition

while:

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while (condition) {
  // code block to be executed
}

do/while:

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do {
  // code block to be executed
}
while (condition);

for-each loop

用來loop陣列(array)

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for (type variableName : arrayName) {
  // code block to be executed
}

範例：

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String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (String i : cars) {
  System.out.println(i);
}

break & continue

  • break: 跳出、終止loop
  • continue: 跳出當前的loop，但會繼續到下一個

array

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};

多維度：
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };