Java - 基礎語法

2019/10/30

Variable

byte

bytes (2^n) → 字节(位元組) / bit → 字元(位元)

type desc
byte 1
boolean 1
char 2
int 4
float 4
long 8
double 8
string depend on charset like ASCII of UTF-8 lead to different bytes

conversion

  • 小 → 大:隱性轉型,大 → 小:強制轉型
// large = small
double d = float f;
float f = (float) double d;
  • 數值可以跟字元相互轉換
  • 布林值不可以相互轉換
  • Java 5 之後會自動轉換包裝類別
int a = 20;
Integer b = new Integer(3);

// converting int into Integer (primitive -> reference)
Integer i = Integer.valueOf(a);

// autoboxing, now compiler will write Integer.valueOf(a) internally
Integer j = a;

// converting Integer into int (reference -> primitive)
int x = b.intValue();

// unboxing, now compiler will write a.intValue() internally
int y = b;

作用域(scope)

in method in block
static variable can access can access
instance variable can access can access
local variable can access can access until block need inside in method
local variable in block N/A this variable only live within the block. example: if, for block

Operator

算數運算符

  • 加法 +
    • 字串相加的情況
       string  + string = string: "3 + 5 = " + 3 + 5 // console: 3 + 5 = 35
       integer + string = string: 3 + 5 + " = 5 + 5" // console: 8 = 5 + 5
      
  • 減法 -
  • 乘法 *
  • 除法 /
  • 餘法 %
    • 先不考慮兩個運算元的正負號,直接做餘數運算
    • 被除數的正負號,就是最終結果的正負號
        17 % 5 = 2
        -17 % 5 = -2
      
  • 自增 ++
    • a++: evaluates a, then increments it (post-incrementation).
    • ++a: increments a, then evaluates it (pre-incrementation).
  • 自減 --
      int a = 1;
      int b = a++; // b = 1, a = 2
      
      a = 1;
      b = ++a; // b = 2, a = 2
      
      int x = 5, y = 5;
      System.out.println(++x); // console 6
      System.out.println(x);   // console 6
      System.out.println(y++); // console 5
      System.out.println(y);   // console 6
    

關係運算符

  • 相等 ==
  • 不相等 !=
  • 大於 >
  • 小於 <
  • 大於或等於 >=
  • 小於或等於 <=

位運算符

Java定义了位运算符,应用于整数类型(int),长整型(long),短整型(short),字符型(char),和字节型(byte)等类型。

位运算符作用在所有的位上,并且按位运算。假设a = 60,b = 13;它们的二进制格式表示将如下:

A = 0011 1100
B = 0000 1101
-----------------
A&B = 0000 1100
A | B = 0011 1101
A ^ B = 0011 0001
~A= 1100 0011

邏輯運算符

  • and or:
    • and 成立在前後條件式都要成立(true),結果才會為真
    • or 成立在前後條件式只要一個成立(true),結果就會為真
  • 短路(&&、||)與非短路(&、|、^):
    • 短路:The Conditional Operators which exhibit “short-circuiting” behavior.
    • 非短路:The bitwise Operator which is not short-circuiting.
  • MySQL and or not:
b
A true false null
true t f n
false f f f
null n f n

賦值運算符

  • =
  • +=
  • -=
  • *=
  • /=
  • %=
  • «=
  • >>=
  • &=
  • ^=
  • |=

條件運算符

variable = booleanExpression ? valueWhenTrue : valueWhenFalse

instanceof 運算符

說明:

object instanceof class → return object is-a the class or not

使用格式:

(object reference variable) instanceof (class/interface type)

簡單例子:

String name = "miku"
boolean result = name instanceof String; // console: true

詳細例子:

class Vehicle {}

public class Car extends Vehicle {
    public static void main(String[] args){
        Vehicle a = new Car();
        boolean result =  a instanceof Car; // console: true
    }
}

Traversal

時間複雜度(time complexity)

See: algorithms time complexity - 時間複雜度

遞歸(recursive function)

// 1. sum of 1 + 2 + 3 + 4 + .... + 18 + 19 + 20
// 2. sum of 1 + 3 + 5 + .... + 17 + 19
// 3. sum of 2 + 4 + 6 + .... + 18 + 20

public class Recursive {
    private static int incrementRecursive(int x) {
        if (x == 1) {
            return 1;
        }
        return incrementRecursive(x - 1) + x;
    }

    private static int oddRecursive(int x) {
        if (x == 1) {
            return 1;
        }
        return oddRecursive(x - 2) + x;
    }

    private static int evenRecursion(int x) {
        if (x == 2) {
            return 2;
        }
        return evenRecursion(x -2) + x;
    }
}

Array

一維數組

  • 基本語法: type[] array_name = new type[]

  • Array → List:
    Integer[] array = new Integer[] {1, 2, 3};
    List<Integer> list = new Arrays.asList(array);
    
  • List → Array:
    List<String> list = new ArrayList<>();
    list.add("element1");
    list.add("element2");
    String[] array = new String[list.size()];
    array = list.toArray(array);
    

二維數組

  • 基本語法: type[][] array_name = new type[row][column]
  • 二維數組:
    • 優點:簡單、易懂,在二維關係比較滿的時候省空間
    • 缺點:在二維關係不滿的時候,浪費空間


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