[toc] https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html ## 8 Different Primitive Data Types | Type | Size(bit) | Limit/Details | Default Value (Field) | | -------- | -------- | -------- | -------------- | | byte | 8 | -128 ~ 127 |0| | short | 16 | -32768 ~ 32767 |0| | int | 32 | -2147483648 ~ 2147483647 |0| | long | 64 | -2^63 ~ 2^63-1 |0L| | float | 32 | (1+8+23) (see IEEE754) |0.0F| | double | 64 | (1+11+52) (see IEEE754) |0.0D| | char | 16 | 0 ~ 65535 |\u0000| | boolean | 1 | true / false |false| - byte 8-bit signed two's complement integer - short 16-bit signed two's complement integer - int 32-bit signed two's complement integer - long 64-bit signed two's complement integer - float single-precision 32-bit IEEE 754 floating point - double double-precision 64-bit IEEE 754 floating point - boolean true and false, the size is not declared btw. - char single 16-bit Unicode character '\u0000' (0) to '\uffff' (65535) --- 宣告Primitive變數的時候, 會使用一個約4bytes大小 + (符合該型別大小->不一定) 的記憶體空間。 [(link)]( https://docs.oracle.com/javase/tutorial/java/javaOO/objectcreation.html) 因此Primitive在賦值時只有值的複製。 範例程式: ``` int i; // 宣告i變數, 值等於初始值(詳見### Default Value) i = 3; // 把i設為3 ``` 等同於: ``` int i = 3; ``` --- ## Literals ### 整數 如果整數L/l結尾, 則為long, 否則為int. (建議使用大寫) ```java 10000; // int 2147483650L; // long 2147483650; // invalid int i = 10000; // 10000 int int i2 = 2147483650L; // invalid int i3 = 2147483650; // invalid long l = 10000; // 10000 long long l2 = 2147483650L; // 2147483650 long long l2 = 2147483650; // invalid ``` --- 使用不同數字系統來表示數值 ```java // number 26 in decimal int decVal = 26; // number 26 in octal int octVal = 032; // number 26 in hexadecimal int hexVal = 0x1a; int hexVal2 = 0x1A; // number 26 in binary int binVal = 0b11010; ``` --- ### 浮點數 如果以以F/f結尾, 則為float, 否則為double, 並且可選擇以D/d結尾. (建議使用大寫) 也可使用E/e來表示科學記號 ```java 123.4 // double 1.234e2 // 科學記號 1.234f // float ``` ### charr, boolean ```java char c = '\u2001'; char c2 = 'a'; char c3 = 492; boolean b = true; boolean b2 = false; ``` ### 底線 可以在數字之間任意新增底線`_`, 除了: - 數字的頭尾 ```java int i = _123; // invalid int i2 = 1_23; // valid int i3 = 123_; // invalid int i4 = 0x_123; // invalid ``` - 小數點左右 ```java float f1 = 3_.1415F; // invalid float f2 = 3._1415F; // invalid ``` - F/L/D之前 ```java long l = 9999_L; // invalid ``` - 插在radix前/中/後 ```java int i4 = _0x52; // invalid int i5 = 0_x52; // invalid int i6 = 0x_52; // invalid ```