Type determination

• java interface query:
  • interfaceDemo instanceof InterfaceDemo
  • subClassInstance instanceof ParentClass
• python
  • isinstance
• golang interface query:
  
  • value, ok := element.(T) element is interface var, T is base type
  • switch value := element.(type)

Cast

• golang not support implicit type cast
• tuncation
  • small type casts to large type is safe
  • large -> small causes tuncations, only keeps the data in low address. The situation depends on what saves in low address (big-endian/little-endian)

{% highlight java %} /*** java ***/ double doubleDemo = 0.1d; int intDemo = (int) doubleDemo ; {% endhighlight %}

{% highlight python %}

python

type(str(123))

bin(17), int('0b10001', 2) oct(20), int('024', 8) hex(22), int('0x16', 16)

suuport string ~ container(list, tuple, dict, set) conversion

str([0, 1, 2]), eval("[0, 1, 2]") {% endhighlight %}

{% highlight go %} /*** go ***/ var int32Demo int32 = 12345 var int64Demo int64 = int64(int32Demo)

switch value := element.(type) { case string: ... case []byte: case int: default: } {% endhighlight %}

Primitive

• Java:
  • null
  • boolean, byte, char(2 bytes), short, int, long, float, double
  • fraction literal is double by default. e.g. 3.14 is double type
• Python: everything is object
  • None
  • bool
  • int, long, float, complex
  • sequence: str, unicode, list, tuple
  • dict
  • set, frozenset

{% highlight python %}

map(bool, [None, 0, "", u"", list(), tuple(), dict(), set(), frozenset()]) [False, False, False, False, False, False, False, False, False] int(True), int(False) (1, 0) {% endhighlight %}

• Go:
  • nil
  • bool
  • int8, uint8, int16, int32, int64, int, uint, uintptr
  • float32, float64
  • complex64, complex128
  • string
  • byte(utf-8), rune(unicode), no char
  • error
  • compound type: array, slice, map, chan, struct, pointer, interface

{% highlight go %} demoInt := 2 if demoInt { fmt.Println("int value can be used by if") } error output: non-bool demoInt (type int) used as if condition {% endhighlight %}

• Javascript:
  • primitive type
    • undefined: undefined
    • null: null - can be treated as a specifal value of Object
    • boolean: true, false
    • number: double (64bits, 8 bytes), NaN, Infinity
    • string: UTF16
  • object type
    • Object
      • wrapper: Number, Boolean, String

{% highlight javascript %} var n = 1, b = true, s = "test" var Num = new Number(n); var Boo = new Boolean(b); var Str = new String(s); {% endhighlight %}

• check by typeof()
• Object type (wrapper for primitive) has methods, except null and undefined