Eight bit integer 11111111 is -1 111 00100000 32 11111111 -------- 00011111 31
Uh oh.
(base) MAC:Mon Apr 12:14:10:HP> jshell
| Welcome to JShell -- Version 15.0.1
| For an introduction type: /help intro
jshell> x = 5
| Error:
| cannot find symbol
| symbol: variable x
| x = 5
| ^
In Java, variables have type. Here we create an
integer variable x with value 5.
jshell> int x = 5;
x ==> 5
The type of a variable canoot be changed during its lifetime. Variables can only bind to objects of their own types.
jshell> x = "cowpies"
| Error:
| incompatible types: java.lang.String cannot be converted to int
| x = "cowpies"
| ^-------^
Now watch this.
jshell> x = 1;
x ==> 1
Here we see all of the variables we have created.
jshell> /vars
| int x = 1
This looks preposterous! It's an infinite loop!
jshell> while(x > 0){ x = x + 1;}
Beezlebub! It's a dark conspiracy.
jshell> x
x ==> -2147483648
No, it is actually just doughnuts. Think of the integers
as being a circle instead of along a line. The type
int is a 32 bit signed integer in
Twos Complement Noatation.
jshell> x = x - 1
x ==> 2147483647
jshell> x
x ==> 2147483647
jshell> byte b = 1;
b ==> 1
jshell> while(b > 0){b++;}
jshell> b
b ==> -128
Bytes are between -128 and 127.
jshell> short s = 1
s ==> 1
jshell> while(s > 0){s++;}
jshell> s
s ==> -32768
A short is a 16 bit signed integer.
jshell> long ell = 1;
ell ==> 1
jshell> for(int k = 0; k < 30; k++){ell *= 2;}
jshell> ell
ell ==> 1073741824
jshell> for(int k = 0; k < 30; k++){ell *= 2;}
jshell> ell
ell ==> 1152921504606846976
jshell> ell *= 2
$19 ==> 2305843009213693952
jshell> ell *= 2
$20 ==> 4611686018427387904
jshell> ell *= 2
$21 ==> -9223372036854775808
That number at the bottom is 263 - 1. Longs are 64 bit signed integers.
There are four integer types byte(8), short(16), int(32), long(64). Here we are making a string.
jshell> String s = "fooment";
s ==> "fooment"
jshell> s
s ==> "fooment"
Boola! Boola!
jshell> 4 < 5
$24 ==> true
jshell> 4 > 5
$25 ==> false
The boolean constants: true, false.
And now for a broken cast.
jshell> (boolean) 1
| Error:
| incompatible types: int cannot be converted to boolean
| (boolean) 1
| ^
Floating point time.
jshell> double z = 4.22422
z ==> 4.22422
jshell> //64 bit floating point number (IEEE 754 standard)
jshell> float boat = 5.2f;
boat ==> 5.2
jshell> //32 bit floating point number (IEEE 754 standard)
Arithmetic is no surprise.
jshell> 4 + 5
$28 ==> 9
jshell> 4 + .3
$29 ==> 4.3
There is a separate character type that has some fluidity with integers. Use single quotes for character literals.
jshell> char x = 'a';
x ==> 'a'
Characters are unicode.
jshell> (int) x
$31 ==> 97
jshell> (char) 945
$32 ==> 'α'
jshell> (char) 946
$33 ==> 'β'
jshell> (char) 947
$34 ==> 'γ'
jshell> (char)5000
$35 ==> 'ᎈ'
jshell> (char)5545
$36 ==> 'ᖩ'
jshell> (char)2000
$37 ==> 'ߐ'
jshell> (char)2001
$38 ==> 'ߑ'
jshell> (char)2002
$39 ==> 'ߒ'
jshell> (char)2003
$40 ==> 'ߓ'
jshell> (char)2004
$41 ==> 'ߔ'
You have met the eight primitive types.
- four integer types: byte, int, short, long
- two floating-point types: float, double
- the boolean type
- the character type
Here are all of the local variables we now have. Note that replies from jshell are also given variables.
jshell> /vars
| byte b = -128
| long ell = -9223372036854775808
| long $19 = 2305843009213693952
| long $20 = 4611686018427387904
| long $21 = -9223372036854775808
| String s = "fooment"
| boolean $24 = true
| boolean $25 = false
| double z = 4.22422
| float boat = 5.2
| int $28 = 9
| double $29 = 4.3
| char x = 'a'
| int $31 = 97
| char $32 = 'α'
| char $33 = 'β'
| char $34 = 'γ'
| char $35 = 'ᎈ'
| char $36 = 'ᖩ'
| char $37 = 'ߐ'
| char $38 = 'ߑ'
| char $39 = 'ߒ'
| char $40 = 'ߓ'
| char $41 = 'ߔ'
jshell> $35
$35 ==> 'ᎈ'
Behold this magic.
jshell> s.toUpperCase()
$43 ==> "FOOMENT"
Ooh, this is not like Python.
jshell> s[0]
| Error:
| array required, but java.lang.String found
| s[0]
| ^--^
Do this. Indexing works just as it does in Python.
jshell> s.charAt(0)
$44 ==> 'f'
jshell> s.charAt(1)
$45 ==> 'o'
jshell>