Что учить после синтаксиса Java?

jshell> "3 бутылки пива по 37 копеек"
$1 ==> "3 бутылки пива по 37 копеек"
 
jshell> "3 бутылки пива по 37 копеек"
$2 ==> "3 бутылки пива по 37 копеек"
 
jshell> var strs = $1.split("\\W")
strs ==> String[18] { "3", "", "", "", "", "", "", "", "", ... "", "", "", "", "", "37" }
 
jshell> var strs = $1.split("\\W+")
strs ==> String[2] { "3", "37" }
 
jshell> Arrays.stream(strs).mapInt(Integer::parseInteger).toArray()
|  Error:
|  invalid method reference
|    cannot find symbol
|      symbol:   method parseInteger()
|  Arrays.stream(strs).mapInt(Integer::parseInteger).toArray()
|                             ^-------------------^
 
jshell> Arrays.stream(strs).mapInt(Integer::parseInt).toArray()
|  Error:
|  cannot find symbol
|    symbol:   method mapInt(Integer::parseInt)
|  Arrays.stream(strs).mapInt(Integer::parseInt).toArray()
|  ^------------------------^
 
jshell> Arrays.stream(strs).mapToInt(Integer::parseInt).toArray()
$5 ==> int[2] { 3, 37 }
 
jshell> var i = $5[0]*$5[1]
i ==> 111
 
jshell> i/10.
$7 ==> 11.1
 
jshell> i/100.
$8 ==> 1.11
 
jshell> var r = i%100
r ==> 11
 
jshell> var r = (i/100)
r ==> 1
 
jshell> var c = i%100
c ==> 11
 
jshell> $1.indexOf('к')
$12 ==> 7
 
jshell> $1.substring($12)
$13 ==> "ки пива по 37 копеек"
 
jshell> $1.indexLast('к')
|  Error:
|  cannot find symbol
|    symbol:   method indexLast(char)
|  $1.indexLast('к')
|  ^----------^
 
jshell> $1.lastIndexOf('к')
$14 ==> 26
 
jshell> $1.substring($14)
$15 ==> "к"
 
jshell> $1.substring($14)
 
jshell> $1.split(" ")
$16 ==> String[6] { "3", "бутылки", "пива", "по", "37", "копеек" }
 
jshell> $16[2].charAt(0)+1
$17 ==> 1088
 
jshell> $16[1].substring(0,2)
$18 ==> "бу"
 
jshell> new StringBuilder($18).reverse()
$19 ==> уб
 
jshell> (char)($17)+$19.toString()
$20 ==> "руб"
 
jshell> r+$20
$21 ==> "1руб"
 
jshell> c+$16[5]
$22 ==> "11копеек"
 
jshell> $21+$1.charAt(1)+$22
$23 ==> "1руб 11копеек"