?????????????????????????Java 8 Streams??????????????????????????????￡???????????????????????????????????????????????Щ??????????????????????е???????????????????????????????????????t??????????????????????κ?????
????????????????У?????????????????Java 8 Streams????????SQL??Ч??????????????и????????????????????Щ??????????????????Щ??????Щ????????????????????????????????????????
????https://dzone.com/articles/java-streams-groupingby-examples
????https://dzone.com/articles/using-java-collectors
???????????????????“??????”???????????????????????????????Excel?????????????????????????????????ο??????????
????http://www.novixys.com/blog/excel-pivot-table-using-apache-poi/
??????CSV???????POJO
?????????????μ?POJO??????Java????Plain Ordinary Java Object??????????????????乤???
public class Player {
private int year;
private String teamID;
private String lgID;
private String playerID;
private int salary;
// defined getters and setters here
}
?????????????????????????CSV???????????????????????Σ????ж??е???Σ????????????л???????????????????????????????????????????????CSV?????????????????????б???С???????????????????????????д??26428?У?
????yearID??teamID??lgID??playerID??salary
????1985??ATL??NL??barkele01??870000
????1985??ATL??NL??bedrost01??550000
????1985??ATL??NL??benedbr01??545000
????1985??ATL??NL??campri01??633333
????1985??ATL??NL??ceronri01??625000
????...
??????????????????????????Streams????CSV???????
Pattern pattern = Pattern.compile("??");
try (BufferedReader in = new BufferedReader(new FileReader(filename));){
List<Player> players = in
.lines()
.skip(1)
.map(line -> {
String[] arr = pattern.split(line);
return new Player(Integer.parseInt(arr[0])??
arr[1]??
arr[2]??
arr[3]??
Integer.parseInt(arr[4]));
})
.collect(Collectors.toList());
}
?????????????????е???
????????????????????????????????????????????????Щ???????????????з????????????????SQL???????Щ?н???????“GROUP BY”??????С?
public class YearTeam
{
public int year;
public String teamID;
public YearTeam(int year??String teamID) {
this.year = year;
this.teamID = teamID;
}
@Override
public boolean equals(Object other)
{
if ( other == null ) return false;
if ( this == other ) return true;
if ( other instanceof YearTeam ) {
YearTeam yt = (YearTeam)other;
if ( year == yt.year && teamID.equals(yt.teamID) )
return true;
}
return false;
}
@Override
public int hashCode()
{
int hash = 1;
hash = hash * 17 + year;
hash = hash * 31 + teamID.hashCode();
return hash;
}
@Override
public String toString()
{
StringBuilder sbuf = new StringBuilder();
sbuf.append('[').append(year).append("?? ").append(teamID)
.append(']');
return sbuf.toString();
}
}
???????????????????????Щ??α????????“public?????У?”????????????????????????ó????????????????????“private????У?”?????????????????getter??/??setter??
?????????????д??equals()??hashCode()???????????洢???Map?????????????key????????á?
??????Streams????????
???????????????CSV???????????д??????POJO??????????????????????е?????????????з??顣
????Map<YearTeam??List<Player>> grouped = in
????.lines()
????.skip(1)
????.map(line -> {
????String[] arr = pattern.split(line);
????return new Player(Integer.parseInt(arr[0])??
????arr[1]??
????arr[2]??
????arr[3]??
????Integer.parseInt(arr[4]));
????})
????.collect(Collectors.groupingBy(x-> new YearTeam(x.getYear()?? x.getTeamID())));
???????????Щ????????????????????????Map????????У?????????????????????顣
??????????????????CSV
?????????????????????????????????CSV??????????????????????Excel????????????????????????????????????summingLong()??Java 8 Streams????????averagingLong()????????????????????????????????????????????summarizingLong()????????????????????????????????e?!
????CSV?е????????
???????????teamID??????????????е????????????????????????????????????????????????TreeSet???????????????????С?
????Set<String> teams = grouped
????.keySet()
????.stream()
????.map(x -> x.teamID)
????.collect(Collectors.toCollection(TreeSet::new));
????System.out.print('??');
????teams.stream().forEach(t -> System.out.print(t + "??"));
????System.out.println();
???????????
????????????????????????????????????????????????????????????????б??????????????????????????????????
Set<Integer> years = grouped
.keySet()
.stream()
.map(x -> x.year)
.collect(Collectors.toSet());
years
.stream()
.forEach(y -> {
System.out.print(y + "??");
teams.stream().forEach(t -> {
YearTeam yt = new YearTeam(y?? t);
List<Player> players = grouped.get(yt);
if ( players != null ) {
long total = players
.stream()
.collect(Collectors.summingLong(Player::getSalary));
System.out.print(total);
}
System.out.print('??');
});
System.out.println();
});
???????????Excel?????
???????????CSV????????Excel?в?????????????
??????Excel??????????????????б????????Щ????????????(?????????????????????“MON”????????Excel????????????????????“????”?е????ɡ??????????????????????)
??????????????????Java??????Collections??????????????????????????????????????????????????????!
???????
?????????????????????????????????????????????????????????????Excel??????????????????????????????Java 8 Streams??????????????????????????????????????????????ù????