We have already seen some examples on Collectors in previous post. In this post, we are going to see Java 8 Collectors groupby example. Groupby is another feature added in java 8 and it is very much similar to SQL/Oracle.
Lets understand more with the help of example: Lets create our model class country as below:
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package
org
.
arpit
.
java2blog
;
public
class
Country
{
String
countryName
;
long
population
;
public
Country
(
)
{
super
(
)
;
}
public
Country
(
String
countryName
,
long
population
)
{
super
(
)
;
this
.
countryName
=
countryName
;
this
.
population
=
population
;
}
public
String
getCountryName
(
)
{
return
countryName
;
}
public
void
setCountryName
(
String
countryName
)
{
this
.
countryName
=
countryName
;
}
public
long
getPopulation
(
)
{
return
population
;
}
public
void
setPopulation
(
long
population
)
{
this
.
population
=
population
;
}
@
Override
public
int
hashCode
(
)
{
final
int
prime
=
31
;
int
result
=
1
;
result
=
prime *
result
+
(
(
countryName
==
null
)
?
0
:
countryName
.
hashCode
(
)
)
;
result
=
prime *
result
+
(
int
)
(
population
^
(
population
>
>
>
32
)
)
;
return
result
;
}
@
Override
public
boolean
equals
(
Object
obj
)
{
if
(
this
==
obj
)
return
true
;
if
(
obj
==
null
)
return
false
;
if
(
getClass
(
)
!
=
obj
.
getClass
(
)
)
return
false
;
Country
other
=
(
Country
)
obj
;
if
(
countryName
==
null
)
{
if
(
other
.
countryName
!
=
null
)
return
false
;
}
else
if
(
!
countryName
.
equals
(
other
.
countryName
)
)
return
false
;
if
(
population
!
=
other
.
population
)
return
false
;
return
true
;
}
public
String
toString
(
)
{
return
"{"
+
countryName
+
","
+
population
+
"}"
;
}
}
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package
org
.
arpit
.
java2blog
;
import
java
.
math
.
BigDecimal
;
import
java
.
util
.
Arrays
;
import
java
.
util
.
List
;
import
java
.
util
.
Map
;
import
java
.
util
.
stream
.
Collectors
;
public
class
Java8CollectorsGroupBy
{
public
static
void
main
(
String
[
]
args
)
{
List
items
=
Arrays
.
asList
(
new
Country
(
"India"
,
20000
)
,
new
Country
(
"China"
,
40000
)
,
new
Country
(
"Nepal"
,
30000
)
,
new
Country
(
"India"
,
50000
)
,
new
Country
(
"China"
,
10000
)
)
;
// Group by countryName
Map
<
String
,
List
>
groupByCountry
=
items
.
stream
(
)
.
collect
(
Collectors
.
groupingBy
(
Country
:
:
getCountryName
)
)
;
System
.
out
.
println
(
groupByCountry
.
get
(
"India"
)
)
;
// Group by CountryName and calculates the count
Map
<
String
,
Long
>
counting
=
items
.
stream
(
)
.
collect
(
Collectors
.
groupingBy
(
Country
:
:
getCountryName
,
Collectors
.
counting
(
)
)
)
;
// Group by countryName and sum up the population
System
.
out
.
println
(
counting
)
;
Map
<
String
,
Long
>
populationCount
=
items
.
stream
(
)
.
collect
(
Collectors
.
groupingBy
(
Country
:
:
getCountryName
,
Collectors
.
summingLong
(
Country
:
:
getPopulation
)
)
)
;
System
.
out
.
println
(
populationCount
)
;
}
}
When you run above class, you will get below output:
[{India,20000}, {India,50000}]
{China=2, Nepal=1, India=2}
{China=50000, Nepal=30000, India=70000}
Reference:
https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html
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