Java Streams API Preview

for today’s post i want to share a series of examples that i have developed while trying the latest features in the java 8 stream api. in my last post i did a comparison of the features with those available in linq, and for today’s post i have decided to go a bit further and try to use the api to work with a small domain model. the examples developed here are based on the same examples presented in the lambdaj project .

the data model

for the examples i will use the following domain model:

you can see a full implementation of all the examples developed here by downloading the following gist .

for the examples presented below assume that  in the context of the code there are three streams always available:

• stream<person> persons .
• stream<car> cars .
• stream<sale> sales .

challenge 1: print all car brands

from a collection of cars print all car brands.

stringjoiner brands = cars.map(car::getbrand)
.collect(tostringjoiner(","));
string allbrands = brands.tostring();

for this example i have also use the new stringjoiner class.

challenge 2: select all sales on toyota

from a collection of sales, select all those that are related to toyota cars.

list<sale> toyotasales;
toyotasales = sales.filter(s -> s.getcar().getbrand().equals("toyota"))
.collect(tolist());
toyotasales.foreach(system.out::println);

for this example i could have also used the foreach method in the stream to get all the sales printed. i did it this way just to illustrate that it is possible to collect all items in the stream into a list and from there we can process them. but ideally, i should have processed the items directly in the stream.

challenge 3: find buys of the youngest person

from a collection of sales, find all those that are from the youngest buyer.

tointfunction<entry<person, list<sale>>> byage;
byage = e -> e.getkey().getage();
byyoungest = sales.collect(groupingby(sale::getbuyer))
.entryset()
.stream()
.sorted(comparing(byage))
.map(entry::getvalue)
.findfirst();
if(byyoungest.ispresent()) {
system.out.println(byyoungest.get());
}

challenge 4: find most costly sale

from a collection of sales, find the most costly of all of them.

optional<sale> mostcostlysale;
comparator<sale> bycost = comparing((todoublefunction<sale>)sale::getcost)
.reverseorder();
mostcostlysale = sales.sorted( bycost )
.findfirst();
if(mostcostlysale.ispresent()) {
system.out.println(mostcostlysale.get());
}

challenge 5: sum of sales from male buyers & sellers

from a collection of sales find the sum of all buys/sells made by men.

doublesum = sales.filter(s -> s.getbuyer().ismale()
&& s.getseller().ismale())
.maptodouble(sale::getcost)
.sum();

challenge 6: find the age of the youngest buyer

from a collection of sales, find the age of the youngest buyer who bought for more than 40,000.

optionalint ageofyoungest;
ageofyoungest = sales.filter(sale -> sale.getcost() > 40000)
.map(sale::getbuyer)
.maptoint(person::getage)
.sorted()
.findfirst();
if(ageofyoungest.ispresent()) {
system.out.println(ageofyoungest.getasint());
}

challenge 7: sort sales by cost

sort a collection of sales by cost.

comparator<sale> bycost= comparing((todoublefunction<sale>) sale::getcost);
list<sale> sortedbycost;
sortedbycost = sales.sorted( bycost )
.collect(tolist());

challenge 8: index cars by brand

from a collection of cars, index cars by the their brand.

map<string,list<car>> bybrand;
bybrand = cars.collect( groupingby(car::getbrand ));

challenge 9: find most bought car

from a collection of sales find the most bought car.

tointfunction<entry<car,list<sale>>> tosize = (e -> e.getvalue().size());
optional<car> mostbought;
mostbought = sales.collect( groupingby(sale::getcar) )
.entryset()
.stream()
.sorted( comparing(tosize).reverseorder() )
.map(entry::getkey)
.findfirst();
if(mostbought.ispresent()) {
system.out.println(mostbought.get());
}

related posts

• java streams preview vs. .net high-order programming with linq
• java infinite streams

futher reading

• jdk 8 early access
• project lambda
• jep 107: bulk data operations for collections
• where is the java collections framework going?