Working with streams - Java 8 in Action: Lambdas, Streams, and Functional-Style Programming - page 131

Trader mario = new Trader("Mario","Milan");
Trader alan = new Trader("Alan","Cambridge");
Trader brian = new Trader("Brian","Cambridge");
List<Transaction> transactions = Arrays.asList(
new Transaction(brian, 2011, 300),
new Transaction(raoul, 2012, 1000),
new Transaction(raoul, 2011, 400),
new Transaction(mario, 2012, 710),
new Transaction(mario, 2012, 700),
new Transaction(alan, 2012, 950)
);
Trader and Transaction are classes defined as follows:
public class Trader{
private final String name;
private final String city;
public Trader(String n, String c){
this.name = n;
this.city = c;
}
public String getName(){
return this.name;
}
public String getCity(){
return this.city;
}
public String toString(){
return "Trader:"+this.name + " in " + this.city;
}
}
public class Transaction{