Java SE 8 Lambdas and Streams - Java: How to Program Early Objects - page 739

Using this in a Lambda That Appears in an Instance Method
As in an anonymous inner class, a lambda can use the outer class's this reference. In an
an anonymous inner class, you must use the syntax OuterClassName .this —otherwise, the
this reference would refer to the object of the anonymous inner class . In a lambda, you refer
to the object of the outer class, simply as this .
Parameter and Variable Names in a Lambda
The parameter names and variable names that you use in lambdas cannot be the same as as
any other local variables in the lambda's lexical scope; otherwise, a compilation error occurs.
17.3.2 Terminal Operations count , min , max , sum and average
Class IntStream provides various terminal operations for common stream reductions on
streams of int values. Terminal operations are eager —they immediately process the items
in the stream. Common reduction operations for IntStream s include:
• count (line 19) returns the number of elements in the stream.
• min (line 21) returns the smallest int in the stream.
• max (line 23) returns the largest int in the stream.
• sum (line 24) returns the sum of all the int s in the stream.
• average (line 26) returns an OptionalDouble (package java.util ) containing
the average of the int s in the stream as a value of type double . For any stream,
it's possible that there are no elements in the stream. Returning OptionalDouble
enables method average to return the average if the stream contains at least one
element . In this example, we know the stream has 10 elements, so we call class
OptionalDouble 's getAsDouble method to obtain the average. If there were no
elements , the OptionalDouble would not contain the average and getAsDouble
would throw a NoSuchElementException . To prevent this exception, you can in-
stead call method orElse , which returns the OptionalDouble 's value if there is
one, or the value you pass to orElse , otherwise.
Class IntStream also provides method summaryStatistics that performs the count , min ,
max , sum and average operations in one pass of an IntStream 's elements and returns the
results as an IntSummaryStatistics object (package java.util ). This provides a signifi-
cant performance boost over reprocessing an IntStream repeatedly for each individual op-
eration. This object has methods for obtaining each result and a toString method that
summarizes all the results. For example, the statement:
System.out.println(IntStream.of(values).summaryStatistics());
produces:
IntSummaryStatistics{count=10, sum=55, min=1, average=5.500000,
max=10}
for the array values in Fig. 17.5.
17.3.3 Terminal Operation reduce
You can define your own reductions for an IntStream by calling its reduce method as
shown in lines 29-31 of Fig. 17.5. Each of the terminal operations in Section 17.3.2 is a