Develop code to extract data from an object using peek() and map() methods including primitive versions of the map() method

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map

<R> Stream<R> map(Function<? super T, ? extends R> mapper)
DoubleStream map(DoubleUnaryOperator mapper)
IntStream map(IntUnaryOperator mapper)
LongStream map(LongUnaryOperator)

peek

Stream<T> peek(Consumer<? super T> action)
DoubleStream peek(DoubleConsumer action)
IntStream peek(IntConsumer action)
LongStream peek(LongConsumer action)

The method peek is an intermediate operation, the most common use for peek is to exam the stream contents without consuming it.

OCPJP>cat ExtractDataDemo.java 

import java.util.stream.*;

import java.util.Arrays;

import java.util.List;

public class ExtractDataDemo {

    public static void main(String[] args) {

        List<String> list = Arrays.asList("a", "bbb", "cc");

        Stream<String> stream = list.stream();

        stream.map(i -> new StringBuffer(i + i).append("ok"))

        .peek(i -> System.out.println("Stream<T>.map.peek: " + i.length())).count();

        

        DoubleStream doubleStream = list.stream().mapToDouble(String::length);

        doubleStream.map(Math::asin).peek(d -> System.out.println("DoubleStream.map.peek: " + d)).findFirst();

        

        IntStream intStream = list.stream().mapToInt(String::length);

        intStream.map(i -> i + 10).peek(i -> System.out.println("IntStream.map.peek: " + i)).summaryStatistics();

        

        LongStream longstream = list.stream().mapToLong(String::length);

        longstream.peek(i -> System.out.println("LongStream.peek.map: " + i)).map(i -> i + 10L).allMatch(i -> i > 10);

    }

}

OCPJP>

OCPJP>

OCPJP>

OCPJP>javac ExtractDataDemo.java 

OCPJP>java ExtractDataDemo

Stream<T>.map.peek: 4

Stream<T>.map.peek: 8

Stream<T>.map.peek: 6

DoubleStream.map.peek: 1.5707963267948966

IntStream.map.peek: 11

IntStream.map.peek: 13

IntStream.map.peek: 12

LongStream.peek.map: 1

LongStream.peek.map: 3

LongStream.peek.map: 2

OCPJP>

notice the effect of findFirst on the DoubleStream's peek method -- Stream is lazy, when the JVM executes the stream, it optimizes all the operations as a whole -- since FindFirst only need one element, peek didn't bother to peek a single extra element.