Exercise/completed #2

tboychuk · 2018-08-14T06:42:51Z

Complete exercises:

MathFunctions
CrazyLambdas
AccountAnalytics
SumOfSquares

- implemented Functions.java

# Conflicts: # account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

# Conflicts: # crazy-lambdas/src/main/java/com/bobocode/CrazyLambdas.java

# Conflicts: # account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

omgkanamikun · 2018-08-14T09:38:26Z

account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

    public Map<Boolean, List<Account>> partitionMaleAccounts() {
-        throw new UnsupportedOperationException("It's your job to implement this method"); // todo
+        return accounts.stream()
+                .collect(partitioningBy(a -> a.getSex().equals(Sex.MALE)));


В мене в IDE метод .collect(partition____) не розпізнає за замовчуванням, та не знаходить параметри,
геттери в Account додавав.
Запрацювало після статік імпорта, але це як підножка при написанні, вчора теж на цьому завис.

You can start by typing Collectors. and it will show you all available methods like partitionBy()

Account class is mapped with lombok annotations. So it requires to enable annotation processing in your IDE + lombok plugin

omgkanamikun · 2018-08-14T12:05:56Z

account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

    public Map<String, List<Account>> groupAccountsByEmailDomain() {
-        throw new UnsupportedOperationException("It's your job to implement this method"); // todo
+        return accounts.stream()
+                .collect(groupingBy(a -> a.getEmail().split("@")[1]));


що значить [1]?

It means, that after splitting email by string @ you will have an array String[]. The first element of that array will contain a username, and the second will contain email domain name.

E.g. "omgkanamikun@gmail.com".split("@") will result in array ["omgkanamikun", "gmail.com"]. So to get a domain name, you need to get an element from array by index 1.

tboychuk

Answered question

tboychuk · 2018-08-15T11:07:18Z

account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

    public Map<Boolean, List<Account>> partitionMaleAccounts() {
-        throw new UnsupportedOperationException("It's your job to implement this method"); // todo
+        return accounts.stream()
+                .collect(partitioningBy(a -> a.getSex().equals(Sex.MALE)));


You can start by typing Collectors. and it will show you all available methods like partitionBy()

tboychuk · 2018-08-15T11:08:25Z

account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

    public Map<Boolean, List<Account>> partitionMaleAccounts() {
-        throw new UnsupportedOperationException("It's your job to implement this method"); // todo
+        return accounts.stream()
+                .collect(partitioningBy(a -> a.getSex().equals(Sex.MALE)));


Account class is mapped with lombok annotations. So it requires to enable annotation processing in your IDE + lombok plugin

tboychuk · 2018-08-15T11:12:02Z

account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

    public Map<String, List<Account>> groupAccountsByEmailDomain() {
-        throw new UnsupportedOperationException("It's your job to implement this method"); // todo
+        return accounts.stream()
+                .collect(groupingBy(a -> a.getEmail().split("@")[1]));


It means, that after splitting email by string @ you will have an array String[]. The first element of that array will contain a username, and the second will contain email domain name.

E.g. "omgkanamikun@gmail.com".split("@") will result in array ["omgkanamikun", "gmail.com"]. So to get a domain name, you need to get an element from array by index 1.

FedosovMax · 2018-08-17T14:13:38Z

account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

-        throw new UnsupportedOperationException("It's your job to implement this method"); // todo
+        return accounts.stream()
+                .map(Account::getBalance)
+                .reduce(BigDecimal.ZERO, BigDecimal::add);


Why do you use ZERO after "BigDecimal" ? It's something about method reduce?

Method reduce is overloaded. So you have two options:

Optional<T> reduce(BinaryOperator<T> accumulator);

T reduce(T identity, BinaryOperator<T> accumulator);

I used the second one, that has an identity (initial) value, because I wanted to receive real value rather than optional

FedosovMax · 2018-08-17T14:40:04Z

account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

-        throw new UnsupportedOperationException("It's your job to implement this method"); // todo
+        return accounts.stream()
+                .map(Account::getFirstName)
+                .flatMapToInt(String::chars)


Can you please explain how works this function? The most important thing, what does method chars?

Sure. Method String#chars() creates a IntStream of characters. (It's because char is basically an int, and JDK doesn't provide a special stream class to hold primitive characters). So, if you want to get a stream of primitive char, you get IntStream.

accounts.stream().map(Account::getFirstName) will return a Stream<String>, so if you will map each string element into a stream of its chars, you will get Stream<IntStream> - a stream of streams.

flatMapToInt(String::chars) is a complex operation that consists of two basic operations:

"MapToInt" which receives a mapper - function that maps String into an IntStream

"flat" transforms Stream<IntStream> into a IntStream, e.g. ((3, 4), (3, 5, 7), (4)) -> (3, 4, 3, 5, 7, 4). Makes it flat.

Now, when you have an IntStream that consists of character codes, you want to transform it into a Stream<Character>, because after you will want to group them into a Map<Character, Long>

mapToObj(c -> (char) c) uses a mapper that cast int to char, and then map char to object Character

Now, when you have a Stream<Character>, you can collect them into a map.

groupingBy(Function.identity(), counting()) will group characters using its real value (because Function.identity() maps each element into itself) as a key, and then will count occurrences of each element (counting()) and use that number as map value

FedosovMax · 2018-08-18T10:12:58Z

account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

+        return accounts.stream()
+                .collect(groupingBy(a -> a.getCreationDate().getMonth(),
+                        mapping(Account::getBalance,
+                                reducing(BigDecimal.ZERO, BigDecimal::add))));


Can't understand what means T identity, and how it works in the method "reduce".
Method ===> "reduce(T identity, BinaryOperator accumulator);"

The value of identity will be used as a first value, when it reduces your stream. In case your stream is empty, reduce() will return identity value.

FedosovMax · 2018-09-18T17:15:14Z

account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

-        throw new UnsupportedOperationException("It's your job to implement this method"); // todo
+        return accounts.stream()
+                .sorted(comparing(Account::getFirstName)
+                        .thenComparing(Account::getLastName))


Почему здесь не получается использовать лямбду а нужно обязательно использовать methodReference? Не дает сделать Идея лямбду.

# Conflicts: # crazy-lambdas/src/main/java/com/bobocode/CrazyLambdas.java

Taras and others added 17 commits May 5, 2018 07:01

Exercise completed

6d57b66

- implemented Functions.java

Merge branch 'master' into exercise/completed

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Complete task

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Merge branch 'master' into exercise/completed

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# Conflicts: # account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

Update form master

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Add Sum of Squares exercise

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Merge branch 'master' into exercise/completed

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Complete the exercise

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Merge branch 'master' into exercise/completed

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Merge branch 'master' into exercise/completed

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Complete the exercise CrazyLambdas.java

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Merge branch 'master' into exercise/completed

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Merge branch 'master' into exercise/completed

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# Conflicts: # crazy-lambdas/src/main/java/com/bobocode/CrazyLambdas.java

Complete the exercise CrazyLambdas.java

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Merge branch 'master' into exercise/completed

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# Conflicts: # account-analytics/src/main/java/com.bobocode/AccountAnalytics.java

Complete AccountAnalytics.java exercise

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Merge branch 'master' into exercise/completed

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tboychuk self-assigned this Aug 14, 2018

tboychuk requested review from FedosovMax, Podolian, omgkanamikun and yura-jt August 14, 2018 06:42

bobocode-projects deleted a comment from Podolian Aug 14, 2018

omgkanamikun reviewed Aug 14, 2018

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tboychuk commented Aug 15, 2018

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FedosovMax reviewed Aug 22, 2018

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FedosovMax reviewed Sep 18, 2018

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tboychuk added 2 commits October 22, 2018 14:49

Merge branch 'master' into exercise/completed

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Merge branch 'master' into exercise/completed

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tboychuk added 8 commits November 8, 2018 00:27

Merge branch 'master' into exercise/completed

56ab37b

Add impl for new method of AccountAnalytics.java

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Merge branch 'master' into exercise/completed

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Implement new methods in AccountAnalytics.java

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Merge branch 'master' into exercise/completed

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Implement new methods in AccountAnalytics.java

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Merge branch 'master' into exercise/completed

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# Conflicts: # crazy-lambdas/src/main/java/com/bobocode/CrazyLambdas.java

Complete new tasks in CrazyLambdas.java

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Exercise/completed #2

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tboychuk commented Aug 14, 2018

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omgkanamikun Aug 14, 2018 •

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