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21 七月 2023
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停止寫循環！在 Java 8 中使用集合的十大最佳實踐

如您所知，我們的習慣是第二天性。一旦你習慣了寫作for (int i = 0; i <......)，你們中的任何人都不想重新學習這個結構（尤其是因為它非常簡單易懂）。然而，循環經常被重複用於執行相同的基本操作，而重複是我們非常希望擺脫的。在 Java 8 中，甲骨文決定幫助我們做到這一點。以下是 10 種最佳收集方法，它們將為您節省大量時間和代碼。

1. Iterable.forEach(消費者<?超T>動作)

名字足以說明問題。它遍歷作為參數傳遞的集合，並為其每個元素執行操作 lambda 表達式。


List <Integer> numbers = new ArrayList<>(Arrays.asList(1,2,3,4,5,6,7));
 numbers.forEach(s -> System.out.print(s + " "));


1 2 3 4 5 6 7

2. Collection.removeIf(Predicate<? super E> filter)

同樣，這裡沒有什麼困難。該方法遍歷集合併移除所有匹配的元素filter。


 List <Integer> numbers = new ArrayList<>(Arrays.asList(1,2,3,4,5,6,7));
 numbers.removeIf(s -> s > 5);
 numbers.forEach(s -> System.out.print(s + " "));

在一行中，我們從列表中刪除所有大於 5 的數字。

3. Map.forEach(BiConsumer <? super K, ? super V> 動作)

該forEach方法不僅適用於實現Collection接口的類，也適用於Map.


 Map <String, String> books = new HashMap<>();
 books.put("War and Peace", "Leo Tolstoy");
 books.put("Crime and Punishment", "Fyodor Dostoevsky");
 books.put("Thinking in Java", "Bruce Eckel");
 books.put("The Brothers Karamazov", "Fyodor Dostoevsky");
 books.put("The Lord of the Rings", "John Tolkien");
 books.forEach((a,b) -> System.out.println("Book title: " + a + ". Author: "+ b));


Book title: The Brothers Karamazov. Author: Fyodor Dostoevsky
Book title: Thinking in Java. Author: Bruce Eckel
Book title: Crime and Punishment. Author: Fyodor Dostoevsky
Book title: War and Peace. Author: Leo Tolstoy
Book title: Lord of the Rings. Author: John Tolkien

4.Map.compute(K鍵,BiFunction<?Super K,?Super V,?extends V>remappingFunction)

看起來有點嚇人，其實很簡單，就像之前的那些一樣。此方法將key的值設置為等於執行的結果mappingFunction。例如：


Map <String, String> books = new HashMap<>();
books.put("War and Peace", "Leo Tolstoy");
books.put("Crime and Punishment", "Fyodor Dostoevsky");
books.put("Thinking in Java", "Bruce Eckel");
books.put("The Brothers Karamazov", "Fyodor Dostoevsky");
books.put("The Lord of the Rings", "John Tolkien");
books.forEach((a,b) -> System.out.println("Book title: " + a + ". Author: "+ b));
 
books.compute("Thinking in Java", (a,b) -> b + ", cool dude");
System.out.println("_______________________");
books.forEach((a,b) -> System.out.println("Book title: " + a + ". Author: "+ b));


Book title: The Brothers Karamazov. Author: Fyodor Dostoevsky
Book title: Thinking in Java. Author: Bruce Eckel
Book title: Crime and Punishment. Author: Fyodor Dostoevsky
Book title: War and Peace. Author: Leo Tolstoy
Book title: Lord of the Rings. Author: John Tolkien
_______________________
Book title: The Brothers Karamazov. Author: Fyodor Dostoevsky
Book title: Thinking in Java. Author: Bruce Eckel, cool dude
Book title: Crime and Punishment. Author: Fyodor Dostoevsky
Book title: War and Peace. Author: Leo Tolstoy
Book title: Lord of the Rings. Author: John Tolkien

《Thinking in Java》的作者絕對牛逼！:)

5. Map.computeIfAbsent(K key, Function <? super K, ? extends V> mappingFunction)

此方法將向中添加一個新元素Map，但前提是它還沒有具有該鍵的元素。分配的值將是執行mappingFunction. 如果具有鍵的元素已經存在，則不會被覆蓋。它只會保持原樣。讓我們回到書本上，嘗試一種新方法：


Map <String, String> books = new HashMap<>();
books.put("War and Peace", "Leo Tolstoy");
books.put("Crime and Punishment", "Fyodor Dostoevsky");
books.put("Thinking in Java", "Bruce Eckel");
books.put("The Brothers Karamazov", "Fyodor Dostoevsky");
books.put("The Lord of the Rings", "John Tolkien");
 
books.computeIfAbsent("Harry Potter and the Prisoner of Azkaban", b -> getHarryPotterAuthor());
books.forEach((a,b) -> System.out.println("Book title: " + a + ". Author: "+ b));

這是我們的mappingFunction：


public static String getHarryPotterAuthor() {
        return "Joanne Rowling";
    }

這是新書：


Book title: The Brothers Karamazov. Author: Fyodor Dostoevsky
Book title: Thinking in Java. Author: Bruce Eckel
Book title: Crime and Punishment. Author: Fyodor Dostoevsky
Book title: War and Peace. Author: Leo Tolstoy
Book title: Harry Potter and the Prisoner of Azkaban. Author: Joanne Rowling
Book title: Lord of the Rings. Author: John Tolkien

6. Map.computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)

這裡我們的原理和一樣Map.compute()，但是只有當的 item 已經存在時才進行計算key。


Map <String, String> books = new HashMap<>();
books.put("War and Peace", "Leo Tolstoy");
books.put("Crime and Punishment", "Fyodor Dostoevsky");
books.put("Thinking in Java", "Bruce Eckel");
books.put("The Brothers Karamazov", "Fyodor Dostoevsky");
books.put("The Lord of the Rings", "John Tolkien");
 
books.computeIfPresent("Eugene Onegin", (a,b) -> b = "Alexander Pushkin");
System.out.println("_________________");
books.forEach((a,b) -> System.out.println("Book title: " + a + ". Author: "+ b));
books.computeIfPresent("The Brothers Karamazov", (a,b) -> b = "Alexander Pushkin");
System.out.println("_________________");
books.forEach((a,b) -> System.out.println("Book title: " + a + ". Author: "+ b));

第一次調用該函數沒有做任何更改，因為我們的Map. 但在第二次調用中，程序將《卡拉馬佐夫兄弟》一書的作者改為亞歷山大·普希金。輸出：


_________________
Book title: The Brothers Karamazov. Author: Fyodor Dostoevsky
Book title: Thinking in Java. Author: Bruce Eckel
Book title: Crime and Punishment. Author: Fyodor Dostoevsky
Book title: War and Peace. Author: Leo Tolstoy
Book title: Lord of the Rings. Author: John Tolkien
 _________________
Book title: The Brothers Karamazov. Author: Alexander Pushkin
Book title: Thinking in Java. Author: Bruce Eckel
Book title: Crime and Punishment. Author: Fyodor Dostoevsky
Book title: War and Peace. Author: Leo Tolstoy
Book title: Lord of the Rings. Author: John Tolkien

7. Map.getOrDefault(Object key, V defaultValue)

此方法返回對應於的值key。如果鍵不存在，則返回默認值。


Map <String, String> books = new HashMap<>();
books.put("War and Peace", "Leo Tolstoy");
books.put("Crime and Punishment", "Fyodor Dostoevsky");
books.put("Thinking in Java", "Bruce Eckel");
books.put("The Brothers Karamazov", "Fyodor Dostoevsky");
books.put("The Lord of the Rings", "John Tolkien");
 
String igor = books.getOrDefault("The Tale of Igor's Campaign", "Unknown author");
System.out.println(igor);

這很方便：


Unknown author

8. Map.merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)

我什至懶得去計算這個方法能為你節省多少行代碼。

如果key您的中不存在Map，或者value此鍵存在null，則該方法會將傳遞的key-value對添加到中Map。
如果key確實存在並且是 its value != null，則該方法將其值更改為執行的結果remappingFunction。
如果remappingFunction返回null，則從key集合中移除。


Map <String, String> books = new HashMap<>();
books.put("War and Peace", "Leo Tolstoy");
books.put("Crime and Punishment", "Fyodor Dostoevsky");
books.put("Thinking in Java", "Bruce Eckel");
books.put("The Brothers Karamazov", "Fyodor Dostoevsky");
books.put("The Lord of the Rings", "John Tolkien");
 
books.merge("Thinking in Java", "Bruce Eckel", (a, b) -> b + " and some coauthor");
books.forEach((a, b) -> System.out.println("Title: " + a + ". Author: "+ b));

輸出：


Title: The Brothers Karamazov. Author: Fyodor Dostoevsky
Title: Thinking in Java. Author: Bruce Eckel and some coauthor
Title: Crime and Punishment. Author: Fyodor Dostoevsky
Title: War and Peace. Author: Leo Tolstoy
Title: Lord of the Rings. Author: John Tolkien

*對不起，布魯斯*

9. Map.putIfAbsent(K鍵,V值)

以前，要將一對添加到 a Map，如果它不存在，您必須執行以下操作：


Map <String, String> map = new HashMap<>();
if (map.get("Lord of the Rings") == null)
    map.put("Lord of the Rings", "John Tolkien");

現在一切都變得容易多了：


Map<String, String> map = new HashMap<>();
map.putIfAbsent("Lord of the Rings", "John Tolkien");

10. Map.replace 和 Map.replaceAll()

最後但並非最不重要的。

Map.replace(K key, V newValue)如果存在這樣的鍵，則將key的值替換為。newValue如果沒有，什麼也不會發生。
Map.replace(K key, V oldValue, V newValue)做同樣的事情，但前提是當前值key等於oldValue。
Map.replaceAll(BiFunction<? super K, ? super V, ? extends V> function)value用函數的結果替換每個。

例如：


Map <String, String> books = new HashMap<>();
books.put("War and Peace", "Leo Tolstoy");
books.put("Crime and Punishment", "Fyodor Dostoevsky");
books.put("Thinking in Java", "Bruce Eckel");
books.put("The Brothers Karamazov", "Fyodor Dostoevsky");
books.put("The Lord of the Rings", "John Tolkien");
 
books.replace("The Brothers Karamazov", "Bruce Eckel", "John Tolkien");
books.forEach((a, b) -> System.out.println("Title: " + a + ". Author: "+ b));


Title: The Brothers Karamazov. Author: Fyodor Dostoevsky
Title: Thinking in Java. Author: Bruce Eckel
Title: Crime and Punishment. Author: Fyodor Dostoevsky
Title: War and Peace. Author: Leo Tolstoy
Title: Lord of the Rings. Author: John Tolkien

沒用！鍵“The Brothers Karamazov”的當前值是“Fyodor Dostoevsky”，而不是“Bruce Eckel”，因此沒有任何變化。


Map  books = new HashMap<>();
books.put("War and Peace", "Leo Tolstoy");
books.put("Crime and Punishment", "Fyodor Dostoevsky");
books.put("Thinking in Java", "Bruce Eckel");
books.put("The Brothers Karamazov", "Fyodor Dostoevsky");
books.put("The Lord of the Rings", "John Tolkien");
 
books.replaceAll((a,b) -> getCoolAuthor());
books.forEach((a, b) -> System.out.println("Title: " + a + ". Author: "+ b));

public static String getCoolAuthor() {
        return "Cool author";
     }


Title: The Brothers Karamazov. Author: Cool author
Title: Thinking in Java. Author: Cool author
Title: Crime and Punishment. Author: Cool author
Title: War and Peace. Author: Cool author
Title: Lord of the Rings. Author: Cool author

Map我們無需任何復雜的構造即可輕鬆更改整體的值！PS 適應新事物總是很困難，但這些變化真的很好。無論如何，我的代碼的某些部分肯定不像以前那樣像意大利麵條了:)祝你學習順利！

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