1.1 查找所有的偶數并求和

public?static?void?p1()?{? List<Integer> numbers = Arrays.asList(1,?2,?3,?4,?5,?6,?7,?8,?9,?10);??int?sum = numbers.stream()? ? ? .filter(num -> num %?2?==?0)? ? ? .mapToInt(Integer::intValue)? ? ? .sum() ;? System.err.printf("result: %s%n", sum) ;}

1.2?查找并打印長度大于 5 的字符串個數

public?static?void?p2() {??List<String> strings =?Arrays.asList(? ??"apple",?"banana",?"grape",?? ??"watermelon",?"kiwi",?"orange");??Long?count = strings.stream()? ? ? .filter(str -> str.length() >?5)? ? ? .count() ;??System.err.printf("result: %s%n", count) ;}

1.3 處理每一個元素最后返回新集合???????

public?static?void?p3()?{? List<Integer> numbers = Arrays.asList(1,?2,?3,?4,?5);? List<Integer> squares = numbers.stream()? ? ? .map(num -> num * num)? ? ? .collect(Collectors.toList()) ;? System.err.printf("result: %s%n", squares) ;}

將每一個元素進行平方操作，最后返回一個新的集合。輸出結果：

result:?[1, 4, 9, 16, 25]

1.4?找出整數列表中的最大元素???????

public?static?void?p4()?{? List<Integer> numbers = Arrays.asList(10,?5,?25,?15,?30);??int?max = numbers.stream()? ? ? .mapToInt(Integer::intValue)? ? ? .max()? ? ? .getAsInt();? System.err.printf("result: %s%n", max) ;}

1.5 將列表中的所有字符串連接成一個字符串???????

public?static?void?p5() {??List<String> fruits =?Arrays.asList("apple",?"banana",?"cherry","coconut",?"apple");??String?concat =fruits.stream()? ? ? .collect(Collectors.joining()) ;??System.err.printf("result: %s%n", concat) ;}

1.6 轉成大寫再排序???????

public?static?void?p6() {??List<String> fruits =?Arrays.asList("apple",?"Banana",?"Grape",?"orange",?"kiwi");??List<String> sortedUppercase = fruits.stream()? ? ? .map(String::toUpperCase)? ? ? .sorted()? ? ? .collect(Collectors.toList());??System.err.printf("result: %s%n", sortedUppercase) ;}

先將字符串轉為大寫，然后在進行排序，輸出結果：

result:?[APPLE, BANANA, GRAPE, KIWI, ORANGE]

1.7 計算double類型平均值???????

public?static?void?p7()?{? List<Double> doubles = Arrays.asList(1.0,?2.0,?3.0,?4.0,?5.0);??double?average = doubles.stream()? ? ? .mapToDouble(Double::doubleValue)? ? ? .average()? ? ? .getAsDouble() ;? System.err.printf("result: %s%n", average) ;}

1.8 刪除重復元素???????

public?static?void?p8() {??List<String> words =?Arrays.asList("apple",?"banana",?"apple",?"orange",?"banana",?"kiwi");??List<String> uniqueWords = words.stream()? ? ? .distinct()? ? ? .collect(Collectors.toList()) ;??System.err.printf("result: %s%n", uniqueWords) ;}

1.9 檢查所有元素是否符合條件???????

public?static?void?p9() {??List<Integer> numbers =?Arrays.asList(2,?4,?6,?8,?10);??boolean?allEven = numbers.stream()? ? ? .allMatch(n -> n%2?==?0) ;??System.err.printf("result: %s%n", allEven) ;}

1.10 檢查集合中是否包含特定元素???????

public?static?void?p10()?{? List<Integer> numbers = Arrays.asList(2,?4,?6,?8,?10);? boolean exists = numbers.stream()? ? ? .anyMatch(n -> n.equals(8)) ;? System.err.printf("result: %s%n", exists) ;}

1.11 查找流中最長的字符串???????

public?static?void?p11() {??List<String> fruits =?Arrays.asList("apple",?"banana",?"cherry",?"coconut",?"apple") ;? int max = fruits.stream()? ? .mapToInt(String::length)? ? .max()? ? .getAsInt() ;??System.err.printf("result: %s%n", max) ;}

1.12 從流中刪除null值???????

public?static?void?p12() {??List<String> fruits =?Arrays.asList("apple",?"banana",?"cherry",?null,"coconut",?"apple");??List<String> nonNullValues = fruits.stream()? ? ? .filter(Objects::nonNull)? ? ? .collect(Collectors.toList()) ;??System.err.printf("result: %s%n", nonNullValues) ;}

過濾為null的值，輸出結果：

result:?[apple, banana, cherry, coconut, apple]

1.13 分組并查找最大值???????

public?static?void?p13() {??List<Employee> employees =new?ArrayList<>() ;? employees.add(new?Employee("Alice","HR",50000.0)) ;? employees.add(new?Employee("Bob","IT",60000.0)) ;? employees.add(new?Employee("Charlie","Finance",55000.0)) ;? employees.add(new?Employee("David","IT",70000.0)) ;? employees.add(new?Employee("Eva",?"HR",?45000.0)) ;? employees.add(new?Employee("Frank","Finance",58000.0));??Map<String,?Optional<Employee>> highestSalaryPerDept = employees.stream()? ? ? .collect(Collectors.groupingBy(? ? ? ? ??Employee::getDepartment,?? ? ? ? ??Collectors.maxBy(Comparator.comparingDouble(Employee::getSalary))? ? ? ));? highestSalaryPerDept.forEach((key, value) -> {? ??System.err.printf("部門: %s, \t最高薪: %s%n", key, value.get()) ;? });}

輸出結果：

2.14 查找列表中第二小的元素???????

public?static?void?p14()?{? List<Integer> numbers = Arrays.asList(2,?4,?6,?8,?10) ;? Optional<Integer> secondSmallest = numbers.stream()? ? ? .distinct()? ? ? .sorted()? ? ? .skip(1)? ? ? .findFirst() ;? System.err.printf("result: %s%n", secondSmallest) ;}

1.15 查找兩個列表的交集???????

public?static?void?p15()?{? List<Integer> list1 = Arrays.asList(1,?2,?3,?4,?5) ;? List<Integer> list2 = Arrays.asList(4,?5,?6,?7,8) ;? List<Integer> intersection = list1.stream()? ? ? .filter(list2::contains)? ? ? .collect(Collectors.toList()) ;? System.err.printf("result: %s%n", intersection) ;}

1.16 并行處理提升性能

使用并行流可以通過BaseStream.parallel或Collection#parallelStream操作，如下計算1億個數的求和???????

public static void main(String[] args) {double[] arr = IntStream.range(0,?100_000_000)? ? ? .mapToDouble(i ->?new?Random().nextDouble() *?100000)? ? ? .toArray() ;? computeSumOfSquareRoots(arr);}
public static void computeSumOfSquareRoots(double[] arr) {double?serialSum = computeSerialSum(DoubleStream.of(arr));? System.out.println("Serial Sum: "?+ serialSum);
double?parallelSum = computeParallelSum(DoubleStream.of(arr));? System.out.println("Parallel Sum: "?+ parallelSum);}
public static double computeSerialSum(DoubleStream stream) {long?startTime = System.currentTimeMillis();double?sum = stream.reduce(0.0D, (l, r) -> l + r) ;long?endTime = System.currentTimeMillis();? System.out.println("Serial Computation Time: "?+ (endTime - startTime) +?" ms");return?sum;}
public static double computeParallelSum(DoubleStream stream) {long?startTime = System.currentTimeMillis();double?sum = stream.parallel().reduce(0, (l, r) -> l + r) ;long?endTime = System.currentTimeMillis();? System.out.println("Parallel Computation Time: "?+ (endTime - startTime) +?" ms");return?sum;}

運行結果???????

SerialComputation?Time:?73?msSerialSum:?5.000114159154823E12ParallelComputation?Time:?38?msParallelSum:?5.000114159151367E12