Preface
When I was studying the code today, I saw the use of streams, so I found a few technical articles about streams on the Internet, run them, and make a record
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1. What is the Fibonacci sequence?
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Each number is the sum of the previous two numbers
such as 1,1,2,3,5,8,13,21,34,55
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2. Implementation code
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Code One:
public static void main(String[] args) {
// a,b,c
int a = 1;
int b = 1;
int c = 0;
System.out.print(a + "\t" + b + "\t");
for (int i = 3; i <= 10; i++) {
// 每次运算之后，将数值向后移位
c = a + b;
a = b;
b = c;
System.out.print(c + "\t");
}
}
Code two:
public static void main(String[] args) {
int[] arr = new int[10];
arr[0] = 1;
arr[1] = 1;
for(int i = 0;i < arr.length;i++) {
if(i > 1) {
arr[i] = arr[i - 2] + arr[i - 1];
}
System.out.print(arr[i] + "\t");
}
}
Code three: (recursive writing)
public class PrintFib {
public static int fib(int num) {
if(num == 1 || num == 2) {
return 1;
}else {
return fib(num - 2) + fib(num - 1);
}
}
//主函数（程序入口）
public static void main(String[] args) {
for(int i = 1;i <= 10;i++) {
System.out.print(fib(i) + "\t");
}
}
}
Code four: (stream operation)
public static void main(String[] args) {
Stream.iterate(new int[]{
0, 1}, n -> new int[]{
n[1], n[0] + n[1]})
.limit(10)
.map(n -> n[1])
.forEach(x -> System.out.print(x+ ","));
}
3. Extension of knowledge
Java 8 stream usage
1. Create a stream
Collection creation
List<String> list = new ArrayList<>();
Stream<String> stream = list.stream(); //获取一个顺序流
Stream<String> parallelStream = list.parallelStream(); //获取一个并行流
Array creation
Integer[] arrs = new Integer[]{
1,2,3,4,5,6};
Stream<Integer> stream = Arrays.stream(arrs);
stream = Stream.of(1,2,3,4,5,6);
The of() method actually calls the Arrays.stream() method inside.
Stream<Integer> stream2 = Stream.iterate(0, (x) -> x + 2).limit(6);
stream2.forEach(System.out::print); // 0 2 4 6 8 10
Stream<Double> stream3 = Stream.generate(Math::random).limit(2);
stream3.forEach(System.out::print); //生成两个随机数
Convert each line of the file into a stream
public static void main(String[] args) {
try {
BufferedReader reader = new BufferedReader(new FileReader("F:\\test_stream.txt"));
Stream<String> lineStream = reader.lines();
lineStream.forEach(System.out::println);
}catch (Exception e){
}
}
Separate strings into streams
public static void main(String[] args) {
Pattern pattern = Pattern.compile(",");
Stream<String> stringStream = pattern.splitAsStream("a,b,c,d");
stringStream.forEach(System.out::print);//abcd
}
2. Operation flow
Screening and slicing
filter: filter some elements in the stream
limit(n): get n elements
skip(n): skip n elements, with limit(n) can realize paging
distinct: pass the hashCode() and equals() of the elements in the stream Remove duplicate elements
public static void main(String[] args) {
Stream<Integer> stream = Stream.of(1, 3, 5, 7, 8, 8, 9, 10);
Stream<Integer> newStream = stream.filter(s -> s > 5) //7 8 8 9 10
.distinct() //7 8 9 10
.skip(2) //9 10
.limit(2); //9 10
newStream.forEach(System.out::println);
}
Class name: The method name is a new syntax introduced by Java 8. System.out returns the PrintStream class, and the println method prints
Mapping
map: Receive a function as a parameter, the function will be applied to each element, and map it into a new element.
flatMap: Receive a function as a parameter, replace each value in the stream with another stream, and then connect all streams into one stream.
public static void main(String[] args) {
List<String> list = Arrays.asList("a,b,c", "1,2,3");
Stream<String> s1 = list.stream().map(s -> s.replaceAll(",", ""));
s1.forEach(System.out::println); // abc 123
Stream<String> s3 = list.stream().flatMap(s -> {
Stream<String> s2 = Arrays.stream(s.split(","));
return s2;
});
s3.forEach(System.out::println); // a b c 1 2 3
}
// 把 Stream<String> 的流转成一个 Stream<Integer> 的流。
public static void main(String[] args) {
List<String> list = new ArrayList<>();
list.add("0");
list.add("00");
list.add("000");
Stream<Integer> stream = list.stream().map(String::length);
stream.forEach(System.out::println);// 1 2 3
}
// peek：如同于map，能得到流中的每一个元素。
// 但map接收的是一个Function表达式，有返回值；
// 而peek接收的是Consumer表达式，没有返回值。
Student s1 = new Student("aa", 10);
Student s2 = new Student("bb", 20);
List<Student> studentList = Arrays.asList(s1, s2);
studentList.stream()
.peek(o -> o.setAge(100))
.forEach(System.out::println);
//结果：
Student{
name='aa', age=100}
Student{
name='bb', age=100}
Sort
public static void main(String[] args) {
List<String> list = Arrays.asList("aa", "ff", "dd");
list.stream().sorted().forEach(System.out::println);// aa dd ff
Student s1 = new Student("a", 1);
Student s2 = new Student("b", 2);
Student s3 = new Student("c", 3);
List<Student> studentList = Arrays.asList(s1, s2, s3);
//先按姓名升序，姓名相同则按年龄升序
studentList.stream().sorted(
(o1, o2) -> {
if (o1.getName().equals(o2.getName())) {
return o1.getAge() - o2.getAge();
} else {
return o1.getName().compareTo(o2.getName());
}
}
).forEach(System.out::println);
}
match
anyMatch: true
if there is one match allMatch: true if all matches
noneMatch: true if none matches
public static void main(String[] args) {
List<String> list = new ArrayList<>();
list.add("11");
list.add("12");
list.add("13");
boolean anyMatchFlag = list.stream().anyMatch(element -> element.contains("2"));
boolean allMatchFlag = list.stream().allMatch(element -> element.length() > 1);
boolean noneMatchFlag = list.stream().noneMatch(element -> element.endsWith("0"));
System.out.println(anyMatchFlag);//true
System.out.println(allMatchFlag);//true
System.out.println(noneMatchFlag);//true
}
Element combination
public static void main(String[] args) {
Integer[] ints = {
0, 1, 2, 3};
List<Integer> list = Arrays.asList(ints);
// 无初始值，数组求和
Optional<Integer> a1 = list.stream().reduce((a, b) -> a + b);
Optional<Integer> b1 = list.stream().reduce(Integer::sum);
System.out.println(a1.orElse(0));//6
System.out.println(b1.orElse(0));//6
//有初始值，初始值相加
int a11 = list.stream().reduce(6, (a, b) -> a + b);
System.out.println(a11);//12
int b11 = list.stream().reduce(6, Integer::sum);
System.out.println(b11);//12
}
3. Stream conversion
Circulation array
String[] strArray = list.stream().toArray(String[]::new);
Circulation collection
List<Integer> list1 = list.stream().map(String::length).collect(Collectors.toList());
List<String> list2 = list.stream().collect(Collectors.toCollection(ArrayList::new));
4. Reference link
Three ways to print Fibonacci numbers using Java
The detailed usage of Java 8 stream will
take you to the Java Stream stream, which is too strong