Java: Stream

数据收集

import java.util.stream.Collectors;
import java.util.stream.Stream;

public class B{
  public static void main(String[] args){
    List<Integer> l1 = Stream.of(1, 2, 3, 4, 5).toList();
    System.out.println("l1.getClass() = " + l1.getClass());

    List<Integer> l2 = Stream.of(1, 2, 3, 4, 5).collect(Collectors.toList());
    System.out.println("l2.getClass() = " + l2.getClass());


    ArrayList<Integer> l3 = Stream.of(1, 2, 3, 4, 5).collect(Collectors.toCollection(() -> {
      return new ArrayList<Integer>();
    }));
    System.out.println("l3.getClass() = " + l3.getClass());

    Collection<Integer> l4 = Stream.of(1, 2, 3, 4, 5).collect(Collectors.toCollection(new Supplier<Collection<Integer>>(){
      @Override
      public Collection<Integer> get(){
        return new ArrayList<Integer>();
      }
    }));
    System.out.println("l4.getClass() = " + l4.getClass());

    LinkedList<Integer> l5 = Stream.of(1, 2, 3, 4, 5).collect(Collectors.toCollection(LinkedList::new));
    System.out.println("l5.getClass() = " + l5.getClass());
  }
}

import java.util.*;
import java.util.function.Supplier;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class B{
  public static void main(String[] args){
    Set<Integer> s1 = Stream.of(1, 2, 3, 4, 5).collect(Collectors.toSet());
    System.out.println("s1.getClass() = " + s1.getClass());
    HashSet<Integer> s2 = Stream.of(1, 2, 3, 4, 5).collect(Collectors.toCollection(() -> {
      return new HashSet<>();
    }));
    System.out.println("s2.getClass() = " + s2.getClass());
    HashSet<Integer> s3 = Stream.of(1, 2, 3, 4, 5).collect(Collectors.toCollection(HashSet::new));
    System.out.println("s3.getClass() = " + s3.getClass());
    Collection<Integer> s4 = Stream.of(1, 2, 3, 4, 5).collect(Collectors.toCollection(new Supplier<Collection<Integer>>(){
      @Override
      public Collection<Integer> get(){
        return new HashSet<>();
      }
    }));
    System.out.println("s4.getClass() = " + s4.getClass());
  }
}

toArray

public class B{
  public static void main(String[] args){
    Object[] objects = Stream.of(1, 2, 3, 4, 5).toArray();
    Integer[] integers = Stream.of(1, 2, 3, 4, 5).toArray(new IntFunction<Integer[]>(){
      @Override
      public Integer[] apply(int value){  // value为数组大小
        System.out.println("\033[37;7m" + value + "\033[0m");
        return new Integer[5];
      }
    });
    System.out.println("integers = " + Arrays.toString(integers));
  }
}

Aggregate

package io.veer.redis.function;


import java.util.Optional;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class addThen{
  public static void main(String[] args){
    Jot[] jots = new Jot[]{
      new Jot("aa", 11),
      new Jot("bb", 22),
      new Jot("bb", 33),
      new Jot("cc", 44)
    };

    Optional<Jot> maxAge = Stream.of(jots
    ).collect(Collectors.maxBy((o1, o2) -> o1.getAge() - o2.getAge()));

    System.out.println("maxAge.get() = " + maxAge.get());

    Optional<Jot> minAge = Stream.of(jots).collect(Collectors.minBy((o1, o2) -> o1.getAge() - o2.getAge()));
    System.out.println("minAge.get() = " + minAge.get());

    Integer sumAge = Stream.of(jots).collect(Collectors.summingInt(jot -> jot.getAge()));
    System.out.println("sumAge = " + sumAge);

    Double avgAge = Stream.of(jots).collect(Collectors.averagingInt(Jot::getAge));
    System.out.println("avgAge = " + avgAge);

    Long count = Stream.of(jots).collect(Collectors.counting());
    System.out.println("count = " + count);
  }
}

class Jot{
  String name;
  Integer age;

  public Jot(String name, Integer age){
    this.name = name;
    this.age = age;
  }

  public String getName(){
    return name;
  }

  public void setName(String name){
    this.name = name;
  }

  public Integer getAge(){
    return age;
  }

  public void setAge(Integer age){
    this.age = age;
  }

  @Override
  public String toString(){
    final StringBuilder sb = new StringBuilder("Jot{");
    sb.append("name='").append(name).append('\'');
    sb.append(", age=").append(age);
    sb.append('}');
    return sb.toString();
  }
}

Group

package io.veer.redis.function;


import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class addThen{
  public static void main(String[] args){
    Jot[] jots = new Jot[]{
      new Jot("aa", 11),
      new Jot("bb", 22),
      new Jot("bb", 33),
      new Jot("cc", 44),
      new Jot("dd", 44)
    };

    Map<String, List<Jot>> map = Stream.of(jots).collect(Collectors.groupingBy(Jot::getName));
    map.forEach((k, v) -> {
      System.out.println("k = " + k);
      System.out.println("v = " + v);
    });

    Map<String, List<Jot>> ageMap = Stream.of(jots).collect(Collectors.groupingBy(jot -> jot.getAge() >= 33 ? "young" : "old"));
    ageMap.forEach((k, v) -> {
      System.out.println("k = " + k);
      System.out.println("v = " + v);
    });

    Map<String, Map<String, List<Jot>>> mapMap = Stream.of(jots).collect(Collectors.groupingBy(Jot::getName, Collectors.groupingBy(jot -> jot.getAge() > 33 ? "young" : "old")));
    mapMap.forEach((k, v) -> {
      System.out.println("k = " + k);
      v.forEach((kk, vv) -> {
        System.out.println("\tkk = " + kk);
        System.out.println("\tvv = " + vv);
      });
    });
  }
}

class Jot{
  String name;
  Integer age;

  public Jot(String name, Integer age){
    this.name = name;
    this.age = age;
  }

  public String getName(){
    return name;
  }

  public void setName(String name){
    this.name = name;
  }

  public Integer getAge(){
    return age;
  }

  public void setAge(Integer age){
    this.age = age;
  }

  @Override
  public String toString(){
    final StringBuilder sb = new StringBuilder("Jot{");
    sb.append("name='").append(name).append('\'');
    sb.append(", age=").append(age);
    sb.append('}');
    return sb.toString();
  }
}

Partition

public class addThen{
  public static void main(String[] args){
    Jot[] jots = new Jot[]{
      new Jot("aa", 11),
      new Jot("bb", 22),
      new Jot("bb", 33),
      new Jot("cc", 44),
      new Jot("dd", 44)
    };

    Map<Boolean, List<Jot>> map = Stream.of(jots).collect(Collectors.partitioningBy(jot -> jot.getAge() > 33));
    map.forEach((k,v)->{
      System.out.println("k = " + k);
      System.out.println("v = " + v);
    });
  }
}

Join

public class addThen{
  public static void main(String[] args){
    Jot[] jots = new Jot[]{
      new Jot("aa", 11),
      new Jot("bb", 22),
      new Jot("bb", 33),
      new Jot("cc", 44),
      new Jot("dd", 44)
    };

    String s = Stream.of(jots).map(Jot::getName).collect(Collectors.joining());
    System.out.println("s = " + s);  // aabbbbccdd
    String ss = Stream.of(jots).map(Jot::getName).collect(Collectors.joining("-"));
    System.out.println("ss = " + ss);  // aa-bb-bb-cc-dd
    String sss = Stream.of(jots).map(Jot::getName).collect(Collectors.joining("-", "@", "%"));
    System.out.println("sss = " + sss);  // @aa-bb-bb-cc-dd%
  }
}

parallel

public class addThen{
  public static void main(String[] args){
    Jot[] jots = new Jot[]{
      new Jot("aa", 11),
      new Jot("bb", 22),
      new Jot("bb", 33),
      new Jot("cc", 44),
      new Jot("dd", 44)
    };

   Stream.of(jots).forEach(jot -> {
     System.out.println("\033[37;7m" + Thread.currentThread() + "\033[0m");
   });
   Stream.of(jots).parallel().filter(s->{
     System.out.println("Thread.currentThread() = " + Thread.currentThread());
     return true;
   }).count();

    LinkedList<Integer> integers = new LinkedList<>();
    Stream<Integer> integerStream = integers.parallelStream();
    integers.add(5);
    integers.add(55);
    integerStream.forEach(b->{
      System.out.println("\033[37;7m" + Thread.currentThread()+ "\033[0m");
      System.out.println("b = " + b);
    });
  }
}

效率对比

public class addThen{
  private static final long count = 999999999;

  public static void main(String[] args){
    Jot[] jots = new Jot[]{
      new Jot("aa", 11),
      new Jot("bb", 22),
      new Jot("bb", 33),
      new Jot("cc", 44),
      new Jot("dd", 44)
    };

    System.out.println("\033[37;7m" + "For loop" + "\033[0m");
    b1();
    System.out.println("\033[37;7m" + "Stream serialization" + "\033[0m");
    b2();
    System.out.println("\033[37;7m" + "Stream parallel" + "\033[0m");
    b3();
  }

  private static void b1(){
    long commence = System.currentTimeMillis();
    long res = 0;
    for(int i = 0; i < count; i++){
      res += i;
    }
    long closure = System.currentTimeMillis();
    System.out.println("res = " + res);
    System.out.printf("elapsed: %d\n", (closure - commence));
  }
  private static void b2(){
    long commence=System.currentTimeMillis();
    long res = LongStream.range(0,count).reduce(0, Long::sum);
    long closure=System.currentTimeMillis();
    System.out.println("res = " + res);
    System.out.printf("elapsed: %d\n", (closure - commence));
  }
  private static void b3(){
    long commence=System.currentTimeMillis();
    long res = LongStream.range(0,count).parallel().reduce(0, Long::sum);
    long closure=System.currentTimeMillis();
    System.out.println("res = " + res);
    System.out.printf("elapsed: %d\n", (closure - commence));
  }
}

多线程安全问题

public class addThen{
  public static void main(String[] args){
    ArrayList<Object> l1 = new ArrayList<>();
    for(int i = 0; i < 1000; i++){
      l1.add(i);
    }
    System.out.println("l1.size() = " + l1.size());

    ArrayList<Object> l2 = new ArrayList<>();

    IntStream.range(0, 1000).parallel().forEach(l2::add);
    System.out.println("l2.size() = " + l2.size());

    ArrayList<Integer> l3 = new ArrayList<>();
    IntStream.range(0, 1000).parallel().forEach(value -> {
      synchronized(addThen.class){
        l3.add(value);
      }
    });
    System.out.println("l3.size() = " + l3.size());

    List<Integer> l4 = IntStream.rangeClosed(1, 1000).parallel().boxed().collect(Collectors.toList());
    System.out.println("l4.size() = " + l4.size());
  }
}