Java: Stream

 

1. 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());
     }
   }

   

    

    

2. 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));
     }
   }

    

3. 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();
     }
   }

    

4. 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();
     }
   }

    

5. 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);
       });
     }
   }

    

6. 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%
     }
   }

    

7. 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);
       });
     }
   }

    

8. 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));
     }
   }

    

9. 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());
     }
   }