Java8 Stream

矫情吗；* 2022-04-24 14:56 206阅读 0赞

# Java8 Stream学习笔记 #

## list ##

### 去重 ###

*  定义对象（重写hashCode（）和equals（）方法）

@Data
    @EqualsAndHashCode
    public class User implements Serializable { 
        private int id;
        private String name;
    }

public static void main(String[] args) { 
       		List<User > newList= new ArrayList<>();
            List<User > list = new ArrayList<>();
            { 
               list.add(new Book(1, "村里有个姑娘叫小芳"));
               list.add(new Book(1, "村里有个姑娘叫小芳"));
               list.add(new Book(2,,"长得好看又善良"));        	
            }
            long l = list.stream().distinct().count();
            System.out.println("去重之后的list长度 :"+l);
            list.stream().distinct().forEach(b -> { 
    			newList.add(b);
    		});
        }

### 按条件去重 ###

#### 第一种方式 ####

@Data
    @EqualsAndHashCode
    public class User implements Serializable { 
        private int id;
        private String name;
        private int age;
    }

public static void main(String[] args) { 
       		List<User > newList= new ArrayList<>();
            List<User > list = new ArrayList<>();
            { 
               list.add(new Book(1, "村里有个姑娘叫小芳",20));
               list.add(new Book(1, "还有个姑娘叫小薇", 20));
               list.add(new Book(2, "长得好看又善良", 20));        	
            }
    		list .stream().collect(collectingAndThen(toCollection(() ->
                    new TreeSet<>(comparing(n -> n.getId() + n.getAge()
                    ))),ArrayList::new)).forEach( user-> { 
                        newList.add(user);
                    }
            );
        }

#### 第二种方式 ####

@Data
    @EqualsAndHashCode
    public class User implements Serializable { 
        private int id;
        private String name;
        private int age;
    }

public static void main(String[] args) { 
       		List<User > newList= new ArrayList<>();
            List<User > list = new ArrayList<>();
            list.add(new Book(1, "村里有个姑娘叫小芳",20));
            list.add(new Book(1, "还有个姑娘叫小薇", 20));
            list.add(new Book(2, "长得好看又善良", 20));   
                 	
     		listData.stream().filter(distinctByKey(n -> n.getId() + n.getAge()))
    	        .forEach(user->
    	                 newList.add(user)
    	         );
        }
    private static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) { 
        Map<Object,Boolean> seen = new ConcurrentHashMap<>();
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
    }

### 排序 ###

1.  sorted() 默认使用自然序排序， 其中的元素必须实现Comparable 接口
2.  sorted(Comparator<? super T> comparator) ：我们可以使用lambada 来创建一个Comparator 实例。可以按照升序或着降序来排序元素。

public class Student implements Comparable<Student> { 
          private int id;
          private String name;
          private int age;
          public Student(int id, String name, int age) { 
              this.id = id;
              this.name = name;
              this.age = age;
          }
          public int getId() { 
              return id;
          }
          public String getName() { 
              return name;
          }
          public int getAge() { 
              return age;
          }
          @Override
          public int compareTo(Student ob) { 
              return name.compareTo(ob.getName());
          }
              @Override
              public boolean equals(final Object obj) { 
                if (obj == null) { 
                   return false;
                }
                final Student std = (Student) obj;
                if (this == std) { 
                   return true;
                } else { 
                   return (this.name.equals(std.name) && (this.age == std.age));
                }
              }
              @Override
              public int hashCode() { 
                int hashno = 7;
                hashno = 13 * hashno + (name == null ? 0 : name.hashCode());
                return hashno;
              }   
      }

import java.util.ArrayList;
    import java.util.Comparator;
    import java.util.List;
    import java.util.stream.Collectors;
    public class SortList { 
        public static void main(String[] args) { 
            List<Student> list = new ArrayList<Student>();
            list.add(new Student(1, "Mahesh", 12));
            list.add(new Student(2, "Suresh", 15));
            list.add(new Student(3, "Nilesh", 10));
    
            System.out.println("---Natural Sorting by Name---");
            List<Student> slist = list.stream().sorted().collect(Collectors.toList());
            slist.forEach(e -> 
            	System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge())
            );
    
            System.out.println("---Natural Sorting by Name in reverse order---");
            slist = list.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());
            slist.forEach(e -> 
            	System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge())
            );        
    
            System.out.println("---Sorting using Comparator by Age---");
            slist = list.stream().sorted(Comparator.comparing(Student::getAge)).collect(Collectors.toList());
            slist.forEach(e -> 
            	System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge())
            );
    
            System.out.println("---Sorting using Comparator by Age with reverse order---");
            slist = list.stream().sorted(Comparator.comparing(Student::getAge).reversed()).collect(Collectors.toList());
            slist.forEach(e -> 
            	System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge())
            );
        }
    }