Java8 流式计算Stream

落日映苍穹つ 2022-12-21 15:51 229阅读 0赞

### Java8 流式计算 Stream ###

*  1.Java8常见的reduce规约

# 1.Java8常见的reduce规约 #

Collectors收集，分组，统计，计算

/** * Java8流式计算 * Java8常见的reduce规约，Collectors收集，分组，统计，计算 */
    public class TestStreamAPI3 { 
    
        List<Employee> emps = Arrays.asList(
                new Employee( 102, "李四", 79, 6666.66, Status.BUSY ),
                new Employee( 101, "张三", 18, 9999.99, Status.FREE ),
                new Employee( 103, "王五", 28, 3333.33, Status.VOCATION ),
                new Employee( 104, "赵六", 8, 7777.77, Status.BUSY ),
                new Employee( 104, "赵六", 8, 7777.77, Status.FREE ),
                new Employee( 104, "赵六", 8, 7777.77, Status.FREE ),
                new Employee( 105, "田七", 38, 5555.55, Status.BUSY )
        );
    
        /** * 3. 终止操作 * 归约：reduce(T identity, BinaryOperator) / reduce(BinaryOperator) ——可以将流中元素反复结合起来，得到一个值。 */
        @Test
        public void test1() { 
            List<Integer> list = Arrays.asList( 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 );
            // 求和
            Integer sum = list.stream().reduce( 0, (x, y) -> x + y );
            System.out.println( sum );
    
            System.out.println( "----------------------------------------" );
            // 工资总和
            Optional<Double> op = emps.stream()
                    .map( Employee::getSalary )
                    .reduce( Double::sum );
            System.out.println( op.get() );
        }
    
        /** * 需求：搜索名字中 “六” 出现的次数 */
        @Test
        public void test2() { 
            Optional<Integer> sum = emps.stream()
                    .map( Employee::getName )
                    .flatMap( TestStreamAPI1::filterCharacter )
                    .map( (ch) -> { 
                        if (ch.equals( '六' )) { 
                            return 1;
                        } else { 
                            return 0;
                        }
                    } ).reduce( Integer::sum );
    
            System.out.println( sum.get() );
        }
    
        /** * 终止操作符：collect——将流转换为其他形式。 * 接收一个 Collector接口的实现，用于给Stream中元素做汇总的方法 */
        @Test
        public void test3() { 
            List<String> list = emps.stream().map( Employee::getName )
                    .collect( Collectors.toList() );
    
            list.forEach( System.out::println );
    
            System.out.println( "----------------------------------" );
    
            Set<String> set = emps.stream()
                    .map( Employee::getName )
                    .collect( Collectors.toSet() );
    
            set.forEach( System.out::println );
    
            System.out.println( "----------------------------------" );
    
            HashSet<String> hs = emps.stream()
                    .map( Employee::getName )
                    .collect( Collectors.toCollection( HashSet::new ) );
    
            hs.forEach( System.out::println );
        }
    
        @Test
        public void test4() { 
            //最大值
            Optional<Double> max = emps.stream()
                    .map( Employee::getSalary )
                    .collect( Collectors.maxBy( Double::compare ) );
            System.out.println( max.get() );
    
            //最小值
            Optional<Employee> op = emps.stream()
                    .collect( Collectors.minBy( (e1, e2) -> Double.compare( e1.getSalary(), e2.getSalary() ) ) );
            System.out.println( op.get() );
    
            //总和
            Double sum = emps.stream()
                    .collect( Collectors.summingDouble( Employee::getSalary ) );
            System.out.println( sum );
    
            //平均值
            Double avg = emps.stream()
                    .collect( Collectors.averagingDouble( Employee::getSalary ) );
    
            System.out.println( avg );
    
            //总和
            Long count = emps.stream()
                    .collect( Collectors.counting() );
    
            System.out.println( count );
    
            System.out.println( "--------------------------------------------" );
    
            DoubleSummaryStatistics dss = emps.stream()
                    .collect( Collectors.summarizingDouble( Employee::getSalary ) );
    
            System.out.println( dss.getMax() );
        }
    
        /** * 分组 */
        @Test
        public void test5() { 
            Map<Status, List<Employee>> map = emps.stream()
                    .collect( Collectors.groupingBy( Employee::getStatus ) );
    
            System.out.println( map );
        }
    
        /** * 多级分组 */
        @Test
        public void test6() { 
            Map<Status, Map<String, List<Employee>>> map = emps.stream()
                    .collect( Collectors.groupingBy( Employee::getStatus, Collectors.groupingBy( (e) -> { 
                        if (e.getAge() >= 60) { 
                            return "老年";
                        } else if (e.getAge() >= 35) { 
                            return "中年";
                        } else { 
                            return "成年";
                        }
                    } ) ) );
    
            System.out.println( map );
        }
    
        /** * 分区 */
        @Test
        public void test7() { 
            Map<Boolean, List<Employee>> map = emps.stream()
                    .collect( Collectors.partitioningBy( (e) -> e.getSalary() >= 5000 ) );
    
            System.out.println( map );
        }
    
        //
        @Test
        public void test8() { 
            String str = emps.stream()
                    .map( Employee::getName )
                    .collect( Collectors.joining( ",", "----", "----" ) );
    
            System.out.println( str );
        }
    
        @Test
        public void test9() { 
            Optional<Double> sum = emps.stream()
                    .map( Employee::getSalary )
                    .collect( Collectors.reducing( Double::sum ) );
    
            System.out.println( sum.get() );
        }
    }

总结  
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