java8 stream的一系列操作去重，过滤，排序，替换，求值

怼烎@ 2022-12-21 15:49 183阅读 0赞

## java8 stream的一系列操作去重，过滤，排序，替换，求值 ##

**java8 stream().forEach、stream().map、stream().filter、stream().sorted的区别和用法**

Java8中流试操作还是很好用的，简洁的同时也在降低可阅读性，权衡利弊，个人喜好吧。

/** * java8 * stream().forEach、stream().map、stream().filter、stream().sorted的区别和用法 * <p> * forEach:原数组遍历。 * map：对原数组加工，可以包含更多操作，转换，过滤，替换，去重等等 * filter：对原数组过滤，符合条件留下 * sorted：排序 * distinct：去重 * **/
    public class ListSreamTest { 
    
        List<Employee> employeeList = Arrays.asList(
                new Employee( "zhaoliu", 77, 7 ),
                new Employee( "zhaoliu", 77, 7 ),
                new Employee( "zhangsan", 18, 9 ),
                new Employee( "lisi", 26, 5 ),
                new Employee( "wangwu", 35, 3 ),
                new Employee( "zhaoliu", 66, 6 )
        );
    
        @Test
        public void listStreamTest() { 
    
            // forEach操作原来的数组
            employeeList.forEach( employee -> { 
                if (employee.getAge() == 18) { 
                    employee.setSalary( 8888 );
                }
            } );
            System.out.println( "forEach遍历：" + employeeList.toString() );
    
            //不影响原数据，生成新数据
            List<Employee> mapEmployeeList = this.employeeList.stream().map( employee -> { 
                if (employee.getAge() == 18) { 
                    employee.setSalary( 8 );
                }
                return employee;
            } ).collect( Collectors.toList() );
            System.out.println( "map加工：" + mapEmployeeList );
    
            //过滤-根据年龄（根据需要指定）
            List<Employee> filterEmployeeList = this.employeeList.stream().filter( employee -> employee.getAge() > 30 ).collect( Collectors.toList() );
            System.out.println( "filter过滤：" + filterEmployeeList );
    
            //升序-根据年龄（根据需要指定）
            List<Employee> sortedEmployeeList = employeeList.stream().sorted( Comparator.comparing( Employee::getAge ) ).collect( Collectors.toList() );
            System.out.println( "sorted升序：" + sortedEmployeeList );
    
            //降序-根据年龄（根据需要指定）
            List<Employee> reversedEmployeeList = employeeList.stream().sorted( Comparator.comparing( Employee::getAge ).reversed() ).collect( Collectors.toList() );
            System.out.println( "sorted降序：" + reversedEmployeeList );
    
            //去重-根据名称（根据需要指定）
            List<String> discantEmployeeList = employeeList.stream().map( employee -> employee.getName() ).distinct().collect( Collectors.toList() );
            System.out.println( "discant去重：" + discantEmployeeList );
            discantEmployeeList.forEach( System.out::println );
    
            //计算-求和
            double salarySum = employeeList.stream().mapToDouble( Employee::getSalary ).sum();
            System.out.println( "工资求和:" + salarySum );
            //计算-最大值
            OptionalDouble max = employeeList.stream().mapToDouble( Employee::getSalary ).max();
            System.out.println( "工资最大值:" + max );
            //计算-最小值
            OptionalDouble min = employeeList.stream().mapToDouble( Employee::getSalary ).min();
            System.out.println( "工资最小值:" + min );
            //计算-平均值
            OptionalDouble average = employeeList.stream().mapToDouble( Employee::getSalary ).average();
            System.out.println( "工资平均值:" + average );
    
            //list转map
            //java.lang.IllegalStateException: Duplicate key问题的解决方案
            //解决方案是：java8中转换成map时，提供了第三个参数，代表的是重复key的值该如何存放
            Map<String, Integer> map = employeeList.stream().collect( Collectors.toMap( Employee::getName, Employee::getAge, (key1, key2) -> key2 ) );
            System.out.println( "list转map：" + map );
    
            //map转list
            List<Employee> employees = map.entrySet().stream().map( e -> new Employee( e.getKey(), e.getValue(), 10 ) ).collect( Collectors.toList() );
            System.out.println( "map转List：" + employees );
    
    
        }
        
        @Test
        public void streamTest() { 
            List<String> list = Arrays.asList( "java", "c++", "c", "python", "php", "c", "c" );
    
            String res = list.stream().map( n -> n.toUpperCase() ).collect( Collectors.joining( "," ) );
            System.out.println( "map操作：" + res );
            List<String> ls = list.stream().filter( n -> n.startsWith( "c" ) ).distinct().collect( Collectors.toList() );
            System.out.println( "filter操作：" + ls );
        }
    }