Stream中Collectors.groupingBy toMap mapping操作实例

短命女 2022-05-19 05:35 208阅读 0赞

groupingBy用于分组，toMap用于list转map

测试代码：

1.建一个实体类，测试中用

package com.xhx.java;
    
    /**
     * xuhaixing
     * 2018/7/20 21:43
     **/
    public class Student {
        private String name;
        private String sex;
        private double money;
    
        public Student(String name, String sex, double money) {
            this.name = name;
            this.sex = sex;
            this.money = money;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public String getSex() {
            return sex;
        }
    
        public void setSex(String sex) {
            this.sex = sex;
        }
    
        public double getMoney() {
            return money;
        }
    
        public void setMoney(double money) {
            this.money = money;
        }
    
        @Override
        public String toString() {
            return "Student{" +
                    "name='" + name + '\'' +
                    ", sex='" + sex + '\'' +
                    ", money=" + money +
                    '}';
        }
    }

2.具体测试代码及输出结果

package com.xhx.java;
    
    import org.junit.Test;
    
    import java.util.*;
    import java.util.function.Function;
    import java.util.stream.Collector;
    import java.util.stream.Collectors;
    
    /**
     * xuhaixing
     * 2018/7/20 21:44
     **/
    public class TestCollectors {
    
        /**
         * Collectors.groupingBy 分组1
         */
        @Test
        public void testGrouping() {
            List<String> items = Arrays.asList("apple", "banana", "apple", "orange", "banana");
            Collector<Object, ?, Map<Object, Long>> objectMapCollector = Collectors.groupingBy(Function.identity(), Collectors.counting());
            Map<Object, Long> collect = items.stream().collect(objectMapCollector);
            System.out.println(collect);
            //{orange=1, banana=2, apple=2}
        }
    
        /**
         * Collectors.groupingBy 分组2
         */
        @Test
        public void testGrouping2() {
            List<Student> students = Arrays.asList(new Student("apple", "男", 10.0),
                    new Student("banana", "男", 10.0),
                    new Student("orange", "男", 20.0),
                    new Student("pipe", "女", 40.0),
                    new Student("pinck", "女", 80.0)
            );
    
            //根据sex分组
            Map<String, List<Student>> collect = students.stream().collect(Collectors.groupingBy(Student::getSex));
            System.out.println(collect);
            //{
            // 女=[Student{name='pipe', sex='女', money=40.0}, Student{name='pinck', sex='女', money=80.0}],
            // 男=[Student{name='apple', sex='男', money=10.0}, Student{name='banana', sex='男', money=10.0}, Student{name='orange', sex='男', money=20.0}]
            // }
    
            //根据sex分组，然后对money求和
            Map<String, Double> collect1 = students.stream().collect(Collectors.groupingBy(Student::getSex, Collectors.summingDouble(Student::getMoney)));
            System.out.println(collect1);
            //{女=120.0, 男=40.0}
        }
    
        /**
         * list、set
         */
        @Test
        public void testGrouping3() {
            List<Student> students = Arrays.asList(new Student("apple", "男", 10.0),
                    new Student("banana", "男", 10.0),
                    new Student("orange", "男", 20.0),
                    new Student("pipe", "女", 40.0),
                    new Student("pinck", "女", 80.0)
            );
            Map<String, List<Student>> collect1 = students.stream().collect(Collectors.groupingBy(Student::getSex));
            System.out.println(collect1);
            //{
            // 女=[Student{name='pipe', sex='女', money=40.0}, Student{name='pinck', sex='女', money=80.0}],
            // 男=[Student{name='apple', sex='男', money=10.0}, Student{name='banana', sex='男', money=10.0}, Student{name='orange', sex='男', money=20.0}]
            // }
    
            Map<String, List<Double>> collect2 = students.stream().collect(Collectors.groupingBy(Student::getSex, Collectors.mapping(Student::getMoney, Collectors.toList())));
            System.out.println(collect2);
            //{女=[40.0, 80.0], 男=[10.0, 10.0, 20.0]}
    
            Map<String, Set<Double>> collect3 = students.stream().collect(Collectors.groupingBy(Student::getSex, Collectors.mapping(Student::getMoney, Collectors.toSet())));
            System.out.println(collect3);
            //{女=[40.0, 80.0], 男=[10.0, 20.0]}
        }
    
        /**
         * toMap 转成map
         */
        @Test
        public void tesToMap(){
            List<Student> students = Arrays.asList(new Student("apple", "男", 10.0),
                    new Student("banana", "男", 10.0),
                    new Student("orange", "男", 20.0),
                    new Student("pipe", "女", 40.0),
                    new Student("pinck", "女", 80.0)
            );
    
    
            Map<String, Double> collect = students.stream().collect(Collectors.toMap(Student::getName, Student::getMoney));
            System.out.println(collect);
            //{orange=20.0, banana=10.0, apple=10.0, pinck=80.0, pipe=40.0}
        }
    
        /**
         * toMap 转成map  key重复
         */
        @Test
        public void tesToMap2(){
            List<Student> students = Arrays.asList(new Student("apple", "男", 10.0),
                    new Student("banana", "男", 10.0),
                    new Student("orange", "男", 20.0),
                    new Student("pipe", "女", 40.0),
                    new Student("pinck", "女", 80.0)
            );
            /*
            java.lang.IllegalStateException: Duplicate key apple
            at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
            at java.util.HashMap.merge(HashMap.java:1254)
            at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
             */
    //        Map<Double,String> collect2 = students.stream().collect(Collectors.toMap(Student::getMoney, Student::getName));
    //        System.out.println(collect2);
    
            Map<Double,String> collect2 = students.stream().collect(Collectors.toMap(Student::getMoney, Student::getName,(oldValue,newValue)->newValue));
            System.out.println(collect2);
            //{80.0=pinck, 40.0=pipe, 20.0=orange, 10.0=banana}  key重复用新值
    
        }
    
        /**
         * toMap 转成map 排序
         */
        @Test
        public void tesToMap3() {
            List<Student> students = Arrays.asList(new Student("apple", "男", 10.0),
                    new Student("banana", "男", 10.0),
                    new Student("orange", "男", 20.0),
                    new Student("pipe", "女", 40.0),
                    new Student("pinck", "女", 80.0)
            );
    
            LinkedHashMap<String, Double> collect = students.stream().sorted(Comparator.comparing(Student::getMoney).reversed())
                    .collect(Collectors.toMap(Student::getName, Student::getMoney, (oldValue, newValue) -> newValue, LinkedHashMap::new));
            System.out.println(collect);
            //{pinck=80.0, pipe=40.0, orange=20.0, apple=10.0, banana=10.0}
        }
    }

需要注意list转set，set中的值不允许有重复，所以只有一个，，list转map，map的key不能重复，所以自己设置选用老的还是新的，还一点需要注意，list转map时key可以为null,value不可以为null,否则会报空指针异常

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