工作记录——List转Map的六种方式Stream

工作记录——List转Map的六种方式Stream

1.key和value都是对象中的某个属性值.

2.key是对象中的某个属性值，value是对象本身（使用返回本身的lambda表达式）

3.key是对象中的某个属性值，value是对象本身（使用Function.identity()的简洁写法

4.key是对象中的某个属性值，value是对象本身，当key冲突时选择第二个key值覆盖第一个key值

5.key是对象中的几个值一起，value是对象本身，当key冲突时选择第二个key值覆盖第一个key值

6.key是几个属性一起，value也是几个属性一起

key和value都是对象中的某个属性值

Map collect = list.stream().collect(Collectors.toMap(PmWorkCellsDto::getColumIndex, PmWorkCellsDto::getWorkCellId));

@Test
    public void testListToMap(){
        List list = new ArrayList();
        for (int i = 0 ;i<5;i++){
            PmWorkCellsDto pmAreasT = new PmWorkCellsDto();
            pmAreasT.setColumIndex(i);
            pmAreasT.setWorkCellId(i+":value");
            list.add(pmAreasT);
        }
        Map collect = list.stream().collect(Collectors.toMap(PmWorkCellsDto::getColumIndex, PmWorkCellsDto::getWorkCellId));
        for(Map.Entry entry:collect.entrySet()){
            System.out.println(entry.getKey()+"--->"+entry.getValue());
        }
    }

0—>0:value

1—>1:value

2—>2:value

3—>3:value

4—>4:value

key是对象中的某个属性值，value是对象本身（使用返回本身的lambda表达式）

Map collect = list.stream().collect(Collectors.toMap(PmWorkCellsDto::getColumIndex, PmWorkCellsDto ->PmWorkCellsDto));

@Test
    public void testListToMap(){
        List list = new ArrayList();
        for (int i = 0 ;i<5;i++){
            PmWorkCellsDto pmAreasT = new PmWorkCellsDto();
            pmAreasT.setColumIndex(i);
            pmAreasT.setWorkCellId(i+":value");
            list.add(pmAreasT);
        }
        Map collect = list.stream().collect(Collectors.toMap(PmWorkCellsDto::getColumIndex, PmWorkCellsDto ->PmWorkCellsDto));
        for(Map.Entry entry:collect.entrySet()){
            System.out.println(entry.getKey()+"--->"+entry.getValue());
        }
    }

key是对象中的某个属性值，value是对象本身（使用Function.identity()的简洁写法

 @Test
    public void testListToMap(){
        List list = new ArrayList();
        for (int i = 0 ;i<5;i++){
            PmWorkCellsDto pmAreasT = new PmWorkCellsDto();
            pmAreasT.setColumIndex(i);
            pmAreasT.setWorkCellId(i+":value");
            list.add(pmAreasT);
        }
        Map collect = list.stream().collect(Collectors.toMap(PmWorkCellsDto::getColumIndex,  Function.identity()));
        for(Map.Entry entry:collect.entrySet()){
            System.out.println(entry.getKey()+"--->"+entry.getValue());
        }
    }

key是对象中的某个属性值，value是对象本身，当key冲突时选择第二个key值覆盖第一个key值

Map collect = list.stream().collect(Collectors.toMap(PmWorkCellsDto::getColumIndex, Function.identity(),(oldValue, newValue) -> newValue));

@Test
    public void testListToMap(){
        List list = new ArrayList();
        for (int i = 0 ;i<5;i++){
            PmWorkCellsDto pmAreasT = new PmWorkCellsDto();
            pmAreasT.setColumIndex(i);
            pmAreasT.setWorkCellId(i+":value");
            list.add(pmAreasT);
        }
        Map collect = list.stream().collect(Collectors.toMap(PmWorkCellsDto::getColumIndex,  Function.identity(),(oldValue, newValue) -> newValue));
        for(Map.Entry entry:collect.entrySet()){
            System.out.println(entry.getKey()+"--->"+entry.getValue());
        }
    }

key是对象中的几个值一起，value是对象本身，当key冲突时选择第二个key值覆盖第一个key值

Map collect = list.stream().collect(Collectors.toMap(x->{

return x.getColumIndex()+x.getWorkCellId();

}, Function.identity(),(oldValue, newValue) -> newValue));

 @Test
    public void testListToMap(){
        List list = new ArrayList();
        for (int i = 0 ;i<5;i++){
            PmWorkCellsDto pmAreasT = new PmWorkCellsDto();
            pmAreasT.setColumIndex(i);
            pmAreasT.setWorkCellId(i+":value");
            list.add(pmAreasT);
        }
        Map collect = list.stream().collect(Collectors.toMap(x->{
            return x.getColumIndex()+x.getWorkCellId();
        },  Function.identity(),(oldValue, newValue) -> newValue));
        for(Map.Entry entry:collect.entrySet()){
            System.out.println(entry.getKey()+"--->"+entry.getValue());
        }
    }

同理如果想要key是几个属性一起，value也是几个属性一起

Map collect = list.stream().collect(Collectors.toMap(x->{

return x.getColumIndex()+x.getWorkCellId();

}, y->{

return y.getColumIndex()+y.getWorkCellId();

},(oldValue, newValue) -> newValue));

@Test
    public void testListToMap(){
        List list = new ArrayList();
        for (int i = 0 ;i<5;i++){
            PmWorkCellsDto pmAreasT = new PmWorkCellsDto();
            pmAreasT.setColumIndex(i);
            pmAreasT.setWorkCellId(i+":value");
            list.add(pmAreasT);
        }
        Map collect = list.stream().collect(Collectors.toMap(x->{
            return x.getColumIndex()+x.getWorkCellId();
        },  y->{
            return y.getColumIndex()+y.getWorkCellId();
        },(oldValue, newValue) -> newValue));
        for(Map.Entry entry:collect.entrySet()){
            System.out.println(entry.getKey()+"--->"+entry.getValue());
        }
    }