How do I create two different compliementary lists using same input

In my previous question - How to filter the age while grouping in map with list I was able to find the name to age groups using List<User> users. Now I am trying to find the different User groups from ages according to the threshold. I tried this

List<User> userAboveThreshold = users.stream().filter(u -> u.getAge() > 21).collect(toList());

List<User> userBelowThreshold = users.stream().filter(u -> u.getAge() <= 21).collect(toList());

This time it works I can see using

userAboveThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));

userBelowThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));

But I have to access the users list again to find the complimentary list. Can this not be done simpler?

asked Dec 20 at 13:26

Mani

2281319

add a comment |

List<User> userAboveThreshold = users.stream().filter(u -> u.getAge() > 21).collect(toList());

List<User> userBelowThreshold = users.stream().filter(u -> u.getAge() <= 21).collect(toList());

This time it works I can see using

userAboveThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));

userBelowThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));

But I have to access the users list again to find the complimentary list. Can this not be done simpler?

asked Dec 20 at 13:26

Mani

2281319

add a comment |

List<User> userAboveThreshold = users.stream().filter(u -> u.getAge() > 21).collect(toList());

List<User> userBelowThreshold = users.stream().filter(u -> u.getAge() <= 21).collect(toList());

This time it works I can see using

userAboveThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));

userBelowThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));

But I have to access the users list again to find the complimentary list. Can this not be done simpler?

asked Dec 20 at 13:26

Mani

2281319

List<User> userAboveThreshold = users.stream().filter(u -> u.getAge() > 21).collect(toList());

List<User> userBelowThreshold = users.stream().filter(u -> u.getAge() <= 21).collect(toList());

This time it works I can see using

userAboveThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));

userBelowThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge()));

But I have to access the users list again to find the complimentary list. Can this not be done simpler?

java java-stream

asked Dec 20 at 13:26

Mani

2281319

asked Dec 20 at 13:26

Mani

2281319

asked Dec 20 at 13:26

Mani

2281319

asked Dec 20 at 13:26

Mani

2281319

asked Dec 20 at 13:26

Mani

2281319

add a comment |

2 Answers
2

active

oldest

votes

`List.removeAll`

You can use removeAll to obtain the complimentary list.

List<User> userBelowThreshold = new ArrayList<>(users); // initiated with 'users'

userBelowThreshold.removeAll(userAboveThreshold);

Note: This would require overridden equals and hashCode implementation for User.

`Collectors.partitioningBy`

On the other hand, if you further want to iterate over the complete users list just once, you can use Collectors.partitioningBy as:

Map<Boolean, List<User>> userAgeMap = users.stream()

        .collect(Collectors.partitioningBy(user -> user.getAge() > 21, Collectors.toList()));

List<User> userAboveThreshold = userAgeMap.get(Boolean.TRUE);

List<User> userBelowThreshold = userAgeMap.get(Boolean.FALSE);

edited Dec 20 at 13:37

answered Dec 20 at 13:27

nullpointer

41.1k1086173

1

For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
– Alexander
Dec 20 at 18:47

add a comment |

You're after the partitioningBy collector:

Map<Boolean, List<User>> result = 

             users.stream().collect(partitioningBy(u -> u.getAge() > 21));

Then use it as follows:

List<User> userAboveThreshold = result.get(true);

List<User> userBelowThreshold = result.get(false);

answered Dec 20 at 13:26

Aomine

38.8k73365

2

@Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
– Eugene
Dec 20 at 13:58

add a comment |

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

`List.removeAll`

You can use removeAll to obtain the complimentary list.

List<User> userBelowThreshold = new ArrayList<>(users); // initiated with 'users'

userBelowThreshold.removeAll(userAboveThreshold);

Note: This would require overridden equals and hashCode implementation for User.

`Collectors.partitioningBy`

On the other hand, if you further want to iterate over the complete users list just once, you can use Collectors.partitioningBy as:

Map<Boolean, List<User>> userAgeMap = users.stream()

        .collect(Collectors.partitioningBy(user -> user.getAge() > 21, Collectors.toList()));

List<User> userAboveThreshold = userAgeMap.get(Boolean.TRUE);

List<User> userBelowThreshold = userAgeMap.get(Boolean.FALSE);

edited Dec 20 at 13:37

answered Dec 20 at 13:27

nullpointer

41.1k1086173

1

For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
– Alexander
Dec 20 at 18:47

add a comment |

`List.removeAll`

You can use removeAll to obtain the complimentary list.

List<User> userBelowThreshold = new ArrayList<>(users); // initiated with 'users'

userBelowThreshold.removeAll(userAboveThreshold);

Note: This would require overridden equals and hashCode implementation for User.

`Collectors.partitioningBy`

On the other hand, if you further want to iterate over the complete users list just once, you can use Collectors.partitioningBy as:

Map<Boolean, List<User>> userAgeMap = users.stream()

        .collect(Collectors.partitioningBy(user -> user.getAge() > 21, Collectors.toList()));

List<User> userAboveThreshold = userAgeMap.get(Boolean.TRUE);

List<User> userBelowThreshold = userAgeMap.get(Boolean.FALSE);

edited Dec 20 at 13:37

answered Dec 20 at 13:27

nullpointer

41.1k1086173

1

For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
– Alexander
Dec 20 at 18:47

add a comment |

`List.removeAll`

You can use removeAll to obtain the complimentary list.

List<User> userBelowThreshold = new ArrayList<>(users); // initiated with 'users'

userBelowThreshold.removeAll(userAboveThreshold);

Note: This would require overridden equals and hashCode implementation for User.

`Collectors.partitioningBy`

On the other hand, if you further want to iterate over the complete users list just once, you can use Collectors.partitioningBy as:

Map<Boolean, List<User>> userAgeMap = users.stream()

        .collect(Collectors.partitioningBy(user -> user.getAge() > 21, Collectors.toList()));

List<User> userAboveThreshold = userAgeMap.get(Boolean.TRUE);

List<User> userBelowThreshold = userAgeMap.get(Boolean.FALSE);

edited Dec 20 at 13:37

answered Dec 20 at 13:27

nullpointer

41.1k1086173

`List.removeAll`

You can use removeAll to obtain the complimentary list.

List<User> userBelowThreshold = new ArrayList<>(users); // initiated with 'users'

userBelowThreshold.removeAll(userAboveThreshold);

Note: This would require overridden equals and hashCode implementation for User.

`Collectors.partitioningBy`

On the other hand, if you further want to iterate over the complete users list just once, you can use Collectors.partitioningBy as:

Map<Boolean, List<User>> userAgeMap = users.stream()

        .collect(Collectors.partitioningBy(user -> user.getAge() > 21, Collectors.toList()));

List<User> userAboveThreshold = userAgeMap.get(Boolean.TRUE);

List<User> userBelowThreshold = userAgeMap.get(Boolean.FALSE);

edited Dec 20 at 13:37

answered Dec 20 at 13:27

nullpointer

41.1k1086173

edited Dec 20 at 13:37

answered Dec 20 at 13:27

nullpointer

41.1k1086173

answered Dec 20 at 13:27

nullpointer

41.1k1086173

answered Dec 20 at 13:27

nullpointer

41.1k1086173

1

For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
– Alexander
Dec 20 at 18:47

add a comment |

1

For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
– Alexander
Dec 20 at 18:47

For those concerned about the cost of using a map here, worry not, Collectors.partitioningBy returns a custom map class that stores the 2 partitions (holding the the true and false member) directly inline.
– Alexander
Dec 20 at 18:47

add a comment |

You're after the partitioningBy collector:

Map<Boolean, List<User>> result = 

             users.stream().collect(partitioningBy(u -> u.getAge() > 21));

Then use it as follows:

List<User> userAboveThreshold = result.get(true);

List<User> userBelowThreshold = result.get(false);

answered Dec 20 at 13:26

Aomine

38.8k73365

2

@Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
– Eugene
Dec 20 at 13:58

add a comment |

You're after the partitioningBy collector:

Map<Boolean, List<User>> result = 

             users.stream().collect(partitioningBy(u -> u.getAge() > 21));

Then use it as follows:

List<User> userAboveThreshold = result.get(true);

List<User> userBelowThreshold = result.get(false);

answered Dec 20 at 13:26

Aomine

38.8k73365

2

@Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
– Eugene
Dec 20 at 13:58

add a comment |

You're after the partitioningBy collector:

Map<Boolean, List<User>> result = 

             users.stream().collect(partitioningBy(u -> u.getAge() > 21));

Then use it as follows:

List<User> userAboveThreshold = result.get(true);

List<User> userBelowThreshold = result.get(false);

answered Dec 20 at 13:26

Aomine

38.8k73365

You're after the partitioningBy collector:

Map<Boolean, List<User>> result = 

             users.stream().collect(partitioningBy(u -> u.getAge() > 21));

Then use it as follows:

List<User> userAboveThreshold = result.get(true);

List<User> userBelowThreshold = result.get(false);

answered Dec 20 at 13:26

Aomine

38.8k73365

answered Dec 20 at 13:26

Aomine

38.8k73365

answered Dec 20 at 13:26

Aomine

38.8k73365

answered Dec 20 at 13:26

Aomine

38.8k73365

2

@Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
– Eugene
Dec 20 at 13:58

add a comment |

2

@Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
– Eugene
Dec 20 at 13:58

@Mani well you should also know that internally it uses a specialized map with two keys only, faster look-up then a HashMap for example
– Eugene
Dec 20 at 13:58

add a comment |

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