Java 8 lambda expression or old way

Question

Hello guys I've got question about Java 8 lambda expression. For example I want to delete following id from class.

UserEntity userEntity = userEntityOptional.get();
for(Long permission_id : deletePermissionDto.getPermissionId())
{
    for(PrivilegeEntity p : userEntity.getPrivilegeEntities())
    {
        if(p.getId().equals(permission_id)){
            userEntity.getPrivilegeEntities().remove(p);
            break;
        }
    }
}

Similar to This code might be implemented in java 8 using a lambda expression.

userEntity.getPrivilegeEntities().removeIf(i -> deletePermissionDto.getPermissionId().stream().anyMatch(i.getId()::equals));

What I curious to know asymptotic of java 8 implementation of the same with the first block of code. And how do you think what the best way of implementation of this code.

This is a good question for discussion. But may be you have posted it to wrong forum :). As this forum is for question - answer only — Ketan
– Ketan, Commented Sep 2, 2020 at 5:14

RubioRic · Accepted Answer · 2020-09-02 07:46:46Z

4

Your second solution is faster than your first one.

First is O(n*m^2) (If it works at all. I suspect IllegalStateException should be thrown if you try to delete any but last permission)
Second is O(n*m)

Where n is size of deletePermissionDto.getPermissionId() and m is size of userEntity.getPrivilegeEntities().

Best solution would be:

HashSet<Long> ids = new HashSet<>(deletePermissionDto.getPermissionId());
userEntity.getPrivilegeEntities()
          .removeIf(x -> ids.contains(x.getId()));

With score of O(n+m)

PS: Answer will be different if you are interested in actual performance.

edited Sep 2, 2020 at 7:46

RubioRic

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answered Sep 2, 2020 at 5:23

talex

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2 Comments

Holger Over a year ago

You seem to assume a particular collection type for the result of userEntity.getPrivilegeEntities(). That’s fair, as the OP didn’t specify one, but you should should note which type you’re assuming. Likewise, copying deletePermissionDto.getPermissionId() into a HashSet is only necessary if it isn’t already a HashSet (I don’t know why the OP uses deletePermissionDto.getPermissionId().stream().anyMatch(i.getId()::equals) instead of just deletePermissionDto.getPermissionId().contains(i.getId()); I don’t know of any collection with a worse-than-linear contains).

talex Over a year ago

@Holger Fair enough. If userEntity.getPrivilegeEntities() already HashSet there is no need in copying it.

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