search in java ArrayList

Question

I m trying to figure out the best way to search a customer in an ArrayList by its Id number  The code below is not working  the compiler tells me that I am missing a return statement   Customer findCustomerByid int id       boolean exist false       if this customers isEmpty              return null             for int i 0 i lt this customers size   i              if this customers get i  getId      id                exist true              break                     if exist                return this customers get id             else               return this customers get id                         the customer class is something like that public class Customer         attributes     int id      int tel      String fname      String lname      String resgistrationDate

User · Answer

Personally I rarely write loops myself now when I can get away with it... I use the Jakarta commons libs:

Customer findCustomerByid(final int id){
    return (Customer) CollectionUtils.find(customers, new Predicate() {
        public boolean evaluate(Object arg0) {
            return ((Customer) arg0).getId()==id;
        }
    });
}

Yay! I saved one line!

User · Answer

You re missing the return statement because if your list size is 0  the for loop will never execute  thus the if will never run  and thus you will never return   Move the if statement out of the loop

User · Answer

Customer findCustomerByid int id       for  int i 0  i lt this customers size    i              Customer customer   this customers get i           if  customer getId      id                return customer                      return null     no Customer found with this ID  maybe throw an exception

User · Answer

Others have pointed out the error in your existing code  but I d like to take two steps further  Firstly  assuming you re using Java 1 5   you can achieve greater readability using the enhanced for loop   Customer findCustomerByid int id           for  Customer customer   customers            if  customer getId      id                return customer                      return null       This has also removed the micro-optimisation of returning null before looping - I doubt that you ll get any benefit from it  and it s more code  Likewise I ve removed the exists flag  returning as soon as you know the answer makes the code simpler   Note that in your original code I think you had a bug  Having found that the customer at index i had the right ID  you then returned the customer at index id - I doubt that this is really what you intended   Secondly  if you re going to do a lot of lookups by ID  have you considered putting your customers into a Map lt Integer  Customer gt

User · Answer

Even if that topic is quite old  I d like to add something  If you overwrite equals for you classes  so it compares your getId  you can use   customer   new Customer id   customers get customers indexOf customer      Of course  you d have to check for an IndexOutOfBounds-Exception  which oculd be translated into a null pointer or a custom CustomerNotFoundException

User · Answer

The compiler is complaining because you currently have the  if exist   block inside of your for loop  It needs to be outside of it   for int i 0 i lt this customers size   i             if this customers get i  getId      id               exist true              break               if exist        return this customers get id     else       return this customers get id       That being said  there are better ways to perform this search  Personally  if I were using an ArrayList  my solution would look like the one that Jon Skeet has posted

User · Answer

In Java 8   Customer findCustomerByid int id        return this customers stream            filter customer - gt  customer getId   equals id            findFirst   get        It might also be better to change the return type to Optional lt Customer gt

User · Answer

I did something close to that  the compiler is seeing that your return statement is in an If   statement  If you wish to resolve this error  simply create a new local variable called customerId before the If statement  then assign a value inside of the if statement  After the if statement  call your return statement  and return cstomerId  Like this   Customer findCustomerByid int id        boolean exist false       if this customers isEmpty              return null             for int i 0 i lt this customers size   i              if this customers get i  getId      id                exist true              break                     int customerId           if exist                customerId   this customers get id             else               customerId   this customers get id                       return customerId

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