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Finding a value in a list

I have a list declared as

        private List<Employees> employees;

and i get values ​​from database using DAO like

        employees= new ArrayList<Employees>();
        employees.addAll(myDAO.getEmployees());

I would like to find the value in employees List

, what is the best approach to find the value in employees List

?

I tried

    Collections.sort(employees);
    int index = Collections.binarySearch(employees, "abc");

However I am getting a cast Exception

Any help is appreciated.

thank

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6 answers 


Collections.sort(employees); // BigO - nlog(n)
int index = Collections.binarySearch(employees, new Employee("abc",...)); // BigO - log(n)

if you sort every time your list is and search for it, the complexity of the code will be nlog(n) + log(n)

where nlog(n)

for sorting a list and log(n)

for a binary search.

It's best if you search your list linearly. the search for a liner will take BigO - n

, which will work better than the previous one.



You are getting a method cast Exception

in Collections#sort

because your list contains null value

which cannot use Employee and raiseClassCastException

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Employees lookingForValue(String value, List<Employees> employees)
{
    for (Employees employee : employees)
    {
       if (value.equals(employee.getFieldValue()))
       {
          return employee;
       }
    }
    return null;
}

using



lookingForValue("abc", employees);
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If you cannot use

myDAO.getEmployees(parameter);

and in your DAO put a where clause then you can search and find the object

Employee emp = null;

    for(Employee e : employees) {
       if(e.getName().equals("X"))
           emp = e;
    }

    if(emp != null) {
      //handle the found employee
    }
    else {
      //employee not in list
    }

Of course, you can also override its equals method, initialize an object with that column and use (I wouldn't recommend it)

myDAO.getEmployees().get(object);
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Go through the list and perform a search action:

String searchString = "abc";
for(Employee employee: employees) {
  if(employee.getName().equals(searchString)) {
    // Found something!
  }
}

If you want to search for all fields Employee

, you may need to create a method in Employee

that checks all fields of an instance:

boolean findString(String searchString) {
  if(getName().equals(searchString)) return true;
  if(getCity().equals(searchString)) return true;
  // etc..
  return false;
}

and use this method in for

-loop.

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If your list is very large, I suggest doing a DAO search - the dbs can be tuned for better performance for such a search (for example select * from employees where name = 'abc'

) and then use the DAO method to return only the items that match.

Another nice alternative is apache-commons Predicate

eg,

matches = CollectionUtils.filter(employees, new Predicate<Employee>()
{
  @Override
  public boolean evaluate(Employee object)

    return "abc".equals(object.getName());
  }
});

Obviously, we parameterize "abc". Make Predicate a named class if you want to reuse it.

Predicate implementations are especially useful if you have many different ways to filter a filter; plus it leaves equal free for correct equality checks.

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You are on the right track. Just override the equals method in Employees (shouldn't it be Employee? Does it describe a collection of employees, or just one?)

Then your code should work. You can also sort your employees as above and provide a comparator:

Collections.sort(fields, new Comparator<Employee>() {
    @Override
    public int compare(Employee o1,Employee o2) {
        // TODO Rerturn 0 if equal or either 1 or -1 depending which of o1 or o2 is the bigger
    }
})

In the last approach, you DO NOT need to override the equals method.

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