If I understand correctly, this is the example scenario:

• listOne [datab] items: [A, B, C, D]
• listTwo [front] items: [B, C, D, E, F]

and what you need to get as an effect is:

• added: [E, F]
• deleted: [A]

First thing first, I would use some type adapter or extend the different types from one common class and override the equals method so you can match them by id and name

Secondly, this is very easy operations on sets (you could use set's but list are fine too). I recommend using a library: https://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/CollectionUtils.html

And now basically:

• added is listTwo - listOne
• deleted is listOne - listTwo

and using java code:

• added: CollectionUtils.removeAll(listTwo, listOne)
• deleted: CollectionUtils.removeAll(listOne, listTwo)

Otherwise, all collections implementing Collection (Java Docs) also has removeAll method, which you can use.

I would create this method:

private MyObject findMatch(List<MyObject> objsFromDB, MyObjRequest req_obj){
    for(MyObject db_obj : objsFromDB){
        if(isSameObj(db_obj, req_obj)){
            return db_obj;
        }
    }
    return null;
}

and refactor accordingly.

Beyond that the only suggestion I would have is perhaps some better variable names (at least use a single naming style; if you don't already have one, use this: http://www.oracle.com/technetwork/java/codeconventions-135099.html#367).

You can convert them to Set collections, and perform a set difference operation on them.

Like this:

Set<Date> ad = new HashSet<Date>(a);
Set<Date> bd = new HashSet<Date>(b);
ad.removeAll(bd);

It's not the most efficient solution but the most terse code would be:

boolean equalLists = listA.size() == listB.size() && listA.containsAll(listB);

Update:

@WesleyPorter is right. The solution above will not work if duplicate objects are in the collection.
For a complete solution you need to iterate over a collection so duplicate objects are handled correctly.

private static boolean cmp( List<?> l1, List<?> l2 ) {
    // make a copy of the list so the original list is not changed, and remove() is supported
    ArrayList<?> cp = new ArrayList<>( l1 );
    for ( Object o : l2 ) {
        if ( !cp.remove( o ) ) {
            return false;
        }
    }
    return cp.isEmpty();
}

Update 28-Oct-2014:

@RoeeGavriel is right. The return statement needs to be conditional. The code above is updated.

EDIT

Here are two versions. One using ArrayList and other using HashSet

Compare them and create your own version from this, until you get what you need.

This should be enough to cover the:

P.S: It is not a school assignment :) So if you just guide me it will be enough

part of your question.

continuing with the original answer:

You may use a java.util.Collection and/or java.util.ArrayList for that.

The retainAll method does the following:

Retains only the elements in this collection that are contained in the specified collection

see this sample:

import java.util.Collection;
import java.util.ArrayList;
import java.util.Arrays;

public class Repeated {
    public static void main( String  [] args ) {
        Collection listOne = new ArrayList(Arrays.asList("milan","dingo", "elpha", "hafil", "meat", "iga", "neeta.peeta"));
        Collection listTwo = new ArrayList(Arrays.asList("hafil", "iga", "binga", "mike", "dingo"));

        listOne.retainAll( listTwo );
        System.out.println( listOne );
    }
}

EDIT

For the second part ( similar values ) you may use the removeAll method:

Removes all of this collection's elements that are also contained in the specified collection.

This second version gives you also the similar values and handles repeated ( by discarding them).

This time the Collection could be a Set instead of a List ( the difference is, the Set doesn't allow repeated values )

import java.util.Collection;
import java.util.HashSet;
import java.util.Arrays;

class Repeated {
      public static void main( String  [] args ) {

          Collection<String> listOne = Arrays.asList("milan","iga",
                                                    "dingo","iga",
                                                    "elpha","iga",
                                                    "hafil","iga",
                                                    "meat","iga", 
                                                    "neeta.peeta","iga");

          Collection<String> listTwo = Arrays.asList("hafil",
                                                     "iga",
                                                     "binga", 
                                                     "mike", 
                                                     "dingo","dingo","dingo");

          Collection<String> similar = new HashSet<String>( listOne );
          Collection<String> different = new HashSet<String>();
          different.addAll( listOne );
          different.addAll( listTwo );

          similar.retainAll( listTwo );
          different.removeAll( similar );

          System.out.printf("One:%s%nTwo:%s%nSimilar:%s%nDifferent:%s%n", listOne, listTwo, similar, different);
      }
}

Output:

$ java Repeated
One:[milan, iga, dingo, iga, elpha, iga, hafil, iga, meat, iga, neeta.peeta, iga]

Two:[hafil, iga, binga, mike, dingo, dingo, dingo]

Similar:[dingo, iga, hafil]

Different:[mike, binga, milan, meat, elpha, neeta.peeta]

If it doesn't do exactly what you need, it gives you a good start so you can handle from here.

Question for the reader: How would you include all the repeated values?

I find the current algorithm of "adding and removing" to be a streamed implementation of non-functional thinking. Rather than adding and removing, I encourage you to think in terms of joining (akin to SQL outer-joining) and filtration, which are more functional concepts. In this interpretation, your "local" collection is on the left-hand side of a left outer join, and your "remote" collection is on the right. This reduces the number of iterations from your six down to two, and produces the same output:

Customer.java

package com.stackexchange.ConsentExample;

import java.util.Collection;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public record Customer(
    Long id,
    String name,
    Integer consent
) {
    @Override
    public String toString() {
        return String.format(
            "Customer{id=%d, name='%s', consent=%d}",
            id, name, consent);
    }

    public static Stream<Customer> join(
        Collection<Customer> locals,
        Collection<Customer> remotes
    ) {
        /* Take local consent if there is no matching remote consent.
           Take remote consent if there is a match. */
        Map<Integer, Customer> remoteByConsent =
            remotes.stream()
            .collect(Collectors.toMap(
                Customer::consent, Function.identity()
            ));

        return locals.stream()
            .map(local ->
                remoteByConsent.getOrDefault(local.consent, local)
            );
    }
}

JoinTest.java

import com.stackexchange.ConsentExample.Customer;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class JoinTest {
    @Test
    public void testJoin() {
        final List<Customer>
        remotes = List.of(
            new Customer(10L, "name1", 12),
            new Customer(11L, "name2", 11),
            new Customer(12L, "name3", 14),
            new Customer(13L, "name4", 16)
        ),
        locals = List.of(
            new Customer(1L, "name1", 12),
            new Customer(2L, "name2", 13),
            new Customer(3L, "name3", 14),
            new Customer(4L, "name4", 15)
        );
        Map<Long, Customer> result =
            Customer.join(locals, remotes)
            .collect(Collectors.toMap(
                Customer::id, Function.identity()
            ));

        assertEquals(4, result.size());

        assertEquals("name1", result.get(10L).name());
        assertEquals(12, result.get(10L).consent());
        assertEquals("name2", result.get(2L).name());
        assertEquals(13, result.get(2L).consent());
        assertEquals("name3", result.get(12L).name());
        assertEquals(14, result.get(12L).consent());
        assertEquals("name4", result.get(4L).name());
        assertEquals(15, result.get(4L).consent());
    }
}