Question: I want to compare two arraylist contents .

I am storing objects in them in this way.

for Arraylist 1:

```
Employee e1=new Employee();

e1.setID("1");
e1.setID("2");

ArrayList<Employee>list1 = new ArrayList<Employee>(); 

if(e1!=null){
    list1.add(e1);
}

```

for Arraylist 2:

```
Employee e2=new Employee();

e2.setID("1");
e2.setID("2");
e2.setID("4");

ArrayList<Employee>list2 = new ArrayList<Employee>(); 

if(e2!=null){
    list2.add(e2);
}

```

Now I am trying to compare the above arraylist contents in this way

```
ArrayList<Employee>unmatchedList = new ArrayList<Employee>();   

for (Employee l1 : list1){                               
    if(!list2.contains(l1.getID())){    
        System.out.println("list2 does not contains this ID"+l1.getID());   
        Employee e3=new Employee();          
        e3.setID(l1.getID());

        if(unmatchedList==null){            
        unmatchedList=new ArrayList<Employee>();            
        unmatchedList.add(e3);          
        }

        if(unmatchedList!=null){                            
            unmatchedList.add(e3);              
        }                       
    }
}

```

But I am not gettting correct unmatchedList contents as "4" only .
I am getting unmatchedList as "1" and "2" which is wrong.
So how can I get unmatched contents only in "unmatchedList"

Answer: 
If your class `Employee` is defined like this : 

```
public class Employee {

private int id;

public Employee(int id){
    this.id = id;
}

public int getId() {
    return id;
}

@Override
public String toString() {
    return "Id : " + this.id;
}

@Override
public boolean equals(Object obj) {
    return (obj instanceof Employee) && this.id == ((Employee)obj).getId();
}

```

in the Main method, you can retrieve the unmatch contents like this : 

```
 public static void main( String[] args )
{
   List<Employee> l1 = new ArrayList<Employee>();
   l1.add(new Employee(1));
   l1.add(new Employee(2));
   l1.add(new Employee(3));
   l1.add(new Employee(4));
   l1.add(new Employee(5));

   List<Employee> l2 = new ArrayList<Employee>();
   l2.add(new Employee(4));
   l2.add(new Employee(5));

   l1.removeAll(l2);
   System.out.println(l1);

}

```

This will print : `[Id : 1, Id : 2, Id : 3]`

Note that for this work you must override the `equals` method.
I also would recommend a Set to do the difference, and if you can use Sets then I would use the difference-method of Guava-Sets:
<http://google-collections.googlecode.com/svn/trunk/javadoc/com/google/common/collect/Sets.html#difference%28java.util.Set,%20java.util.Set%29>