length – arrays (int[], double[], String[]) —- to know the length of the arrays.

length() – String related Object (String, StringBuilder etc)to know the length of the String

size()– Collection Object (ArrayList, Set etc)to know the size of the Collection

length is not a method, so it completely makes sense that it will not work on objects. Its only works on arrays.

Points to Note

1. ArrayList allows NULL values many times
2. Set and HashMap allows NULL once
3. In HashMap the values will be overwritten when keys are same

List An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.

MapAn object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

Test.java

public class Test
{
 public static void main(String[] args) 
 {
 List<String> arrNames = new ArrayList<String>();
 
 arrNames.add("Hello");
 arrNames.add(null);
 arrNames.add(null);
 arrNames.add("Hi"); 
 System.out.println("List - "+arrNames.size());
 
 Map hmTest = new HashMap();
 hmTest.put("1", "mugil");
 hmTest.put(null, null);
 hmTest.put(null, null);
 hmTest.put("1", "vannan");
 System.out.println("HashMap - "+ hmTest.size());
 
 Set<Integer> setTest = new HashSet<Integer>();
 
 setTest.add(new Integer("10"));
 setTest.add(new Integer("20"));
 setTest.add(new Integer("10"));
 
 System.out.println("Set - "+ setTest.size());
 }
}

Output

List - 4
HashMap - 2
Set - 2