Difference between Value Object pattern and Data Transfer pattern

Question

In which scenario can I use those design patterns in n-tier architecture?

Answer 1

There are several good answers here, but I'll add one to capture a key difference:

Value objects do not have an identity. That is, any comparison between two instances of a value object that contain should indicate that they are equal. Data Transfer Objects, while only being used to hold values, do have an identity. Comparing two instances of a DTO that have the same values, but were created independently, will not indicate that they are equal.

Example:

DTO dto1 = new DTO(10);
DTO dto2 = new DTO(10);
dto1.equals(dto2); //False (if equals is not overridden)
dto1 == dto2; //False

VO vo1 = VO.getInstance(10);
VO vo2 = VO.getInstance(10);
vo1.equals(vo2); //True
vo1 == vo2; //True

It's slightly difficult to implement the Value Object pattern in Java, since the == operator always compares object identity. One way to do so would be to implement an object cache that returns the same object for each value.

public class VO {
  Map> cache = new LinkedHashMap>(); 
  public static VO getInstance(int value) {
     VO cached = cache.get(value);
     if(cached == null) {
        cached = new VO(value);
        cache.put(value, new WeakReference(cached));
     }
     return cached.get();
  }

  private int value;
  private VO(int value) {
    this.value = value;
  }
}