对象集包含按值包含对象的对象（非引用）（java 7）

Question

I have my object with some feilds.

public class MyObject{

   private String a;
   private String b;

}

I have a Set contains objects like this :

Set<MyObject> thirdSet = new HashSet<MyObject>();

Set<MyObject> firstSet=getFirstSet();

Set<MyObject> secondSet = getSecondeSet();


for (MyObjectobj : firstSet) {
  if (!secondSet.contains(obj)) {
    thirdSet.add(obj);
  }
}

I need to select all obj that not contains in my secondSet into thridSet (obj with value not by reference) Is it possible or using collection is more better?

Answer 1

You'll need to override both equals and hashcode methods in your object. I'd recommend using the java 7 Objects utility methods if you can to prevent NullPointerExceptions.

@Override
public boolean equals(Object other) {
    if (!(other instanceof MyObject)) {
        return false;
    }
    MyObject that = (MyObject) other;
    return Objects.equals(a, that.a) && Objects.equals(b, that.b);
}

@Override
public int hashcode() {
    Objects.hash(a, b);
}

I'd also recommend taking a look at the third party library Guava if possible which would simplify your code.

Set<MyObject> thirdSet = new HashSet<>(Sets.difference(firstSet, secondSet));

Note wrapping it in a new HashSet so it can be modified (if you don't need to modify it you can remove that)

Answer 2

You should override Object#equals and Object#hashCode in MyObject.java.

@Override
public boolean equals(Object o) {
    if (!(o instanceof MyObject)) {
        return false;
    }

    MyObject m = (MyObject) o;

    return a.equals(m.a) && b.equals(m.b);
}

@Override
public int hashCode() {
    return Objects.hash(a, b);
}

Also if you're allowed to use external libraries, you should check out Guava's Sets#difference .