如何定义我自己的元素类以与Set一起使用

Question

I have the following code:

public class MyElement {
    String name;
    String type;

    MyElement(String name, String type) {
        this.name = name;
        this.type = type;
    }
}

public class Test {

    public static void main(String[] args) {
        Set<MyElement> set = new HashSet<MyElement>();
        set.add(new MyElement("foo", "bar"));
        set.add(new MyElement("foo", "bar"));
        set.add(new MyElement("foo", "bar"));
        System.out.println(set.size());
        System.out.println(set.contains(new MyElement("foo", "bar")));
    }
}

which when executed comes back with:

3

false

I would have expected the result to be 1 and true. Why are my elements not being recognised as being the same and how do I rectify this? Thanks, Wayne.

Answer 1

You need to implement equals(Object o) and hashCode() on MyElement per the general contract. Absent that Set.contains() will use the default implementation which compares the memory address of the objects. Since you're creating a new instance of MyElement in the contains call it comes back as false.

Answer 2

You should override an equals(MyElement me) function. Equals returns a boolean

Otherwise, you are checking that two items are the same instance of an object, not that their internal content is the same.

MyElement(String name, String type) {
    this.name = name;
    this.type = type;
}

public boolean Equals<MyElement>(MyElement me) {
    return this.name.equals(me.name) && this.type.equals(me.type);
}