Removing a user defined object in a HashSet in Java

Question

I have created a simple class called Idea with a HashSet of ColumnPositions(another class I created with just an x and y field) . The equals and hashcode method in ColumnPosition looks like this:

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + x;
    result = prime * result + y;
    System.out.println("hashCode: " + result);
    return result;
}

@Override
public boolean equals(Object obj) {
    if (obj instanceof ColumnPosition) {
        ColumnPosition cp = (ColumnPosition) obj;
        if (cp.x != this.x || cp.y != this.y) {
            return false;
        }
    }
    return true;
}

However, when I go to test the Idea class and try to remove a ColumnPosition object within the HashSet of the Idea object. It doesn't work. What's wrong?? Thanks!

public void test_HashSetRemoveColumnPosition() {
    ColumnPosition column_03 = new ColumnPosition(0, 3);

    Set<ColumnPosition> columns = new HashSet<ColumnPosition>();
    columns.add(column_03);

    this.idea.getColumnPositions().add(column_03);
    assertEquals(1, columns.size());

    assertTrue(this.idea.getColumnPositions().remove(column_03)); // this is passing
    assertEquals(0, columns.size()); // this is failing because columns.size() still equals 1. WHAT?! How is this possible if the remove method returned true?
}

Answer 1

确保this.idea.getColumnPositions()与columns是同一对象。

Answer 2

You have 2 hashsets: the local variable in your test and a field in your test class. They are not the same so when you remove from one, the other is not modified...