Java物件相等比較(Equals)

以下程式碼顯示如何實現equals()和hashCode()方法

class Point {
  private int x;
  private int y;

  public Point(int x, int y) {
    this.x = x;
    this.y = y;
  }

  /* implement the equals() method */
  public boolean equals(Object otherObject) {
    // Are the same?
    if (this == otherObject) {
      return true;
    }

    // Is otherObject a null reference?
    if (otherObject == null) {
      return false;
    }

    // Do they belong to the same class?
    if (this.getClass() != otherObject.getClass()) {
      return false;
    }

    // Get the reference of otherObject in a SmartPoint variable
    Point otherPoint = (Point) otherObject;

    // Do they have the same x and y co-ordinates
    boolean isSamePoint = (this.x == otherPoint.x && this.y == otherPoint.y);

    return isSamePoint;
  }

  /*
   * implement hashCode() method of the Object class, which is a requirement
   * when you implement equals() method
   */
  public int hashCode() {
    return (this.x + this.y);
  }
}

public class Main {
  public static void main(String[] args) {
    Point pt1 = new Point(10, 10);
    Point pt2 = new Point(10, 10);
    Point pt3 = new Point(12, 19);
    Point pt4 = pt1;

    System.out.println("pt1 == pt1: " + (pt1 == pt1));
    System.out.println("pt1.equals(pt1): " + pt1.equals(pt1));

    System.out.println("pt1 == pt2: " + (pt1 == pt2));
    System.out.println("pt1.equals(pt2): " + pt1.equals(pt2));

    System.out.println("pt1 == pt3: " + (pt1 == pt3));
    System.out.println("pt1.equals(pt3): " + pt1.equals(pt3));

    System.out.println("pt1 == pt4: " + (pt1 == pt4));
    System.out.println("pt1.equals(pt4): " + pt1.equals(pt4));
  }
}

上面的程式碼生成以下結果。

pt1 == pt1: true
pt1.equals(pt1): true
pt1 == pt2: false
pt1.equals(pt2): true
pt1 == pt3: false
pt1.equals(pt3): false
pt1 == pt4: true
pt1.equals(pt4): true

這裡是equals()方法實現的規範。 假設x，y和z是三個物件的非空參照。

• 自反性。 表示式x.equals(x)應該返回true。
• 對稱性。 如果x.equals(y)返回true，那麼y.equals(x)也必須返回true。
• 傳遞性。如果x.equals(y)返回true，那麼y.equals(z)也返回true，則x.equals(z)必須返回true。
• 一致性。如果x.equals(y)返回true，它應該保持返回true，直到x或y的狀態被修改。 如果x.equals(y)返回false，它應該保持返回false，直到x或y的狀態被修改。
• 與null參照的比較：任何類的物件不應等於null參照。 表示式x.equals(null)應始終返回false。
• 與hashCode()方法的關係：如果x.equals(y)返回true，x.hashCode()必須返回與y.hashCode()相同的值。