如何修复我的重写equals（）方法

Question

• I'm writing a override method for .equals() to compare two different classes of shapes.

• The equals() will compare the dimensions of those shapes (EX: 4 X 4 X 4, equal to 4 X 4 X 4)

• Having trouble, already looked at other examples, and they don't apply to my program.

• Please help with the return statement and if I set up other parts of it, that would also help :)

By the way, I don't have the ability to use Eclipse.

Here's some code:

Method - My return statement is not done, I was trying to figure that out still.

   @Override
   public boolean equals( Object ob )
 {
    // instanceof is not in the AP Java subset
    if ( ! (ob instanceof Rectangle3) )
    {
        return false;
    }
    Rectangle3 that = (Rectangle3)ob;
    return Objects.equals()
    return t.getReal() == getReal() && t.getImag() == getImag();
 }

Rectangle class -

private int length;
private int width;
public Rectangle3(int l, int w)
{
    length = l;
    width = w;
}
public int getLength()
{
    return length;
}
public int getWidth()
{
    return width;
}

public String toString()
{
    return "(" + length + " X " + width + ")";
}

Box class -

 public class Box3 extends Rectangle3
 {
// instance variables 
private int height;

/**
 * Constructor for objects of class box
 */
public Box3(int l, int w, int h)
{
    // call superclass
    super(l, w);
    // initialise instance variables
    height = h;
}
// return the height
public int getHeight()
{
    return height;
}
public String toString()
{
    return "(" + getLength() + " X " + getWidth() + " X " + height + ")";
}

} 

Part of main -

public static void main(String []args)
{
    Rectangle3 one = new Rectangle3(5, 20);
    Box3       two = new Box3(5, 5, 5);
    Cube3    three = new Cube3(5, 5, 5);
    // print
    System.out.println("          Dimensions: ");
    showEffectBoth(one);
    showEffectBoth(two);
    showEffectBoth(three);
}

public static void showEffectBoth(Rectangle3 r)
{
    System.out.println(r.getClass().getName() + " - " + r);

}

Answer 1

You've chosen wrong structure for inheritance of classes. The parent class should be Box3 . Rectangle3 is a Box3 with height = 0; Cube3 is Box3 with equal sides:

class Box3{
    int length;
    int height;
    int width;

    Box3()
    {
    }

    Box3(int length, int height, int width)
    {
        this.length = length;
        this.height = height;
        this.width = width;
    }

}

class Rectangle3 extends Box3{
    Rectangle3(int length, int width)
    {
        super(length, 0, width);
    }
}

class Cube3 extends Box3{
    Cube3(int side)
    {
        super(side, side, side);
    }
}

And then you can add the following code to Box3 class for comparing Box3 instances:

@Override
public boolean equals(Object ob)
{
    if (ob == null) return false;
    if (ob == this) return true;
    if (!(ob instanceof Box3))return false;
    Box3 that = (Box3) ob;
    return that.getHeight() == this.getHeight()
        && that.getWidth() == this.getWidth()
        && that.getLength() == this.getLength();
}

You should also override hashCode method, something like this:

@Override
public int hashCode() {
    int hash = 1;
    hash = hash * 17 + this.length;
    hash = hash * 31 + this.width;
    hash = hash * 13 + this.height;
    return hash;
}

Answer 2

Your existing equals method is slightly confusing, I will explain whats wrong with it in the last section, but theres a lot in there that is unclear.

Writing an equals method.

An equals method is something which outputs true if two objects are equal by whatever definition you choose , so you have said that they need to both be rectangles and have the same class (Rectangle) and have equal dimentions. Here would be an equals method

public class Rectangle3{
    private int length;
    private int width;
    public Rectangle3(int l, int w)
    {
        length = l;
        width = w;
    }
    public int getLength()
    {
        return length;
    }
    public int getWidth()
    {
        return width;
    }
    @Override
    public boolean equals(Object other){
        if (!(other instanceof Rectangle3)){
             return false;
        }

        Rectangle3 otherRectangle=(Rectangle3)other;
        return this.length==otherRectangle.length && this.width==otherRectangle.width;

    }
}

Having seen Rectangle3 you should now be able to create one for Box3 as well.

Overriding the hashcode

Remember that when you override the equals method you need to override the hashcode also; this is basically a quick check of equality. A hashcode must return the same number for two equal objects and should probably equal non equal numbers for non equal objects. An example might be as follows:

@Override
public int hashCode() {
    int hash = 7;
    hash = 37 * hash + this.length;
    hash = 37 * hash + this.width;
    return hash;
}

A good IDE will be able to ble to generate a new HashCode for you. Hashcodes are used by many collections (such as HashSet), failing to include it will cause you bugs down the road.

Your equals method

   @Override
   public boolean equals( Object ob )
 {
    // this is good, first check if its the right type
    if ( ! (ob instanceof Rectangle3) )
    {
        return false;
    }

    //poor naming convention but again the right idea, we need to cast to Rectangle3 to use its methods
    Rectangle3 that = (Rectangle3)ob;
    return Objects.equals()

    //this just shouldn't be here, you've also missed a ;
    return Objects.equals()

    //t is not defined, possible you mean that, but in any case 
    //getReal() and getImag() are not defined in the rectangle class
    return t.getReal() == getReal() && t.getImag() == getImag();
 }

This is the equals method for Rectangle3 , you'll need to create annother equals method for Box3 , assuming Rectangle3's equals isn't suffificient (which it won't be as it doesn't include the 3rd dimention)