在不同包中的子类中为equals（）使用super（） [英] Using super() for equals() in a subclass in a different package

问题描述

我有一个类Vehicle，它在包A和一个类Car，它在包B，我想使用equals方法，并利用继承的super（），但我不知道如何做到这一点。

当我尝试在main中运行该文件时，我得到：

 线程main中的异常java.lang.NullPointerException 
 at vehicle.Vehicle.equals（Vehicle.java:97）
 at car.Car.equals（Car.java:104）
 Main.main（Main.java:48）
  

这是代码：

  public boolean equals（Vehicle other）{
 if（this.type.equals（other.type））{
 if（this.year == other.year&& this.price == other.price）{
 return true; 
} else {
 return false; 
} 
} else {
 return false; 
} 
} 
 //等于in Car 
 public boolean equals（Car other）{
 if（this.type.equals（other.type））{
 if（this.speed == other.speed&& this.door == other.door）{
 if（super.equals（other））{
 return true; 
} else {
 return false; 
} 
} else {
 return false; 
} 
} else {
 return false; 
} 
} 
  

解决方案

code> equals（）方法根据 null 合约应传回 false c>作为参数传递

对于任何非空引用值 x ， x.equals（null）应返回 false 。

在每个 equals（）方法的开头添加：

  if（other == null）{
 return false; 
} 
  

其次，您必须覆写 code>，不重载它：

  public boolean equals（Object other）
  

最后，你需要 instanceof 和向下转换， >



和BTW这个：

  if（this.speed == other.speed 
 {
 if（super.equals（other））
 {
 return true; 
} 
 else 
 {
 return false; 
} 
} 
 else 
 {
 return false; 
} 
  

等效于：

  if（this.speed == other.speed&& this.door == other.door）
 {
 return super.equals ）; 
} 
 else 
 {
 return false; 
} 
  

，其中可以减少为：

  return this.speed == other.speed&& this.door == other.door&&超级
  




I have a class Vehicle which is in package A and a class Car which is in package B and I want to use equals method and take advantage of inheritance by using super(), but I don't know how to do this.

When I try to run the file in main I get this:

Exception in thread "main" java.lang.NullPointerException
    at vehicle.Vehicle.equals(Vehicle.java:97)
    at car.Car.equals(Car.java:104)
    at Main.main(Main.java:48)

Here is the code:

public boolean equals(Vehicle other) {
    if (this.type.equals(other.type)) {
        if (this.year == other.year && this.price == other.price) {
            return true;
        } else {
            return false;
        }
    } else {
        return false;
    }
}
//equals in Car
public boolean equals(Car other) {
    if (this.type.equals(other.type)) {
        if (this.speed == other.speed && this.door == other.door) {
            if (super.equals(other)) {
                return true;
            } else {
                return false;
            }
        } else {
            return false;
        }
    } else {
        return false;
    }
}

解决方案

equals() method as per the contract should return false when null passed as an argument:

For any non-null reference value x, x.equals(null) should return false.

Add this at the very beginning of every equals() method:

if(other == null) {
  return false;
}

Secondly you have to override equals(), not overload it:

public boolean equals(Object other)

Finally you'll need instanceof and downcasting to make this all work.

And BTW this:

if (this.speed == other.speed && this.door == other.door)
{
    if(super.equals(other))
    {
        return true;
    }
    else
    {
        return false;
    }
}
else
{
    return false;
}

is equivalent to:

if (this.speed == other.speed && this.door == other.door)
{
    return super.equals(other);
}
else
{
    return false;
}

which in turns can be reduced to:

return this.speed == other.speed && this.door == other.door && super.equals(other);

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