[英]contains() method in List not working as expected
contains()
方法的api说
“如果此列表包含指定的元素,则返回true。更正式地说,当且仅当此列表包含至少一个元素e使得(o == null?e == null:o.equals(e))时,返回true。
我在类中覆盖了equals()
方法,但是当我检查时contains()
仍返回false
我的代码
class Animal implements Comparable<Animal>{
int legs;
Animal(int legs){this.legs=legs;}
public int compareTo(Animal otherAnimal){
return this.legs-otherAnimal.legs;
}
public String toString(){return this.getClass().getName();}
public boolean equals(Animal otherAnimal){
return (this.legs==otherAnimal.legs) &&
(this.getClass().getName().equals(otherAnimal.getClass().getName()));
}
public int hashCode(){
byte[] byteVal = this.getClass().getName().getBytes();
int sum=0;
for(int i=0, n=byteVal.length; i<n ; i++)
sum+=byteVal[i];
sum+=this.legs;
return sum;
}
}
class Spider extends Animal{
Spider(int legs){super(legs);}
}
class Dog extends Animal{
Dog(int legs){super(legs);}
}
class Man extends Animal{
Man(int legs){super(legs);}
}
原谅类背后的坏概念,但是我只是测试对我的概念的理解。
现在,当我尝试此操作时,即使覆盖equals,它也会打印false
List<Animal> li=new ArrayList<Animal>();
Animal a1=new Dog(4);
li.add(a1);
li.add(new Man(2));
li.add(new Spider(6));
List<Animal> li2=new ArrayList<Animal>();
Collections.addAll(li2,new Dog(4),new Man(2),new Spider(6));
System.out.println(li2.size());
System.out.println(li.contains(li2.get(0))); //should return true but returns false
您重载equals
而不是覆盖equals
。 要覆盖Object
的equals
方法,必须使用相同的签名,这意味着参数必须为Object
类型。
改成:
@Override
public boolean equals(Object other){
if (!(other instanceof Animal))
return false;
Animal otherAnimal = (Animal) other;
return (this.legs==otherAnimal.legs) &&
(this.getClass().getName().equals(otherAnimal.getClass().getName()));
}
如果满足以下所有条件,则在类C中声明的实例方法m1覆盖在类A中声明的另一个实例方法m2:
C is a subclass of A.
The signature of m1 is a subsignature of the signature of m2.
Either:
m2 is public, protected, or declared with default access in the same package as C, or
m1 overrides a method m3 (m3 distinct from m1, m3 distinct from m2), such that m3 overrides m2.
签名必须相同,才能覆盖您忽略的情况!
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.