尝试使用.contains（）Java方法

Question

This is my "main" code:

ArrayList<Person> people = new ArrayList<Person>();

  people.add(new Person("Aaron Morgan", 31));
  people.add(new Person("Faye Palmer", 27));
  people.add(new Person("Dexter Koh", 33));
  people.add(new Person("Sallie Axl", 25));

  if (people.contains(new Person("Adam Kelly", 48)))
  {
     System.out.print("MATCH FOUND");
  }

    else
  {
     System.out.print("MATCH NOT FOUND");
  }

I'm attempting to create a method which will compare "Adam Kelly" with the list of people in the array list. I can take in the "Adam Kelly" into the method fine, however I'm not sure how I'm meant to compare it in the class/method.

Can anyone shed any light on this?

Answer 1

Implement .equals() like this in your Person class:

@Override
public boolean equals(Object o) {
 if(o == null) { return false; }
 if(!(o instanceof Person)) { return false; }

 Person person = (Person) o;
 return this.name.equals(person.getName()) && this.age == person.getAge();
}

The .contains() method will look for an existing value based on their .equals() representation. The standard implementation for a reference type is simply the address location of the object in memory. Since you have two different objects this will return a different address and thus it won't be found.

What you have to do is create your own implementation that uses the fields you want.

Answer 2

Constains depends on the Object's equals() implementation. So you need to write an equal implementation to Person(). Either way contains() on list is not very optimal and has O(n) runtime. It's much better to use a Hash based collection, namely Set() and make sure Person has both valid hashCode() and equals() implementation.

Answer 3

In your People class you need to implement an equals method.

public boolean equals(Object obj) {
  if (obj instanceof Person) {
    Person p = (Person) obj;
    return ((this == obj) || (this.getName().equals(p.getName()) && 
           this.getAge() == p.getAge()));
  }
  return false;
}

Answer 4

必须编写自定义的equals方法，在其中比较属性

equals(Object obj) 

Answer 5

Basically you need to implement object equality by using Java's equals and hashcode in the Object class. Otherwise it will use the Object class implementation that only checks references by using the == operator.

Something like this in your class Person:

class Person {

    // your fields and stuff

    @Override
    public boolean Equals(Object o) {
        if (o == null) {
            return false;
        }
        if (!(o instanceof Person)) {
            return false;
        }

        Person that = (Person) o;

            if(this.name != null){
                return (this.name.equals(that.name)) && (this.age == that.age);
            }

            return false;
    }

    @Override
    public int hashCode(){
          // use a library here!
    }

} 

Answer 6

There's an easy way to make that. First you have to create an iterator for your ArrayList.

Iterator it = people.iterator();

then explore all elements and compare

while ( it.hasNext() ) {
  Object obj = it.next();
  Person person= (Person)obj;
  if(person.getName().equals("Adam Kelly") && person.getAge().equals("43"))
  {
   System.out.print("MATCH FOUND");
   }
else{
   System.out.print("MATCH NOT FOUND");
  }
}

hope this solve your problem.