Container Class and its equals(), hashCode() methods

1.Why we need Container

For example, if we need to manage the employees' names of an company, if we don't know Container, we can only use an Array to store all the names of the employees. But it's not convenient, since the name may change alot. Sometime we need delete the name since the employee will leave the company, sometimes we need to add some names when we hire some new employees. It's difficult to delete a particular field in an Array and we can't add several fields in the array to store new names when the array is full. In java we have Container to solve these problems.

(important containers)

2. it's equals() and hashCode() methods

Like its name, a Container is a container to store Object. we can store different kinds of object in one contaion (not like array which can only store one kind of object). As we all know, in the Object class, the equals() method compare the reference of two objects. But in some other types the method is overwrited to compare their values, such as String, Integer objects.

If we want to add an object to an Container, we can use add(Object) method, and if we want to remove a object from the Container, we should use remove(Object) method. the remove(Object) realize its function by calling the equals() method.it compare two objects' reference by default which is meaningless. we need to rewrite the equals() method of our objects that need to be added into containerto compaare their values not reference, so does the hashCode() method. Because, two objects' hash Code must be the same if they equals to each other.

For example:

package com.test;

import java.util.Collection;
import java.util.HashSet;

public class BasicContainer {

	public static void main(String[] args) {
		Collection c = new HashSet();
		c.add("hello");
		c.add(new Name("nick","lau"));
		c.add(new Integer(100));
		
		c.remove("hello");
		c.remove(new Integer(100));
		System.out.println(c.remove(new Name("nick","lau")));
		System.out.println(c);

	}

}

the result: false 

                   nick lau

Since the String and Integer object has already overwrited the equals() method. they comapre the value of each object. c.remove("hello") compare all the value in Contaioner c to see which object's value is equals "hello", if find then delete the object in c. so dose the c.remove(new Integer(100)). Since in the Name object, we didn't overwrite the equals() method. it compare the reference of all the objects in c to see if there is one whose reference is the same with the new created Name("nick","lau"). Obviously, there is not. So the object can't be deleted from the conatainer c and it is printed out at last.

if we rewrite Name's equals() function:

package com.test;

import java.util.Collection;
import java.util.HashSet;

public class BasicContainer {

	public static void main(String[] args) {
		Collection c = new HashSet();
		c.add("hello");
		c.add(new Name("nick","lau"));
		c.add(new Integer(100));
		
		c.remove("hello");
		c.remove(new Integer(100));
		System.out.println(c.remove(new Name("nick","lau")));
		System.out.println(c);

	}

}

class Name{
	private String firstName,lastName;
	public Name(String _firstName, String _lastName){
		this.firstName = _firstName;
		this.lastName = _lastName;
	}

	//rewrite the equals method
	public boolean equals(Object obj){
		if(obj instanceof Name){
			Name name = (Name)obj;
			return(firstName.equals(name.firstName))&&(lastName.equals(name.lastName));
                 //since firstName is a String, so we are using String's equals() function.
                }
		return super.equals(obj);
	}
	
	//rewrite the hashCode() method
	public int hashCode(){
		return firstName.hashCode();
		// it means use "String"'s hashCode()function which is already rewritten.
	}

}

Since the class Name is now rewrited, when

c.remove(new Name("nick","lau"))

it will go through the container to compair this Name( nick, lau) and the new Name(nick, lau), since we are using the rewritted equals() function, the two objects are the same, so the container will delete this object. So there will not be any object at last in the container.