Generics - Part 1

Before diving into Generics we have to get a better understanding of some Object's methods like hashCode() and equals().

Overriding equals()

The main purpose of this method is compare if the objects are meaningfully equivalent, if you don't override this method you cannot use the object as a key in a hashtable, it's because when you don't override a equals() method it uses the operator == to do a comparison.

Let's override a equals() method.

	package br.com.ocjp.generics;
	public class Employee {
	private String name;
	private int age;
	public String getName() {
	return name;
	}
	public void setName(String name) {
	this.name = name;
	}
	public int getAge() {
	return age;
	}
	public void setAge(int age) {
	this.age = age;
	}
	public boolean equals(Object o) {
	if ( (o instanceof Employee) && ( ((Employee)o).getName().equals(this.getName()) &&
	((Employee)o).getAge() == this.getAge() ) ) {
	return true;
	} else {
	return false;
	}
	}
	}

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	package br.com.ocjp.generics;
	public class AppEquals {
	public static void main(String[] args) {
	Employee e = new Employee();
	e.setName("David");
	e.setAge(34);
	Employee e1 = new Employee();
	e1.setName("Anthony");
	e1.setAge(34);
	Employee e2 = new Employee();
	e2.setName("David");
	e2.setAge(34);
	System.out.println(e.equals(e1));
	System.out.println(e.equals(e2));
	}
	}

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Overriding hashCode()

Hascode is used by some Collections, it provides a way to store the objects in a collection as well as it helps to locale the objets within the collections.  If two objects are equal, their hashcodes must be equal as well.

Implementing hashCode()

	package br.com.ocjp.generics;
	public class Employee {
	private String name;
	private int age;
	public String getName() {
	return name;
	}
	public void setName(String name) {
	this.name = name;
	}
	public int getAge() {
	return age;
	}
	public void setAge(int age) {
	this.age = age;
	}
	@Override
	public int hashCode() {
	final int prime = 31;
	int result = 1;
	result = prime * result + age;
	result = prime * result + ((name == null) ? 0 : name.hashCode());
	return result;
	}
	}

view raw gistfile1.java hosted with ❤ by GitHub

Let's review equals and hashCode methods

After this brief introduction needed to work with Collections you're able to dive into Collections which will be covered in the next section.