Java 17 Recipes

5-11. Comparing Objects
 Problem
Your application requires comparing two or more objects to see whether they are 
the same.
 Solution 1
To determine whether the two object references point to the same object, use the == 
and != operators. The following solution demonstrates the comparison of two object 
references to determine whether they refer to the same object.
public static void main(String[] args){
// Compare if two objects contain the same values
Team team1 = new Team();
Team team2 = new Team();
team1.setName("Jokers");
team1.setCity("Crazyville");
team2.setName("Jokers");
team2.setCity("Crazyville");
if (team1 == team2){
System.out.println("These object references refer to the same 
object.");
} else {
System.out.println("These object references do NOT refer to the 
same object.");
}
// Compare two objects to see if they refer to the same object
Team team3 = team1;
Team team4 = team1;
if (team3 == team4){
System.out.println("These object references refer to the same 
object.");
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} else {
System.out.println("These object references do NOT refer to the 
same object.");
}
}
The following are the results of running the code.
These object references do NOT refer to the same object.
These object references refer to the same object.
 Solution 2
To determine whether the two objects contain the same values, use the equals() 
method. The object being compared must implement equals() and hashCode()for this 
solution to work properly. The following is the code for the Team class that overrides 
these two methods.
public class Team implements TeamType, Cloneable {
private List
players;
private String name;
private String city;
// Used by the hashCode method for performance reasons
private volatile int cachedHashCode = 0;
/**
* @return the players
*/
public List
getPlayers() {
return players;
}
/**
* @param players the players to set
*/
public void setPlayers(List
players) {
this.players = players;
}
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/**
* @return the name
*/
public String getName() {
return name;
}
/**
* @param name the name to set
*/
@Override
public void setName(String name) {
this.name = name;
}
/**
* @return the city
*/
public String getCity() {
return city;
}
/**
* @param city the city to set
*/
@Override
public void setCity(String city) {
this.city = city;
}
public String getFullName() {
return this.name + " - " + this.city;
}
/**
* Overrides Object's clone method
*
* @return
*/
public Object clone() {
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try {
return super.clone();
} catch (CloneNotSupportedException ex) {
return null;
}
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj instanceof Team) {
Team other = (Team) obj;
return other.getName().equals(this.getName())
&& other.getCity().equals(this.getCity())
&& other.getPlayers().equals(this.getPlayers());
} else {
return false;
}
}
@Override
public int hashCode() {
int hashCode = cachedHashCode;
if (hashCode == 0) {
String concatStrings = name + city;
if (players.size() > 0) {
for (Player player : players) {
concatStrings = concatStrings
+ player.getFirstName()
+ player.getLastName()
+ player.getPosition()
+ String.valueOf(player.getStatus());
}
}
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hashCode = concatStrings.hashCode();
}
return hashCode;
}
}
The following solution demonstrates the comparison of two objects that contain the 
same values.
public static void main(String[] args){
// Compare if two objects contain the same values
Team team1 = new Team();
Team team2 = new Team();
// Build Player List
Player newPlayer = new Player("Josh", "Juneau");
playerList.add(0, newPlayer);
newPlayer = new Player("Jonathan", "Gennick");
playerList.add(1, newPlayer);
newPlayer = new Player("Joe", "Blow");
playerList.add(1, newPlayer);
newPlayer = new Player("John", "Smith");
playerList.add(1, newPlayer);
newPlayer = new Player("Paul", "Bunyan");
playerList.add(1, newPlayer);
team1.setName("Jokers");
team1.setCity("Crazyville");
team1.setPlayers(playerList);
team2.setName("Jokers");
team2.setCity("Crazyville");
team2.setPlayers(playerList);
if (team1.equals(team2)){
System.out.println("These object references contain the same 
values.");
} else {
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System.out.println("These object references do NOT contain the 
same values.");
}
}
The following are the results of running this code.
These object references do NOT refer to the same object.
These object references contain the same values.
These object references refer to the same object.