Comparing Object References

A variable whose type is a class stores a reference, which locates an object kept elsewhere in memory. A music app's Song class shows what that means for comparisons.

Method Header	Description
`public Song(String title, int seconds)`	Creates a song with the given title and length in seconds.
`public int getSeconds()`	Returns this song's length in seconds.
`public boolean equals(Object other)`	Tests whether `other` is a `Song` whose title and length both match this song's.

Same object, or same contents

Song opener = new Song("Opening", 200);
Song encore = new Song("Opening", 200);

Each new creates an object, so there are two Song objects, with the same title and the same length. Applied to references, == asks whether two references are to one object. Here opener == encore is false, since the two variables refer to different objects.

Whether two songs carry the same information is a question for equals. For Song, the documentation says the title and the length must both match, so opener.equals(encore) is true. A song created as new Song("Opening", 180) would not be equal to opener, since the lengths differ.

Two references to one object

The app's Player class keeps a reference to the song it is playing.

Method Header	Description
`public void play(Song s)`	Starts playing `s`.
`public Song getPlaying()`	Returns the song being played, or `null` if nothing is playing.
`public Song getQueued()`	Returns the song queued to play next, or `null` if nothing is queued.

Suppose player refers to a Player object. After player.play(opener);, the call player.getPlaying() returns the reference stored in the player, and player.getPlaying() == opener is true: both are references to one object. Comparing it with encore by == gives false, even though the two songs are equivalent.

A reference to no object

A reference that is null refers to no object, and some methods return null on purpose. Suppose now that player's queue is empty. Then player.getQueued() == null is true. A comparison with null never fails, but calling a method through a null reference throws a NullPointerException, since there's no object to run it on. So a program that uses the queued song tests for null first.

Song next = player.getQueued();
if (next == null)
{
    System.out.println("Queue is empty");
}
else
{
    System.out.println(next.getSeconds());
}

The call next.getSeconds() is in the else body, so it runs only when next refers to a song.