Foreach loop in java for a custom object list

Question

I have an ArrayList for type Room (my custom object) Defined as below

ArrayList<Room> rooms = new ArrayList<Room>();

After then adding a series of objects to the ArrayList I want to go through them all and check various things. I am not a keen user of java but I know in many other programming languages a foreach loop would be the most simple way of doing this.

After a bit of research I found the following link which suggests the code below. How does the Java 'for each' loop work?

for(Iterator<String> i = someList.iterator(); i.hasNext(); ) {
  String item = i.next();
  System.out.println(item);
}

But as far as I can tell this cant be used for an Arraylist of a custom object.

Can, and if so how can I implement a foreach loop for an ArrayList of a custom object? Or how could I otherwise process each item?

Did you try it? Of course it works with custom objects. Try it out. — Rohit Jain
– Rohit Jain, Commented Nov 22, 2012 at 22:03
google search "java foreach" brings up the following page: docs.oracle.com/javase/1.5.0/docs/guide/language/foreach.html A bit of research? Took me 2 seconds. — Ingo
– Ingo, Commented Nov 22, 2012 at 22:06

ShyJ · Accepted Answer · 2012-11-22 22:04:28Z

39

Actually the enhanced for loop should look like this

for (final Room room : rooms) {
          // Here your room is available
}

answered Nov 22, 2012 at 22:04

ShyJ

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4 Comments

mr.user1065741 Over a year ago

in the code above what is the 'final' for? The example below doesn't seem to have this.

ShyJ Over a year ago

Using final keyword is considered a good practice whenever you're using a variable which holds a constant value or a reference (like in your case) which should not change. In this for loop it prevents the code inside it to accidently assign a different Room to the room local variable.

Cubic Over a year ago

@ShyJ It doesn't really add anything though. Assigning room does no harm, and making room final doesn't prevent any harm that could be done by mutating a room.

ShyJ Over a year ago

@Cubic Well, without final you could actually have a room = new Room () statement in the loop body and operate on a completely different room.

Skelly · Accepted Answer · 2022-03-16 03:49:59Z

12

You can also use Java 8 stream API and do the same thing in one line.

If you want to print any specific property then use this syntax:

ArrayList<Room> rooms = new ArrayList<>();
rooms.forEach(room -> System.out.println(room.getName()));

OR

ArrayList<Room> rooms = new ArrayList<>();
rooms.forEach(room -> {
    // here room is available
});

if you want to print all the properties of Java object then use this:

ArrayList<Room> rooms = new ArrayList<>();
rooms.forEach(System.out::println);

edited Mar 16, 2022 at 3:49

Skelly

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answered Sep 1, 2018 at 19:51

Amar Prakash Pandey

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Comments

Illyes Istvan · Accepted Answer · 2012-11-22 22:07:41Z

6

for(Room room : rooms) {
  //room contains an element of rooms
}

answered Nov 22, 2012 at 22:07

Illyes Istvan

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3 Comments

mr.user1065741 Over a year ago

What is the difference between this sample and the code in the other answer that includes the word 'final'?

Illyes Istvan Over a year ago

If a variable is final, it means you cant change its value.

Pinkesh Sharma Over a year ago

So that while iterating you are not allowed to update its value.

Jiri Kremser · Accepted Answer · 2012-11-22 22:06:35Z

2

You can fix your example with the iterator pattern by changing the parametrization of the class:

List<Room> rooms = new ArrayList<Room>();
rooms.add(room1);
rooms.add(room2);
for(Iterator<Room> i = rooms.iterator(); i.hasNext(); ) {
  String item = i.next();
  System.out.println(item);
}

or much simpler way:

List<Room> rooms = new ArrayList<Room>();
rooms.add(room1);
rooms.add(room2);
for(Room room : rooms) {
  System.out.println(room);
}

answered Nov 22, 2012 at 22:06

Jiri Kremser

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Comments

Andriya · Accepted Answer · 2015-07-06 11:15:42Z

0

If this code fails to operate on every item in the list, it must be because something is throwing an exception before you have completed the list; the likeliest candidate is the method called "insertOrThrow". You could wrap that call in a try-catch structure to handle the exception for whichever items are failing without exiting the loop and the method prematurely.

answered Jul 6, 2015 at 11:15

Andriya

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