What does <?> sign mean in ArrayList

Question

In an ArrayList for android such as the simplelistadapter I commonly see ArrayList<?> and in some tutorials I've reviewed the <?> is replaced by some value. But I'm still not quite sure what variable or qualifier this stands for.

It simply means that the ArrayList is of the type ? (ie: the type that the ArrayList describes or wraps is of type ?) which means that it can be any non-primitive type, as mentioned below. If you had an ArrayList<?>, it could contain a Long, a String, and even a Map. An ArrayList<Integer>, however, could only contain Integer values. — Naftuli Kay
– Naftuli Kay, Commented Mar 5, 2012 at 18:48
possible duplicate of What does List<?> mean in java generics? — Ken Redler
– Ken Redler, Commented Mar 6, 2012 at 5:16

Nitzan Tomer · Accepted Answer · 2012-03-05 18:45:55Z

5

It's called Generics.

For example, you can do this:

ArrayList<Integer> list = new ArrayList(); list.add(new Integer(1)); list.add(new Integer(2)); ..... Integer i = (Integer)list.get(1);

Or you can do:

ArrayList<Integer> list = new ArrayList<Integer>(); list.add(new Integer(1)); list.add(new Integer(2)); ..... Integer i = list.get(1);

As you can see, no need for casting.

answered Mar 5, 2012 at 18:45

Nitzan Tomer

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4 Comments

Melvin Lai Over a year ago

Can an ArrayList store an array? ArrayList<Array> ?

Nitzan Tomer Over a year ago

Yeah, it can "store" any type, even ArrayList<ArrayList<String>> works.

Melvin Lai Over a year ago

I see, does that make the ArrayList 2D? And seeing that String, Integers are Objects. I assuming, ArrayList<Objects> works too right? :)

Nitzan Tomer Over a year ago

Yeah, I guess that you can look at it as a 2D array kind of thing. You can go to 3D, 4D, etc.. if you want/need to. And yes, you can put Object there or any other type (not primitives).

Luchian Grigore · Accepted Answer · 2012-03-05 18:42:44Z

2

It means the ArrayList can contain any non-primitive type.

answered Mar 5, 2012 at 18:42

Luchian Grigore

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Comments

aidanok · Accepted Answer · 2012-03-05 20:12:46Z

The is a wildcard character for a generic type. Normally you declare an array list like:

ArrayList<String>

Where the type is specified exactly. The list will contain Strings. But sometimes you want to make a method or a class that takes an ArrayList of any type, or you want a field that points an ArrayList of any type

public void removeFirstItem(ArrayList<?> target) { ... }

Now this method can take an ArrayList<String>, or an ArrayList<Long>, etc, and do some operation on it.

Similiary you can have a local variable:

ArrayList<?> someList; someList = new ArrayList<String>(); someList = new ArrayList<Long>);

This works, whereas:

ArrayList<String> someList = new ArrayList<String>(); someList = new ArrayList<Long>();

Will not, since someList is specified as an ArrayList<String>, so only ArrayList<String> can be assigned to it.

Eng.Fouad · Accepted Answer · 2012-03-05 18:43:58Z

0

Similar to ArrayList<? extends Object>, which means it can contain any object inherits from Object class (i.e. All objects in Java).

answered Mar 5, 2012 at 18:43

Eng.Fouad

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3 Comments

Naftuli Kay Over a year ago

Hate to nitpick, but <em>all non-primitive objects in Java</em> would be more technically correct.

Eng.Fouad Over a year ago

@TKKocheran but, what do you mean by "non-premitive" objects? Is there primitive objects in Java?

Naftuli Kay Over a year ago

You can't make an ArrayList<int>, you can only make an ArrayList<Integer>, because an int is a primitive type and can't be null, but an Integer is a real Java type and can, in fact, be null.

Community · Accepted Answer · 2017-05-23 11:55:53Z

The syntax is a "wildcard". It's needed with Java generics:

A wildcard can be "bounded" or "unbounded". The whole issue of compile/design time generic typing is closely associated with the runtime JVM issue of "type erasure":

'Hope that helps .. pSM

Shawn Shroyer · Accepted Answer · 2012-03-05 18:49:41Z

This is a question regarding generics. The <(Kind of Object)> syntax represents that only a certain kind of object (class instance) may be passed as an argument. When you have the syntax, you are saying that any kind of class instance may be passed.

How about an Object class? The class contains 4 Strings in it. Can it still be used in an ArrayList?

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