Initializing a collection

Question

What is the difference between the following two statements, in initializing an ArrayList?

ArrayList<String> a = new ArrayList<String>();
ArrayList<String> a = new ArrayList<>();

well. first one was allowed in java 6 whereas second one is allowed in java 7, I am also curious to know the detailed reason for it — Prasad Kharkar, Jul 11 '13 at 10:42
@PrasadKharkar you mean "the _second one_ is not allowed in java 6", right? — fge, Jul 11 '13 at 10:42
@fge yeah made silly mistake while commenting. I mean. java 6 doesn't allow second version and java 7 allows it. — Prasad Kharkar, Jul 11 '13 at 10:44
But isn't there any detailed explanation for this? Or is it just the syntax change? Is this only the difference? — Prasad Kharkar, Jul 11 '13 at 10:45
@PrasadKharkar this is due to type inference ([link](http://docs.oracle.com/javase/tutorial/java/generics/genTypeInference.html)), which is new in Java 7 — fge, Jul 11 '13 at 10:47
Also read [this](http://tronicek.blogspot.in/2011/03/do-we-really-need-in-diamond-operator.html) blog. — AllTooSir, Jul 11 '13 at 10:48

score 13 · Accepted Answer · answered Jul 11 '13 at 10:42

13

Before Java 1.7, only this one is permitted:

ArrayList<String> a = new ArrayList<String>();

And in 1.7, this is added, which is the same but shorter: (all programmers are lazy)

ArrayList<String> a = new ArrayList<>();

answered Jul 11 '13 at 10:42

johnchen902

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+1 for "all programmers are lazy". Best example the self explaining variable `a` ;-) – Martin Seeler Jul 11 '13 at 10:46
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@Chasmo +1 for you comment too.. – Prasad Kharkar Jul 11 '13 at 10:48
Actually known as "Diamond Operator" in Java 7 >. – axcdnt Jul 11 '13 at 11:29
Why not just do away with the diamonds on the RHS if it's inferred? – Tyler Pfaff Apr 27 '15 at 17:32
Compatibility. It meant another thing (raw type). – johnchen902 Apr 29 '15 at 02:13

Reimeus · Answer 2 · 2013-07-11T10:50:12.283

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The latter uses an inferred type introduced in Java 7. The syntax (known as diamond operator) is illegal for Collections before Java 1.7 so the former is used for those earlier versions.

The diamond operator reduces the verbosity of the declaration.

edited Jul 11 '13 at 10:50

answered Jul 11 '13 at 10:41

Reimeus

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score 3 · Answer 3 · answered Jul 11 '13 at 10:42

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There is no difference. The second option (called Diamond Operator) is a shortcut. The compiler will infer that the type parameter of the generic ArrayList must be String.

answered Jul 11 '13 at 10:42

Pyranja

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score 0 · Answer 4 · answered Jul 11 '13 at 10:42

0

Second option was uses a concept that was introduced in java 7 - inferred types. Apart from that and assuming you are using java 7 the effect of the two calls should be the same. On earlier java versions you can not use second version of your code.

answered Jul 11 '13 at 10:42

Ivaylo Strandjev

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Initializing a collection

4 Answers4