This question already has answers here:
What is the point of the diamond operator (<>) in Java?
(7 answers)
Closed 2 years ago.
Hi Guys I have a question about HashMap declaration in Java. I'd like to know what is the difference between:
HashMap<String,Integer> map = new HashMap<>();
and
HashMap<String,Integer> map = new HashMap<String,Integer>();
Most coding websites teach the second one. However, the first one works perfectly fine. Is there a reason we should use second one?
There is technically none, however since Java 1.7 it's a convention to use this <> diamond operator, it reduces the text size.
This is called Type Inference.
Compilers from releases prior to Java SE 7 are able to infer the actual type parameters of generic constructors, similar to generic methods. However, compilers in Java SE 7 and later can infer the actual type parameters of the generic class being instantiated if you use the diamond (<>).
new HashMap<String,Integer>() - The type parameter is mandatory before Java 7
new HashMap<>() - The type parameter is inferred as of Java 7
It's recommended to use the "diamond operator" (<>) due to the brevity. Effectively, there is no difference.
There is no difference between the two of those since form the Java 7. Previously before Java 7 you must mention the Generic type parameters like in your second case in both sides.But to make it easier so that developer can write less code after Java 7 only you need to define it once in the left side.
Before Java Version 7 you need to code like below.
HashMap<String,Integer> map = new HashMap<String,Integer>();//For java versions below 1.7
After Java version 7
HashMap<String,Integer> map = new HashMap<>();//For java versions above 1.7
Related
This question already has an answer here:
Can Java 10 type inference for local variables infer void?
(1 answer)
Closed 4 years ago.
I was going through new features in java 10.
But I could not understand what is local-variable type inference?
Can someone please explain.
Now we can write var x = new HashMap<String,String>(); instead of a more verbose Map<String,String> x = new HashMap<String,String> (); and the information about type is not duplicated. It is one step ahead of Java 7 type inference for generic instance creation , a.k.a. diamond.
Usage of var is possible within a method's scope only (hence the name: local variable) and it is allowed only in the statements where the type can be inferred (hence the suffix: type inference).
Parsing a var statement, the Java 10 compiler infers the type from the right hand side (RHS) expression. This means that the variable declared using var needs to be immediately assigned and even this:
var readMe;
readMe = "notAGoodVariableName";
OR this:
var readMe = null;
is NOT allowed.
Also, please note that since it makes code a little less explicit, if used in statements like var x = getCapitalized('abc'), it can create confusions for the code reader.
Finally, var is not a keyword, but a reserved type name. Not being a keyword ensures that all the legacy applications don't break. But being a reserved type name still means that there would be a single breaking point and the legacy applications will have to rename all classes/interfaces which are named exactly as var while upgrading to Java 10 (a very rare and anti-naming-convention case).
There are some rules to be followed to use it correctly, hence read more at:
http://openjdk.java.net/jeps/286
https://developer.oracle.com/java/jdk-10-local-variable-type-inference
This question already has answers here:
What is the diamond operator in Java?
(2 answers)
Closed 4 years ago.
What's the most right and recommended java expression:
new ArrayList<>();
Or
new ArrayList<String>();
My question goes on any Object that contains any type (like Map).
The first way is valid from Java 7 and you need not to have type init which called as Diamond Operator.
You can replace the type arguments required to invoke the constructor of a generic class with an empty set of type parameters (<>) as long as the compiler can infer the type arguments from the context. This pair of angle brackets is informally called the diamond.
The purpose of the diamond operator is to simplify instantiation of generic classes. So just to keep the things simple prefer first way.
Since Java 7, the Diamond operator is used to reduce the verbosity.
If you use version >=7, it is recommended to use the first one.
Go through this.
From Java >= 7 none of them is better than other. The compiler will basically handle them both the same way.
Before Java 7 you had to specificy your Generic Type.
List<String> myStrings = new ArrayList<String>();
But since Java 7, you can do:
List<String> myStrings = new ArrayList<>();
And the compiler is gonna find out the rigth target type for your Collection and inject into your Collection. this is called Type Inference for Generic Instance Creation
Again none is better or recommended above the other, it is just to facilitate your job, so that you write less code. If you are a new java programmer trying to understand the language, you should start with the former. If you are a experienced programmer you do the latter
This question already has answers here:
Is there a difference between explicitly putting the type into the diamond operator vs letting java figure it out?
(4 answers)
Closed 5 years ago.
What is purpose of re-specifying the type on the right / constructor side of a Java Collection declaration + instantiation?
For example, how is
List<MyClass> a = new ArrayList<MyClass>();
different / better / worse than
List<MyClass> a = new ArrayList<>();
My understanding is the the second form became legal in Java 7, but I still see a lot of examples in Java 7 and 8 where the first form is used.
It is not worse or different. It is the same.
A new and easy way introduced by compiler to reduce human effort of defining of type.
This question already has answers here:
What is the diamond operator in Java?
(2 answers)
Closed 7 years ago.
just like the title said, in Java, is there any difference between the two statements?
Usually I can see the two statements both. So I suppose there may be no difference. But could you help to confirm and explain why they are identical?
ArrayList<String> al = new ArrayList<String>();
ArrayList<String> al = new ArrayList<>();
Thanks!
They are identical. new ArrayList<String>(); was required prior to JDK 7. In JDK 7 they introduced the 'diamond operator', where the type is inferred.
So, if you are using JDK 7+ you can simply use new Arraylist<>()
You can see this in the Java Language Specification under Class Instance Creation Expressions, or search for 'Diamond Operator'
This is a Diamond Operator.
The Diamond Operator reduces some of Java's verbosity surrounding generics by having the compiler infer parameter types for constructors of generic classes.
http://www.javaworld.com/article/2074080/core-java/jdk-7--the-diamond-operator.html
In java 7 and later there is no difference. The latter is considered more concise.
The so called diamond operator <> is helping here for the type to be inferred.
Is the explicit specification of array lists considered as a bad practice since Java7? Why is it unneccessary from this version?
List<String> foo = new Arraylist<String>(); // before
List<String> bar = new Arraylist<>(); // from Java7
Is it just because it's defined in the List?
Thanks!
Is the explicit specification of array lists considered as a bad practice since Java7?
It's not "bad practice" - it's just a little more longwinded than it might be. The two statements have exactly the same effect, so there's no downside to the longer version in that sense. You definitely don't need to go around "fixing" all your old code - but you may choose to use the shorter form when you're editing existing code anyway, or writing new code.
Why is it unneccessary from this version?
Because Java 7 introduced the "diamond operator" (not actually an operator at all) precisely to avoid having to specify type arguments redundantly. From the Oracle documentation on "Type Inference for Generic Instance Creation":
You can replace the type arguments required to invoke the constructor of a generic class with an empty set of type parameters (<>) as long as the compiler can infer the type arguments from the context. This pair of angle brackets is informally called the diamond.
This isn't the only new feature in Java 7 from a language perspective - again, see the documentation for details.
I dont think its a bad practice, its just a way of giving ease to the programmer.. Consider this
Map<List<String>,Map<String,Integer>> superMap = new HashMap<>();
the same should have been written as
Map<List<String>,Map<String,Integer>> superMap = new HashMap<List<String>,Map<String,Integer>>();
in Java 6, where the later generic substitution is redundant
The new format just helps you to save some keystrokes. So in Java 7 when you write
List<String> bar = new Arraylist<>();
which is considered equal to
List<String> foo = new Arraylist<String>();
Compiler will automatically infer the types from the left hand side when diamond operator is used.