Is there auto type inferring in Java?

问题:

Is there an auto variable type in Java like you have in C++?

An example:

for ( auto var : object_array)
    std::cout << var << std::endl;

for( auto var : object_array)
    var.do_something_that_only_this_particular_obj_can_do();

I know that there is an enhanced for loop in Java, but is there an auto? If not, is there a hack to doing this? I am referring to the new feature in C++11

回答1:

Answered before the question was EDITED :

No there is no auto variable type in Java. The same loop can be achieved as:

for ( Object var : object_array)
  System.out.println(var);

Java has local variables, whose scope is within the block where they have been defined. Similar to C and C++, but there is no auto or register keyword. However, the Java compiler will not allow the usage of a not-explicitly-initialized local variable and will give a compilation error (unlike C and C++ where the compiler will usually only give a warning). Courtesy: Wikipedia.

No, there isn't any mainstream type-inference in Java like C++ . There was an RFE but this was closed as "Will not fix", reason given was :

Humans benefit from the redundancy of the type declaration in two ways. First, the redundant type serves as valuable documentation - readers do not have to search for the declaration of getMap() to find out what type it returns. Second, the redundancy allows the programmer to declare the intended type, and thereby benefit from a cross check performed by the compiler.

回答2:

Might be Java 10 has what you (and I) want, through the var keyword.

var list = new ArrayList<String>();  // infers ArrayList<String>
var stream = list.stream();          // infers Stream<String>

From JDK Enhancement Proposals 286

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Update: Yap, that feature made it into the Java 10 release!

回答3:

Java 7 introduces the diamond syntax

Box<Integer> integerBox = new Box<>(); // Java 7

As compared to old java

Box<Integer> integerBox = new Box<Integer>(); // Before Java 7

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The critical reader will notice that this new syntax doesn't help with writing the for loops in the original question. That's correct and fully intentional it seems. See the other answer that cites Oracle's bug database.

回答4:

In Java 8, you can use lambda type inference to avoid declaring the type. The analogue to the questioner's examples would be:

object_array.forEach(var -> System.out.println(var)); 
object_array.forEach(var -> var.do_something_that_only_this_particular_obj_can_do());

both of which can also be simplified using method references:

object_array.forEach(System.out::println); 
object_array.forEach(ObjectType::do_something_that_only_this_particular_obj_can_do);

回答5:

In short, no, there is no auto type. If all you are doing is printing the value though, you could just refer to the value as an Object.

回答6:

It's not a pure Java solution, however adding a library called lombok will enable the magic below to compile and work very much similar to auto keyword in C++

List<String> strList = Arrays.asList("foo", "bar", "baz");
for (val s: strList){
    System.out.println(s.length());
}