为什么方法'Arrays.asList（）'不接受类型为{“”，“”}的参数，而是接受（“”，“”）？

Question

I am not new to Java Collections , but I have a confusion about the following scenario.

In my project I've implemented a code like this:

List<String> obj_lStr= new ArrayList<String>();
String[] obj_arrStr = {"someString", "noString", "abcString"};
obj_lStr.addAll(Arrays.asList(obj_arrStr));

but during code review my project lead gave me the instruction to change this code and implement it without using String[] obj_arrStr .

Then I changed my code to this:

obj_lStr.addAll(Arrays.asList( { "someString", "noString", "abcString" }));

but I got compilation errors:

Main.java:13: error: illegal start of expression
    x.addAll(Arrays.asList({"someString", "noString", "abcString"}));
                           ^
Main.java:13: error: ')' expected
    x.addAll(Arrays.asList({"someString", "noString", "abcString"}));
                            ^
Main.java:13: error: ';' expected
    x.addAll(Arrays.asList({"someString", "noString", "abcString"}));

and I change my line of code to this:

obj_lStr.addAll(Arrays.asList("someString", "noString", "abcString"));

then the compilation error is gone.

Question: Why is it so? Why asList() method raises a compilation error with {"","",""} , but not for ("","","") ?

Answer 1

Arrays.asList takes either an array, or all elements of the array as argument.

So you need either

Arrays.asList(new String[]{"someString", "noString", "abcString"})

or

Arrays.asList("someString", "noString", "abcString")

Answer 2

the signature of asList() has the answer : public static <T> List<T> asList(T... a)

it takes a varargs , so obj_lStr.addAll(Arrays.asList("someString", "noString", "abcString")); compiles bcoz your arguments will be turned into an array.

but here obj_lStr.addAll(Arrays.asList( { "someString", "noString", "abcString" })); , you need to specify the type of the array. so you can do this : obj_lStr.addAll(Arrays.asList(new String[] { "someString", "noString", "abcString" })); .

the {} initialization syntax for arrays is available only during initilization , for ex:

int[] a ={1,2,4}; // works

but

int[] a new int[3];
a = {1,2,4}; // doesn't work

Answer 3

It's just to do with where in Java code you're allowed to omit the type and allow the array to be inferred -- specifically, you can only use array = {foo, bar...} when you're assigning directly into an array of the appropriate type, not where you're passing it around elsewhere.

In any event, what you should be writing is

Arrays.asList("someString", "noString", "abcString")

with no {} braces at all.

Answer 4

you can either feed an String Array to Arrays.asList :

Arrays.asList(new String[] {"a","b","c"});

or with multiple Strings (varags)

Arrays.asList("a","b","c");

But just

Arrays.asList({"a","b","c"});

isnt working, as {"a","b","c"} does not represent an object an Java

Answer 5

If you try looking at the javadocs , you'll learn that asList has the signature

public static <T> List<T> asList(T... a)

Now, the argument declaration T... a , uses a feature of java called variable arguments , which basically just means that the argument a , can either be an array of type T, or, it can be a sequence of objects all of type T.

Hope that helps.