Compile error while converting array to Set in java

Question

I was trying to convert int array to Set<Integer>.

int[] arr = {5, 2, 7, 2, 4, 7, 8, 2, 3};
Set<Integer> s = new HashSet<Integer>(Arrays.asList(arr));

But the compiler doesn't accept the above code. It says: "The constructor HashSet(List) is undefined." Well, I thought that int should be autoboxed.

I slightly modified the code, change int to String:

String[] arr = {"hello", "world"};
Set<String> s = new HashSet<String>(Arrays.asList(arr));

This code works okay.

I tried the following, change int to Integer:

Integer[] arr = {5, 2, 7, 2, 4, 7, 8, 2, 3};
Set<Integer> s = new HashSet<Integer>(Arrays.asList(arr));

This is passed the compilation.

My question is: why doesn't java compiler accept the first code?

This might help you. stackoverflow.com/questions/25890806/… — ares
– ares, Commented Sep 24, 2014 at 10:05

Ruchira Gayan Ranaweera · Accepted Answer · 2014-09-24 09:50:14Z

3

int[] arr = {5, 2, 7, 2, 4, 7, 8, 2, 3};
Set<Integer> s = new HashSet<Integer>(Arrays.asList(arr));

Because your Set is type Integer(Object) and array is type int(primitive).

Read Arrays.asList()

Arrays.asList() // takes in put as Generics

Generics in Java is not applicable to primitive types as in int. That's why you are getting this error.

edited Sep 24, 2014 at 9:50

answered Sep 24, 2014 at 9:39

Ruchira Gayan Ranaweera

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3 Comments

Suresh Atta Over a year ago

Arrays.asList(arr) returns a list of Integer's. Collections wont support primitives.

Ruchira Gayan Ranaweera Over a year ago

@sᴜʀᴇsʜᴀᴛᴛᴀ Issue is not with return type, issue in input type in asList()

Ruchira Gayan Ranaweera Over a year ago

@TheLostMind Issue is not with return type, issue in input type in asList()

Chris K · Accepted Answer · 2014-09-24 09:52:13Z

Your problem is the call to Arrays.asList(arr). It has gotten confused by the use of a primitive array. Java treats primitives and objects differently.

asList only knows about arrays of objects, and in your case it is treating the entire array as a single element. That is, asList(arr) is returning Set<int[]>, such that the following is true:

Set<int[]> s = new HashSet<Integer>(ints);

Which is not what you meant.

There is no auto boxing of primitive arrays in Java. The quickest fix is to use Integer rather than int for the input array:

Integer[] arr = {5, 2, 7, 2, 4, 7, 8, 2, 3};   // java supports autoboxing when declaring an array 
Set<Integer> s = new HashSet<Integer>(Arrays.asList(arr));

otherwise you will have to iterate and add the elements yourself.

int[] arr = {5, 2, 7, 2, 4, 7, 8, 2, 3};
Set<Integer> s = new HashSet<Integer>();        
for ( int v : arr ) {
    s.add(v);  // autoboxing of a single int is supported by Java, and happens here
}

Mangesh Mandavgane · Accepted Answer · 2014-09-24 12:44:39Z

1

In java Containers want "Objects" and primitives don't derive from Object. Generics allow you to pretend there is no wrapper, but you still pay the performance price of boxing. This is IMPORTANT to many programmers.

List list; That is also a reason why there are wrapper classes for primitive types:

int -> Integer

boolean -> Boolean

double -> Double

byte -> Byte etc...

answered Sep 24, 2014 at 12:44

Mangesh Mandavgane

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