CODEWITHSUNDEEP
javatype-conversion
leigero
leigero

Reputation: 3283

Two contradictory errors about Type mismatch in Java?

What on earth could be causing this? I'm getting two contradicting errors on one method I'm trying to create in a java program

public void numberOfTrianglesIncidentToVertex(){
  for(List<Integer> pairs: neighbors.get(2)){     // Type mismatch: cannot convert from element type Integer to List<Integer>

  }
    int fail = neighbors.get(2);       // Type mismatch: cannot convert from List<Integer> to int
}

The neighbors variable is declared in a super class as follows:

List<List<Integer>> neighbors 
= new ArrayList<List<Integer>>();

I don't know why it would tell me on one line that its an Integer and can't be converted to a List of integers and then on the very next line just change its mind and say the exact opposite. which is it?

Upvotes: 1

Views: 147

Answers (2)

Tala
Tala

Reputation: 8928

 neighbors.get(2)

returns you List<Integer>. Second warning is clear about that.

Type mismatch: cannot convert from element type Integer to List

To iterate over this list you'll need to iterate over i - integer.

 for(Integer pairs: neighbors.get(2))

Upvotes: 1

Rohit Jain
Rohit Jain

Reputation: 213223

Given your declaration of neighbors, the following invocation:

neighbors.get(2);

will give you a List<Integer>.

Now in 1st snippet, you are trying to iterate over the return value. So, when you iterate over List<Integer>, you get back values of type Integer. And you are using List<Integer> type loop variable. Hence that error message. You can't assign an Integer reference to a List<Integer> reference.

You should change your loop to:

for(int val: neighbors.get(2)) {  // 'int' works in place of 'Integer', due to unboxing
}

However, if you iterate on neighbors, your loop will work fine, because then you will get List<Integer> reference on iterating.

for(List<Integer> val: neighbors) { 
}

In 2nd snippet, you are directly assigning the fetched value - List<Integer> to an int primitive. Which obviously you can't do. They aren't at all compatible. Hence the error message. The assignment should be like:

List<Integer> list = neighbors.get(2);

Upvotes: 5

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