计数为零时如何不返回任何内容？

Question

I want the function countChar to only return a value if the count is greater than zero, otherwise i want it to just return nothing.

I understand that the method cannot return null as null can't be an Int. Is there another way to effectively return a "null" value?

public static int countChar(String str, char ch){ 
int count = 0;
for (int i=0;i<str.length();i++) {
  if (str.charAt(i)==ch)
    count++;
  }
  if (count>0) {
    return count;
  }else{
    return null;
  }
}

error: incompatible types: cannot be converted to int return null; ^ I would like it to return absolutely nothing instead of zero.

Answer 1

Use Integer instead of int , everything else remains unchanged:

public static Integer countChar(String str, char ch){...} 

Answer 2

But then the method naming wouldn't make sense- countChars counts the number of times a char occurs in a string. If a particular char doesn't occur then 0 is a perfectly reasonable response. Null isn't really a reasonable response - if you want null then have the method that calls countChars get a 0 back and proceed as if it were null:

int result = countChars("Hello world", 'z');
if(result == 0)
  someMethod(null);
else
  someMethod(result.ToString());

Even throwing an exception would be better than returning null because it at least tells the developer who is using your code, what is wrong and gives some clue on how to fix it. Asking code to do something that shouldn't really return a blank response, and getting a blank response can be incredibly frustrating to work with

Answer 3

You can change it in two ways.

1. By changing the comparing logic as given. It won't return anything if it is zero.

if (count > 0) {
    return count;
  }
~~~~~~~~~~~~~~~~~~~~~~~~~~~

OR

2. You can return null but change the return type of method from primitive to nonprimitive.

~~~~~~~~~~~~~~~~~~~~
public static Integer countChar(String str, char ch){

---
---
if (count>0) {
    return count;
  }

    return null; 
} 
~~~~~~~~~~~~~~~~~~~