为什么在所有if-else分支都具有return语句的情况下，方法中的if-else if-else（在循环内）给出“ return语句丢失”的信息？

Question

Why compiler gives me "return statement missing" erorr?

All possible conditional branches (if-else) contain a return statement.

But if I have the same if-else-if-else construct alone (not inside for() loop) - it compiles fine, without producing this "return statement missing" erorr?

public Resume get(String uuid) {

        // compiler shall know that array is not empty!
        final Resume[] storage = new Resume[100]; // same for static final

        // is executed at least once - see above!
        for(Resume resume : storage) {
            if (resume == null) {
                return null;                
            } else if (resume.uuid.equals(uuid)){
                return resume;
            }  else {
                return null;
            }            
        }
        // return statement is missing !!!      
    }

I could understand the compiler - it is afraid that the loop would never execute if the array named storage is empty. Why compiler ignores the size of the array? But it is just a half of problem with advanced for loop.

Let's take a regular for loop, totally independant of storage array size.

public Resume get(String uuid) {
        Resume resume = new Resume();

        // being a static final field produces same compile error...
        final int LIMIT = 100;

        // LIMIT is known to compiler, since it is final primitive.
        // for loop execution not dependent on storage array being empty
        for (int i = 0; i < LIMIT; i++) {       
            if (resume == null) {
                return null;                
            } else if (resume.uuid.equals(uuid)){
                return resume;
            }  else {
                return null;
            }            
        }
        // return null;   //statement is missing !!!; //       
    }

What explains this second for loop problem?

PS I think it is because things like "i < LIMIT" (even LIMIT being final) do not evaluate to true during compilation, true appears only at runtime , so at compile-time compiler is afraid just-in-case ;)

Answer 1

Even though you assign a non empty array to the storage variable, the compiler doesn't take that into account when considering whether all execution paths have a return statement.

As far as the compiler is concerned, it's possible that the for loop will have 0 iterations, and therefore there must be a return statement after the loop.

In fact, even if you use only constants in the loop's condition, the compiler will still require a return statement after the loop. The following method won't pass compilation:

public static int test() 
{
    for (int i = 0; i < 5; i++) {
        return 3;
    }
}

Answer 2

What if your storage is empty, you will never go into loop, thus you wouldn't reach any of your return statements.

At compile time you couldn't guarantee that your collections would never be empty, thus compiler need a return statement for this branch of execution.

Answer 3

如果storage为空，则不会发生退货。

Answer 4

所有可能的条件分支都在for循环中，如果您的存储数组为空而不是return语句，该怎么办呢，这就是编译器给您此错误的原因

Answer 5

Change your code like this because if storage is empty, it pass the for :

public Resume get(String uuid) {

        // compiler shall know that array is not empty!
        final Resume[] storage = new Resume[100]; // same for static final

        // is executed at least once - see above!
        for(Resume resume : storage) {
            if (resume == null) {
                return null;                
            } else if (resume.uuid.equals(uuid)){
                return resume;
            }  else {
                return null;
            }            
        }
        return null;      
    }