What will be the output of the following program? import java.util.List; import java.util.LinkedList; public class Q13 { public static void main(String[] args) { List list = new LinkedList(); System.out.println(list.size()); } }

Question

BlackTom AI · Accepted Answer

A List can reference a LinkedList, and calling size() on a newly constructed LinkedList returns 0. 1. Relevant knowledge points and concepts - Java generics and wildcards: List accepts any List whose element type is a subtype of Number (covariant read-only view). - LinkedList() constructs an empty linked list of Integer. - size() returns the number of elements stored in the list. - Compile-time rules: you cannot add elements through a reference typed List (except null), but reading methods like size() are allowed. - No runtime exception is thrown by these constructs unless an explicit null or other runtime fault occurs. 2. Step-by-step reasoning Step 1: Evaluate the right-hand side: new LinkedList() creates a new empty LinkedList instance whose element type is Integer. At creation its internal element count is 0. Step 2: Type compatibility: Integer extends Number, so List list = new LinkedList() is a legal assignment because the wildcard ? extends Number matches Integer. Step 3: The code calls list.size(). The size() method is defined in the List interface and returns the current element count of the underlying LinkedList object. Step 4: Since the list was just constructed and no elements were added, size() returns 0. Step 5: System.out.println prints the integer value 0. 3. Intermediate values / calculations - new LinkedList() -> createdList with elementCount = 0 - list.size() -> returns 0 4. Why the correct answer is correct Because the list reference points to an empty LinkedList, and size() reports the number of elements (zero). The wildcard assignment is valid, so the program compiles and runs, producing the numeric output 0. 5. Why the other options are incorrect - "null": The variable references an actual LinkedList object, not null. - "Compile error": The generic assignment is legal; calling size() is allowed on List. - "Runtime error": No runtime exception occurs; the code runs normally. 6. Conclusion The program prints 0, which matches the provided correct answer "0".

What will be the output of the following program? import java.util.List; import java.util.LinkedList; public class Q13 { public static void main(String[] args) { List<? extends Number> list = new LinkedList<Integer>(); System.out.println(list.size()); } }单项选择题

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