为什么我的方法覆盖会产生类型冲突？

Question

I need help to understand how the testing should go through. I should implement the test method in Stacktest.java (have already done that). Then extend StackTest with LinkedlistTest (I have already done that). Then add super.setUp() as the first line in LinkedListTest.setUp (I have done that).

But the one overriden method is getIntegerStack that I implement in the LinkedListTest , but then I get an error:

'getIntegerStack()' in 'LinkedListTest' clashes with 'getIntegerStack()' in 'StackTest'; attempting to use incompatible return type

I dont know how to fix it. The code:

• StackTest.java is an abstract test class.
• For each implementation of Stack , one may simply extend StackTest with an implementing test class. The only method that should be overridden is StackTest.getIntegerStack , which should simply return an instance of a class that implements Stack<Integer> . See the setUp method in StackTest and try to understand how this works.
• In your case, you should extend StackTest with a test class called LinkedListTest .
• you must add a call to super.setUp() ; as the first line in LinkedList.setUp() .

TODO: How should I make the getIntegerStack() do work.

import org.junit.Test;
import org.junit.Before;
import static org.junit.Assert.*;

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.CoreMatchers.*;
import java.util.Arrays;
import java.util.NoSuchElementException;
import java.util.Stack;

/**
 * Test class for LinkedList
 */
public class LinkedListTest extends StackTest{

    /* A sequence of integers */
    private int[] elements;

    /* An empty linked list */
    private LinkedList<Integer> list;

    @Before
    public void setUp() {
        super.setUp();
        list = new LinkedList<Integer>();
        elements = new int[]{-919,  388,   67, -248, -309, -725,  904,   53,
                90, -469, -559,  256,  612,  366, -412, -221,
                347, -921, -978,  324, -858,  480, -443,  891,
                329,   -5,  878, -538,  445, -366,  760,   52};
    }

    /**
     * This is the only method that implementing classes need to override.
     *
     * @return An instance of Stack.
     */
    @Override
    protected Stack<Integer> getIntegerStack() {
        return null;
    }
}

Stack interface:

  * A interface of stack
  */
 public interface Stack <T> {
     /**
      * Adds the element to the top of the stack.
      */
     void push (T elem);
    
      T pop();

      T top();

     int size();

     boolean isEmpty();
 }

import java.util.EmptyStackException;
import java.util.NoSuchElementException;
/**
 * A singly linked list.
 *
 */
public class LinkedList<T> implements Stack <T> {
    private ListElement<T> first;   // First element in list.
    private ListElement<T> last;    // Last element in list.
    private int size;  // Number of elements in list.

    /**
     * A list element.
     */
    private static class ListElement<T>{
        public T data;
        public ListElement<T> next;

        public ListElement(T data) {
            this.data = data;
            this.next = null;
        }
    }

    /**
     * Creates an empty list.
     */
    public LinkedList() {
        this.first = null;
        this.last = null;
        this.size = 0;
    }

    /**
     * Inserts the given element at the beginning of this list.
     *
     * @param element An element to insert into the list.
     */
    public void addFirst(T element) {
        ListElement<T> firstElement = new ListElement<>(element);
        if (this.size == 0){
            this.first = firstElement;
            this.last = firstElement;
        }
        else{
            firstElement.next = this.first;
            this.first = firstElement;
        }
        this.size ++;
    }

    /**
     * Inserts the given element at the end of this list.
     *
     * @param element An element to insert into the list.
     */
    public void addLast(T element) {
        ListElement<T> lastElement = new ListElement<>(element);
        if(this.size ==0){
            this.first = lastElement;
        }
        else{
            this.last.next = lastElement;
        }
        this.last = lastElement;
        this.size ++;
    }

    /**
     * @return The head of the list.
     * @throws NoSuchElementException if the list is empty.
     */
    public T getFirst() {
        if (this.first != null){
            return this.first.data;
        }
        else{
            throw new NoSuchElementException();
        }
    }

    /**
     * @return The tail of the list.
     * @throws NoSuchElementException if the list is empty.
     */
    public T getLast() {
        if(this.last != null){
            return this.last.data;
        }
        else{
            throw new NoSuchElementException();
        }
    }

    /**
     * Returns an element from a specified index.
     *
     * @param index A list index.
     * @return The element at the specified index.
     * @throws IndexOutOfBoundsException if the index is out of bounds.
     */
    public T get(int index) {
        if(index < 0|| index >= this.size){
            throw new IndexOutOfBoundsException();
        }
        else{
            ListElement<T>element = this.first;
            for(int i = 0; i < index; i++){
                element = element.next;
            }
            return element.data;
        }
    }

    /**
     * Removes the first element from the list.
     *
     * @return The removed element.
     * @throws NoSuchElementException if the list is empty.
     */
    public T removeFirst() {
        if(this.first != null || this.size != 0){
            ListElement<T> list = this.first;
            this.first = first.next;
            size --;
            if(size() == 0){
                last = null;
            }
            return list.data;
        }
        else{
            throw new NoSuchElementException();
        }
    }

    /**
     * Removes all of the elements from the list.
     */
    public void clear() {
        this.first = null;
        this.last = null;
        this.size =0;
    }

    /**
     * Adds the element to the top of the stock.
     * @param elem
     */
    @Override
    public void push(T elem) {
        ListElement <T> list = new ListElement<>(elem);
        if( first == null){
            first = list;
            last = first;
        } else{
            list.next = first;
            first = list;
        }
        size ++;
    }

    /**
     * Removes and returns the top element in stack,
     * that is the element that was last added.
     * Throws an EmptyStackException if stack is empty.
     * @return the top element in the stack.
     */
    @Override
    public T pop(){
        if(isEmpty()){
            throw new EmptyStackException();
        }else{
            ListElement <T> list = first;
            first = first.next;
            size --;

            return list.data;
        }
    }

    /**
     * returns the top element in the stack without removing it.
     * Throws an EmptyStackException if stack is empty.
     * @return the top element.
     */
    @Override
    public T top() {
        if(isEmpty()){
            throw new EmptyStackException();
        }else{
            return first.data;
        }
    }

    /**
     * Returns the number of elements in the stock
     * @return The number of elements in the stock.
     */
    public int size() {

        return this.size;
    }

    /**
     * Note that by definition, the list is empty if both first and last
     * are null, regardless of what value the size field holds (it should
     * be 0, otherwise something is wrong).
     *
     * @return <code>true</code> if this list contains no elements.
     */
    public boolean isEmpty() {

        return first == null && last == null;
    }

    /**
     * Creates a string representation of this list. The string
     * representation consists of a list of the elements enclosed in
     * square brackets ("[]"). Adjacent elements are separated by the
     * characters ", " (comma and space). Elements are converted to
     * strings by the method toString() inherited from Object.
     *
     * Examples:
     *  "[1, 4, 2, 3, 44]"
     *  "[]"
     *
     * @return A string representing the list.
     */
    public String toString() {
        ListElement<T> listOfElements = this.first;
        String returnString = "[";
        while(listOfElements != null) {
            returnString += listOfElements.data;
            if(listOfElements.next != null){
                returnString += ", ";
            }
            listOfElements = listOfElements.next;
        }
        returnString += "]";
        return returnString;
    }
}

import org.junit.Test;
import org.junit.Before;
import org.junit.Rule;
import org.junit.rules.Timeout;
import static org.junit.Assert.*;
import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.CoreMatchers.*;
import java.lang.Integer;
import java.util.EmptyStackException;
import java.util.stream.IntStream;

/**
 * Abstract test class for Stack implementations.
 *
 * Implementing test classes must only implement the getIntegerStack
 * method. Be careful not to override ANY other methods!
 *
 */
public abstract class StackTest{
    @Rule public Timeout globalTimeout = Timeout.seconds(5);

    private Stack<Integer> stack;
    private int[] valuesInStack;
    private int initialStackSize;
    private Stack<Integer> emptyStack;

    @Before
    public void setUp() {
        valuesInStack = new int[] {3, 4, 1, -123, 4, 1};
        initialStackSize = valuesInStack.length;
        stack = getIntegerStack();
        pushArrayToStack(valuesInStack, stack);
        emptyStack = getIntegerStack();
    }

    /**
     * Push an array to the stack, in order.
     *
     * @param array An int array.
     * @param stack A Stack.
     */
    private void pushArrayToStack(int[] array, Stack<Integer> stack) {
        for (int i = 0; i < array.length; i++) {
            stack.push(array[i]);
        }
    }

    /**
     * This is the only method that implementing classes need to override.
     *
     * @return An instance of Stack.
     */
    protected abstract Stack<Integer> getIntegerStack();

    @Test
    public void topIsLastPushedValue() {
        // Arrange
        int value = 1338;

        // Act
        emptyStack.push(value);
        stack.push(value);

        int emptyStackTop = emptyStack.top();
        int stackTop = stack.top();

        // Assert
       assertThat(emptyStackTop, equalTo(value));
     assertThat(stackTop, equalTo(value));
    }

    // HELPERS

    /**
     * Pops the desired amount of elements.
     *
     * @param stack A Stack.
     * @param amountOfElements The amount of elements to pop.
     */
    private void popElements(Stack<Integer> stack, int amountOfElements) {
        for (int i = 0; i < amountOfElements; i++) {
            stack.pop();
        }
    }

    /**
     * Class used for stream operations when both actual and expected values
     * need to be gather in conjunction.
     */
    private class ResultPair<T> {
        public final T actual;
        public final T expected;

        public ResultPair(T actual, T expected) {
            this.actual = actual;
            this.expected = expected;
        }
    }
}

Answer 1

Your LinkedListTest class contains the erroneous line:

import java.util.Stack;

Because of that, the getIntegerStack() method returns a java.util.Stack object instead of an object of the Stack interface you created yourself.

Just dropping that import statement will fix your problem.

Unrelated to your problem, you can also let getIntegerStack() return any subclass of your Stack interface. In this case it would be more natural to let it return your LinkedList :

/**
 * In the context of `LinkedListTest` we return a `LinkedList`.
 *
 * @return An instance of Stack.
 */
@Override
protected LinkedList<Integer> getIntegerStack() {
    return new LinkedList<Integer>();
}