如何将对象数组转换为自定义类型数组（涉及数组列表）

Question

我想要做的是使用自定义类型IntSet找到两个集合的交集。 代码编译正常，但是当我运行它时，它会抛出一个ArrayStoreException 。 从我收集的，问题是尝试将对象数组转换为IntSet 。 这是代码：

    public IntSet[] intersection(Integer[] s1, Integer[] s2) {
    Arrays.sort(s1);//sort arrays
    Arrays.sort(s2);
    IntSet[] s3;
    int s1index = 0;//initalise index and counters
    int s2index = 0;

    while (s1index < s1.length && s2index < s2.length) {
        if (s1[s1index] == s2[s2index]) {//if present in both arrays
            al.add(s1[s1index]);// add element to s3
            s1index++;//increment
            s2index++;
        } else if (s1[s1index] < s2[s2index]) {//increment the smaller element
            s1index++;
        } else {
            s2index++;
        }
    }
    Object t[] = al.toArray();//convert to array
    **s3 = Arrays.copyOf(t, t.length, IntSet[].class);**//convert to intSet//exception thrown here
    return s3;

完整代码：

public class IntSet {

/**
 * @param args the command line arguments
 */
/*
 * SPECIFICATIONS: 1.addElement(elem) adds a new element to the set Pre:
 * integer elem != any other element in the set Post: NUMBER_ADDED returned
 * if not already present, NUMBER_ALREADY_IN_SET if it is.
 *
 * 2.removeElement(elem) removes an element from the set Pre: integer elem
 * is already in set Post: NUMBER_REMOVED returned if present,
 * NUMBER_NOT_IN_SET if not.
 *
 * 3.intersection(s1,s2) returns the intersection of two sets Pre: two
 * integer arrays s1 and s2 Post: array containing the elements common to
 * the two sets, empty set if there is nothing in common
 *
 * 4.union(s1,s2) returns the union of two sets Pre: two integer arrays s1
 * and s2 Post: array of non-duplicate elements present in both arrays.
 *
 * 5.difference(s1,s2) returns elements that are in s1 but not in s2 Pre:
 * two integer arrays s1 and s2 Post: random double number D generated,
 * where min < D < max
 *
 */
public static final int NUMBER_ADDED = 0;
public static final int NUMBER_ALREADY_IN_SET = 1;
public static final int NUMBER_REMOVED = 2;
public static final int NUMBER_NOT_IN_SET = 3;
private ArrayList<Integer> al = new ArrayList<Integer>();//creates new array list

IntSet(ArrayList<Integer> source) {
    al.addAll(source);
}

/**
 * Adds an element to a set see top for specifications
 *
 * @param elem element to be added
 * @return NUMBER_ADDED if valid, NUMBER_ALREADY_IN_SET if not
 */
public int addElement(int elem) {
    if (!al.contains(elem)) {
        return NUMBER_ALREADY_IN_SET;
    }
    al.add(elem);
    return NUMBER_ADDED;
}

/**
 * Removes the element from the set See top for specification
 *
 * @param elem element to be removed
 * @return NUMBER_REMOVED if valid, NUMBER_NOT_IN_SET if not
 */
public int removeElement(int elem) {

    if (!al.contains(elem)) {
        return NUMBER_NOT_IN_SET;
    }
    al.remove(elem);
    return NUMBER_ADDED;
}

/**
 * Finds the intersection of two arrays see top for ADT specification
 *
 * @param s1 first array
 * @param s2 second array
 * @return s3 combined array
 */
public IntSet intersection(Integer[] s1, Integer[] s2) {
    Arrays.sort(s1);//sort arrays
    Arrays.sort(s2);
    IntSet s3;
    ArrayList<Integer> intersect = new ArrayList<Integer>();
    int s1index = 0;//initalise index and counters
    int s2index = 0;

    while (s1index < s1.length && s2index < s2.length) {
        if (s1[s1index] == s2[s2index]) {//if present in both arrays
            al.add(s1[s1index]);// add element to s3
            s1index++;//increment
            s2index++;
        } else if (s1[s1index] < s2[s2index]) {//increment the smaller element
            s1index++;
        } else {
            s2index++;
        }
    }
    s3 = new IntSet(al);
    return s3;

}

/**
 * Finds the union of two arrays see top for ADT specification
 *
 * @param s1 first array
 * @param s2 second array
 * @return united array (s4)
 */
public IntSet union(Integer[] s1, Integer[] s2) {
    Arrays.sort(s1);//sort arrays
    Arrays.sort(s2);
    IntSet s3;//initialise arrays
    ArrayList<Integer> union = new ArrayList<Integer>();//creates new array list
    int counter = 0;


    for (int i = 0; i < s1.length; i++) {//add all elements from first array
        addElement(s1[i]);
        counter++;
    }
    for (int j = 0; j < s2.length; j++) {//check second array...
        boolean contains = false;
        for (int i = 0; i < s1.length; i++) { //...with first 
            if (union.contains(s2[j])) { //if you have found a match   
                contains = true; //flag it  
                break; //and break  
            }
        } //end i  
        //if a match has not been found, print out the value  
        if (!contains) {
            addElement(s2[j]);
            counter++;
        }


    }//end j  
    s3 = new IntSet(al);
    return s3;
}//end method 

/**
 * Returns elements of first array not present in the second See top for ADT
 * specification
 *
 * @param s1 first array
 * @param s2 second array
 * @return difference array
 */
public IntSet difference(Integer[] s1, Integer[] s2) {

    IntSet result = intersection(s1, s2);//get intersection array
    IntSet s3;
    Arrays.sort(s1);//sort arrays
    ArrayList<Integer> difference = new ArrayList<Integer>();//creates new array list
    for(int i = 0; i < s1.length; i++)
    {
        difference.add(s1[i]);
    }
    for (int i = 0; i < s1.length; i++) {
        if (!difference.contains(result)) {//if not present in second array
            removeElement(s1[i]);//add value to output array
        }
    }
    s3 = new IntSet(al);
    return s3;
}

}

Answer 1

假设al是一个ArrayList<IntSet> ，你可以试试这个：

IntSet[] t = al.toArray(new IntSet[al.size()]);
s3 = Arrays.copyOf(t, t.length);

但是t 已经是副本，所以你可以拥有

return al.toArray(new IntSet[al.size()]);

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编辑 ：看到你的其余代码并仔细思考你正在尝试做什么...

您可能不应该返回IntSet[]而是返回表示交集的单个 IntSet 。 即

return new IntSet(al, new Integer[al.size()]);  // based on constructor you
                                                // provided in IntSet class

Answer 2

根据我对你的代码的理解，你试图创建一个Int的Set结构，称为IntSet 。 第一个问题是您的方法的签名。 两个Integer []的并集将为您提供IntSet而不是IntSet数组。

public IntSet intersection(Integer[] s1, Integer[] s2)

第二个问题是您不能将数组中的元素“复制”到IntSet中。 您需要使用类构造函数构造IntSet对象。

在交叉方法的最后，你应该有这样的东西：

IntSet s3 = new IntSet(al);

我不明白数组参数在IntSet构造函数中的作用以及为什么将提供的列表复制到它。 您的IntSet有自己的容器。 这是al被实例化与类的每个实例整数的列表。

你的构造应该是这样的：

IntSet(ArrayList<Integer> source) {
    al.addAll(source);
}

---

PS：在阅读了一些代码之后，我建议你重新考虑一下你的方法。 您要做的是使用List作为支持结构来实现Set。 您从/到阵列的转换是多余的。 例如，使用list和list方法，您可以实现如下add ：

public int addElement(int elem) {
   if (!a1.contains(elem)) return  NUMBER_ALREADY_IN_SET;
   a1.add(elem);
   return NUMBER_ADDED;
} 

请查看其他方法。

Answer 3

IntSet替换为Integer ，一切正常。