Question

我无法弄清楚迭代器如何用于替换元素并计算被替换的元素数量。这是我到目前为止的代码：

public class example {
    public static void main (String args[]){
        //create an ArrayList
        ArrayList<String> list = new ArrayList<String>();


        //add elements to the array list
        list.add("A");
        list.add("B");
        list.add("C");
        list.add("D");
        list.add("B");
        list.add("B");

        //use iterator to display original list
        Iterator iter = list.iterator();
        while (iter.hasNext()){
            Object element = iter.next();
            System.out.println (element + " ");
            }

        // call replace         
        String b = "B";
        String x = "X";
        ArrayList<String> myList = new ArrayList<String>();
        replace (b, x, myList);
    }

    public static <E> int replace(E match, E replacement, ArrayList<E> myList) {

        //throw exceptions if null
            if (match == null){
                throw new IllegalArgumentException ("match cannot be null");
            }
            if (replacement == null){
                throw new IllegalArgumentException ("replacement cannot be null");
            }
            if (myList == null){
                throw new IllegalArgumentException ("myList cannot be null");
            }
            //return 0 if myList is empty
            boolean emptylist = myList.isEmpty();
            if (emptylist = true){
                System.out.println("0");
            }


    }

我已经使用迭代器打印出列表中的元素，但现在我必须使用迭代器来替换并返回替换次数。在这种情况下，我想用“X”代替“B”并计算“X”的数量。我假设我想把迭代器放在泛型方法中，但我真的不知道要进入哪个方向......

Answer 1

你遍历列表，当你找到匹配的元素时，你set替换它：

public static <E> int replace(E match, E replacement, ArrayList<E> myList) {

    //throw exceptions if null
    if (match == null){
        throw new IllegalArgumentException ("match cannot be null");
    }
    if (replacement == null){
        throw new IllegalArgumentException ("replacement cannot be null");
    }
    if (myList == null){
        throw new IllegalArgumentException ("myList cannot be null");
    }

    int counter = 0;
    ListIterator<E> iter = myList.listIterator();
    while (iter.hasNext()) {
        E val = iter.next();
        if (val.equals(match)) {
            iter.set(replacement);
            ++counter;
        }
    }
    return counter;
}

Answer 2

尝试使用此处记录的ListIterator课程：http://docs.oracle.com/javase/6/docs/api/java/util/ListIterator.html

set方法的文档描述了如何修改列表元素。

Answer 3

你不需要迭代器来替换元素，当你在它上面循环时添加或删除元素时需要它。你可以用简单的for循环进行替换。

public class example {
  public static void main (String[] args) throws java.lang.Exception
    {
         ArrayList<String> list = new ArrayList<String>();

        //add elements to the array list
        list.add("A");
        list.add("B");
        list.add("C");
        list.add("D");
        list.add("B");
        list.add("B");

        String b = "B";
        String x = "X";

       System.out.print(replace (b, x, list));            

    }

    public static <E> int replace(E match, E replacement, ArrayList<E> myList) {
        //throw exceptions if null
            if (match == null || replacement == null || myList == null ){
                throw new IllegalArgumentException ("match cannot be null");
            }

            int i=0;
            int replaceCount=0;            
            for(E str : myList){
                if(str.equals(match)){
                    myList.set(i,replacement);
                    replaceCount++;
                }
                i++;
            }
        return replaceCount;
        }
}

如何使用迭代器来替换和计数

3 个答案: