在java中使用错误startsWith

Question

defined variable:

 LinkedList list1=new LinkedList();

Object get() in list1 obtains a node of list1

Object remove() in list1 deletes a node of list1

count() is length of list1

for(int i=1;i<list1.count();i++){
  if(list1.get(i).startsWith('"',0)) //Error here
    list1.remove(i);
}

Error: cannot find symbol

symbol: method charAt(int)
location: class Object

how to fix this problem? I would like to delete the node in list1 which starts with (").

Answer 1

1. startsWith is a method in the String class; you are using the raw LinkedList type, and thus it is treated like a LinkedList<Object> . If you wish to use Strings, make it a LinkedList<String> .
2. startsWith accepts only String arguments, not char arguments. Use startsWith("\\"") instead.
3. The second parameter you've supplied to startsWith is superfluous; providing no second parameter will assume starting position as 0.
4. You have an extraneous semicolon after your if statement. This will cause the if body to be treated as empty. Definitely remove this semicolon and optionally use curly braces.

---

Your modified solution may look something like this:

LinkedList<String> list1 = new LinkedList<String>();

// [...] Populate the list accordingly here

for(int i=1; i < list1.count(); i++){
    if (list1.get(i).startsWith("\"")) {
        list1.remove(i);
    }
}

---

Additional notes:

1. Your for loop starts at index 1. Note that this will not remove over the first element. I'm sure if this is your desired behavior.
2. As you remove elements in the list, the indices of the latter elements in the list will change.

For example:

[ "a", "b", "c", "d" ]
   ^
(remove element at index 0)

[ "b", "c", "d" ]
        ^
(remove element at index 1... uh oh, we missed "b"!)

[ "b", "d" ]
              ^
(remove element at index 2... ERROR; index out of bounds)

Answer 2

If it's a list of strings, you need to tell the compiler so. Java does not duck-type, you have to tell the compiler explicitly that the Object is a String.

List<String> list1=new LinkedList<String>();

When it's declared that way, get(i) will return a reference of type String and you can use its methods.

Answer 3

1. If you don't specify the list type, Java will treat is as a list of Object s, and there is no Object.charAt() method. Add the <String> type:

    LinkedList<String> list1 = new LinkedList<String>();

   Or, absent that, add a cast to the get call:

    if (((String) list1.get(i)).startsWith("\\"", 0))

2. The String.startsWith() method takes a String argument, not a char.

    if (list1.get(i).startsWith("\\""))

3. List indices are 0-based.

    for (int i = 0; i < list1.count(); i++) ^

4. You shouldn't use get(int) with a LinkedList. This method is for random access . Linked lists can be traversed in order quickly, but random access is slow. You should either iterate over the list sequentially, or use an ArrayList which does support fast random access.

    for (String str: list1) { } // or List<String> list1 = new ArrayList<String>();

5. Removing elements from a list while you're iterating over it is tricky. The best way to do it is to use an iterator.

    Iterator<String> it = list1.iterator(); while (it.hasNext()) { String str = it.next(); if (str.startsWith("\\"")) { it.remove(); } }

Answer 4

you are trying to get startWith on Object , typecast as String.

if(((String)list1.get(i)).startsWith('"',0))

Answer 5

if(list1.get(i).startsWith("\\"",0))

Answer 6

List<String> itemsToBeRemoved = new ArrayList<String>();

for(int i=1;i<list1.count();i++){
    String str = list1.get(i);
    if(str.startsWith("\""))
    itemsToBeRemoved.add(str);
}
list1.removeAll(itemsToBeRemoved);

Answer 7

You should specify the object types that your LinkedList will hold or perform a cast when trying to call the charAt method. Something like:

LinkedList<String> list1=new LinkedList<String>();

Or casting to String

if (((String) list1.get(i)).startsWith("\""))