arraylist

问题 I have a bunch of indexes and I want to remove elements at these indexes from an ArrayList . I can't do a simple sequence of remove() s because the elements are shifted after each removal. How do I solve this? 回答1: Sort the indices in descending order and then remove them one by one. If you do that, there's no way a remove will affect any indices that you later want to remove. How you sort them will depend on the collection you are using to store the indices. If it's a list, you can do this:

问题 String[] array = {"a","c","b"}; ArrayList<String> list = new ArrayList<String>(); list.add("a"); list.add("b"); list.add("c"); System.out.println(array); System.out.println(list); For list [a, b, c] is output while for array some address is output. When we want to output the array values, we can use Arrays.toString(array); which works just like list . I just wonder why we can't call toString() directly on array to get the values. Isn't it more intuitive and convenient to do so? What results

问题 int[] alist = new int [3]; alist.add("apple"); alist.add("banana"); alist.add("orange"); Say that I want to use the second item in the ArrayList. What is the coding in order to get the following output? output: banana 回答1: You have ArrayList all wrong, You can't have an integer array and assign a string value. You cannot do a add() method in an array Rather do this: List<String> alist = new ArrayList<String>(); alist.add("apple"); alist.add("banana"); alist.add("orange"); String value = alist

问题 I have a group of classes that all implement a validation interface which has the method isValid() . I want to put a group of objects--all of different classes--into an ArrayList, loop through them and call isValid() on each. Here's my code Email email = new email(); Address address = new Address(); ArrayList<? extends Validation> myValidationObjects = new ArrayList(); But when I try to do: myValidationObjects.add(email); I get: The method add(capture#2-of ? extends Validation) in the type

问题 Does anyone have any idea, why removeRange method in AbstractList (and also in ArrayList) is protected ? It looks like a quite well-defined and useful operation, but still, to use it, we're forced to subclass the List implementation. Is there some hidden rationale? Seems quite inexplicable to me. 回答1: Yes, because that's not how you remove a range from outside code. Instead, do this: list.subList(start, end).clear(); This actually calls removeRange behind the scenes. † The OP asks why

问题 I'm confused about why I keep getting a java.lang.StackOverFlow error when I try to write a constructor for MyArrayList, a personal version of the arraylist class in java. I know StackOverFlow errors happen when there is a recursive call, but that doesn't appear to be the case for the code I have written? Can anybody help explain why I'm getting this error? This is my code, I've included all of the constructors I have written, but the first MyArrayList() is the one that the error indicates in

问题 Say I have an ArrayList: <string1.4> <string2.4> <string3.4> and I wish to return the the first element of the arrayList when I say arrayList.containsSubString('string1'); How could this be done other than iterating through each of the elements of the arrayList and checking if string1 is a substring of that element string? 回答1: The only way I can think of is doing something like: strs.get(strs.indexOf(new Object() { @Override public boolean equals(Object obj) { return obj.toString().contains

问题 I have this arraylist: // Add predators predators = new ArrayList(); for (int i = 0; i < predNum; i++) { Creature predator = new Creature(random(width), random(height), 2); predators.add(predator); } How can the statement be structured so that the last element from the predators arraylist is removed every 500 frames? Does it need a loop of some sort? if (frameCount == 500){ predators.remove(1) } 回答1: If you already have a variable that keeps track of what frame you are on, you can use this if

问题 "Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0" is the main error I get when I compile this method: public static ArrayList<ArrayList<Integer>> createSparseArray(int len, double den) { int counter = 0; ArrayList<Integer> placeHolder = new ArrayList<Integer>(); for (int j = 0; j < len; j++) { double randomNumber = Math.random(); if (randomNumber < den) { counter++; placeHolder.add(j); } } ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>

问题 I'm trying to create a set of arrays of ints, the thing is that if I try to do: HashSet<int[]> s = new HashSet<int[]>(); int a1[] = {1,2,3}; int a2[] = {1,2,3}; s.add(a1); s.add(a2) System.out.println(s.size()); Then s has two objects, but there should be only one. Note: it doesn't matter if it is HashSet< Integer[]>. It just doesn't work. Now If I try to do this with an ArrayList< Integer>, something like: HashSet<ArrayList<Integer>> s = new HashSet<ArrayList<Integer>>(); ArrayList<Integer>