This question already has answers here: What is PECS (Producer Extends Consumer Super)? (16 answers) Closed 3 years ago. What is the difference between List<? super T> and List<? extends T> ? I used to use List<? extends T>, but it does not allow me to add elements to it list.add(e), whereas the List<? super … Read more
How might I convert an ArrayList<String> object to a String[] array in Java? 1 17
How would you initialise a static Map in Java? Method one: static initialiser Method two: instance initialiser (anonymous subclass) or some other method? What are the pros and cons of each? Here is an example illustrating the two methods: import java.util.HashMap; import java.util.Map; public class Test { private static final Map<Integer, String> myMap = new … Read more
We all know you can’t do the following because of ConcurrentModificationException: for (Object i : l) { if (condition(i)) { l.remove(i); } } But this apparently works sometimes, but not always. Here’s some specific code: public static void main(String[] args) { Collection<Integer> l = new ArrayList<>(); for (int i = 0; i < 10; ++i) … Read more
Is there some way of initializing a Java HashMap like this?: Map<String,String> test = new HashMap<String, String>{“test”:”test”,”test”:”test”}; What would be the correct syntax? I have not found anything regarding this. Is this possible? I am looking for the shortest/fastest way to put some “final/static” values in a map that never change and are known in … Read more
I am relatively new to Java, and often find that I need to sort a Map<Key, Value> on the values. Since the values are not unique, I find myself converting the keySet into an array, and sorting that array through array sort with a custom comparator that sorts on the value associated with the key. … Read more
I wanted to create a list of options for testing purposes. At first, I did this: ArrayList<String> places = new ArrayList<String>(); places.add(“Buenos Aires”); places.add(“Córdoba”); places.add(“La Plata”); Then, I refactored the code as follows: ArrayList<String> places = new ArrayList<String>( Arrays.asList(“Buenos Aires”, “Córdoba”, “La Plata”)); Is there a better way to do this? 3 33
I’ve always been one to simply use: List<String> names = new ArrayList<>(); I use the interface as the type name for portability, so that when I ask questions such as this, I can rework my code. When should LinkedList be used over ArrayList and vice-versa? 3 33
If I have an object implementing the Map interface in Java and I wish to iterate over every pair contained within it, what is the most efficient way of going through the map? Will the ordering of elements depend on the specific map implementation that I have for the interface? 4 46
What are the differences between a HashMap and a Hashtable in Java? Which is more efficient for non-threaded applications? 3 35
