Declaring the ArrayList

Question about the task

Under discussion

I thought that when you declared an ArrayList that you had to say what kind of variable would be in it. That wasn't done here, e.g. ArrayList<Integer>, so can someone explain when you have to and when you don't have to declare the types in the ArrayList?? thanks

Using an ArrayList and LinkedList, perform 10,000 insertions, deletions, and get and set calls.

Requirements:

The program should not display text on the screen.
The program should not read values from the keyboard.
The Solution class must contain only 5 methods.
The insert10000(List list) method must insert 10,000 items into the list.
The get10000(List list) method should call get 10,000 times on the list.
The set10000(List list) method should call set 10,000 times on the list.
The remove10000(List list) method must remove 10,000 items from the list.

package com.codegym.task.task08.task0808; import java.util.ArrayList; import java.util.LinkedList; import java.util.List; public class Solution { public static void main(String[] args) throws Exception { ArrayList<Integer> arrayList = new ArrayList<Integer>(10000); insert10000(arrayList); get10000(arrayList); set10000(arrayList); remove10000(arrayList); // LinkedList LinkedList<Integer> linkedList = new LinkedList<Integer>(); insert10000(linkedList); get10000(linkedList); set10000(linkedList); remove10000(linkedList); } public static void insert10000(List list) { for (int i = 0; i < 10000; i++) list.add(i); } public static void get10000(List list) { for (int i = 0; i < 10000; i++) list.get(i); } public static void set10000(List list) { for (int i = 0; i < 10000; i++) list.set(i, 1 + 1); } public static void remove10000(List list) { for (int i = 0; i < 9999; i++) list.remove(i); } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

12 August 2020, 18:40

When you neglect to specify the type of list that you are creating, Java will auto implement the list as a generic list of Objects. So writing it in with no type is the same exact thing as writing this:

List<Object> variableName;

Everything in Java is an Object, so having a List of Objects would mean that the list could contain literally anything, so code like this would compile and work:

List temp = new ArrayList();
temp.add(1); // adds a number (defaults to integer)
temp.add("Code Gym"); // adds a string
temp.add(new short[]{1,2,3}); // adds an array
temp.add(new ArrayList<>()); // adds another ArrayList
temp.add(1.23); // add another number (this defaults to double)

Now the problem is that you can't pull an item out of this list and have the computer know what it is. The second item in the list is a string, we know that. So if you were to try to get that second item like this:

String secondItem = temp.get(1);

an error would occur because the computer only knows that it is an Object and has no idea that it is a String. Now you would have to write additional lines of code to test that it is a String and then convert it to a String so that you could actually use it from that point forward as a String. ArrayLists are designed with the idea that you would use them to store multiple items of one specific type of object, such as strings. When you declare the type, only that type of object can be put in, therefore when you get items from a declared List, only that type of object would be returned. So in the line of code that fails above, if the list was declared like this:

List<String> temp = new ArrayList();

and only Strings were entered into it, then that line of code would not fail.

Gil Allen

Level 8 , Johnstown, United States

12 August 2020, 21:54

Thank you very much. very clear now!