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.get(1); //returns the 2nd item from list, in this case "banana"
Indexing is counted from 0 to N-1 where N is size() of list.
GeeksforGeeks
geeksforgeeks.org › java › arraylist-get-method-java-examples
ArrayList get(index) Method in Java with Examples - GeeksforGeeks
December 10, 2024 - The get(index) method of ArrayList in Java is used to retrieve the element at the specified index within the list.
Top answer 1 of 7
110
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.get(1); //returns the 2nd item from list, in this case "banana"
Indexing is counted from 0 to N-1 where N is size() of list.
2 of 7
20
Read more about Array and ArrayList
List<String> aList = new ArrayList<String>();
aList.add("apple");
aList.add("banana");
aList.add("orange");
String result = alist.get(1); //this will retrieve banana
Note: Index starts from 0 i.e. Zero
- Resource
A better way to find the index of an element in an array?
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3
August 27, 2022Java while loop to find index of elements in ArrayList
ArrayList.indexOf(Object o) will always return the index of the first occurrence of the given object, so what you observe is happening by design; to get the indices of the others, you would have to remove the item from the list once you find it so that when you call indexOf again, the next occurrence is seen. Alternatively, you could scan through the list and compare elements directly instead of using indexOf. That way, you wouldn't have to modify the list at all, since you'd simply be looking at each item and comparing with a reference value. More on reddit.com
13
1
May 4, 2017Java: Is Arraylist better than Arrays?
The biggest difference is that you can add to and remove from a list, whereas arrays have a fixed size. More on reddit.com
28
19
June 23, 2024Two dimensional ArrayList, need help with constructor
There are probably better ways to solve your problem, but here is the answer to your question: List> list1 = new ArrayList>(); for (int i = 0; i < 10; i++) { List list2 = new ArrayList(); for (int j = 0; j < 10; j++) { list2.add(i * j); } list1.add(list2); } Integer value = list1.get(1).get(5); More on reddit.com
5
0
February 17, 2012Videos
01:37
#121 Java ArrayList indexOf() – Find Element Position with Examples ...
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Does ArrayList has indexing in Java (Core Java Interview Question ...
09:40
Learn Java arraylists in 9 minutes! 📃 - YouTube
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Searching and removing values from an ArrayList in Java - YouTube
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How a Java HashMap internal implementation works | TheServerSide
11:54
#92 ArrayList in Java - YouTube
Codecademy
codecademy.com › docs › java › arraylist › .indexof()
Java | ArrayList | .indexOf() | Codecademy
February 12, 2023 - The .indexOf() method returns the index of the first occurrence of the specified element in an ArrayList. If the element is not found, -1 is returned. ... Looking for an introduction to the theory behind programming?
TutorialsPoint
tutorialspoint.com › get-the-index-of-a-particular-element-in-an-arraylist-in-java
Java ArrayList indexOf() Method
September 13, 2023 - We're adding couple of Integers to the ArrayList object using add() method calls per element and using indexOf(object) method, we're checking index of an element and printing it. package com.tutorialspoint; import java.util.ArrayList; public class ArrayListDemo { public static void main(String[] args) { // create an empty array list ArrayList<Integer> arrayList = new ArrayList<>(); // use add() method to add elements in the arrayList arrayList.add(0); arrayList.add(1); arrayList.add(2); arrayList.add(3); arrayList.add(4); arrayList.add(5); arrayList.add(6); // let us print index of 5 System.out.println("Index of 5 = " + arrayList.indexOf(5)); } }
Vultr
docs.vultr.com › java › standard-library › java › util › ArrayList › indexOf
Java ArrayList indexOf() - Find Element Index | Vultr Docs
September 27, 2024 - Create an instance of ArrayList and add some elements to it. Use the indexOf() method to find the index of a specified element. ... import java.util.ArrayList; ArrayList<String> list = new ArrayList<>(); list.add("Apple"); list.add("Banana"); ...
GeeksforGeeks
geeksforgeeks.org › java › java-util-arraylist-indexof-java
Java Arraylist indexOf() Method - GeeksforGeeks
December 10, 2024 - Example 1: Here, we will use the indexOf() method to find the index of a specific element in an ArrayList. ... // Java program to demonstrate the // use of indexOf() method import java.util.ArrayList; public class GFG { public static void ...
How to do in Java
howtodoinjava.com › home › collections framework › java arraylist › java arraylist.indexof()
Java Arraylist.indexOf() - Get Element Index - HowToDoInJava
January 13, 2023 - To get the index of the last occurrence of the same element, use the lastIndexOf() method. The indexOf() returns the index of the first occurrence of the specified element in this list. It will return '-1' if the list does not contain the element.
W3Resource
w3resource.com › java-tutorial › arraylist › arraylist_indexof.php
Java ArrayList.indexOf() Method
August 19, 2022 - Return Value: The index of the first occurrence an element in ArrayList, or -1 if the element does not exist. ... The following example creates an ArrayList containing various colors. It then tries to determine the position of a certain color. import java.util.*; public class test { public ...
Programiz
programiz.com › java-programming › library › arraylist › indexof
Java ArrayList indexOf()
returns the position of the specified element from the arraylist · Note: If the specified element doesn't exist in the list, the indexOf() method returns -1. import java.util.ArrayList; class Main { public static void main(String[] args) { // create an ArrayList ArrayList<Integer> numbers ...
Medium
medium.com › @AlexanderObregon › javas-arraylist-indexof-method-explained-6bb47ce4c894
Java’s ArrayList.indexOf() Method Explained | Medium
August 19, 2024 - This method accepts a single argument, Object o, which represents the element you wish to find within the ArrayList. If the specified element is present in the list, indexOf() returns the zero-based index of its first occurrence.
W3Schools
w3schools.com › java › ref_arraylist_indexof.asp
Java ArrayList indexOf() Method
public int indexOf(Object item) Java Arrays Tutorial · Java ArrayList Tutorial · ❮ ArrayList Methods · ★ +1 · Sign in to track progress · REMOVE ADS · PLUS · SPACES · GET CERTIFIED · FOR TEACHERS · BOOTCAMPS · CONTACT US · × · If you want to use W3Schools services as an educational institution, team or enterprise, send us an e-mail: sales@w3schools.com ·
Java Code Geeks
javacodegeeks.com › home › core java
Java ArrayList Insert/Replace At Index - Java Code Geeks
January 2, 2022 - ArrayList.add(int index, E element) method takes the index of the array list where the new value to be inserted and E element that the new value to be added to the existing values of ArrayList. add(index, value) method is used to insert the ...
How to do in Java
howtodoinjava.com › home › collections framework › java arraylist › arraylist get() – get element at index
Java ArrayList - Get Element at Index
January 12, 2023 - For example, if ArrayList holds 10 objects, then a valid argument index will be between 0 to 9 (both inclusive). The get() method returns the reference of the object present at the specified index.
TutorialsPoint
tutorialspoint.com › java › util › arraylist_get.htm
Java ArrayList get() Method
The Java ArrayList get(int index) method returns the element at the specified position in this list. Index starts from 0 like first element can be retrieved using get(0) method call and so on.
BeginnersBook
beginnersbook.com › 2013 › 12 › java-arraylist-get-method-example
Java ArrayList get() Method example
ArrayList get(int index) method is used for fetching an element from the list. We need to specify the index and get method returns the value present at the specified index.
Coderanch
coderanch.com › t › 622130 › java › ArrayList-Find-Indexes-Occurrence
ArrayList: Find all Indexes of an Occurrence [Solved] (Beginning Java forum at Coderanch)
October 19, 2013 - Well done It would have been easier had the indexOf() method been overloaded to take a starting index. ... You can of course subclass ArrayList and add that method.
Reddit
reddit.com › r/javahelp › a better way to find the index of an element in an array?
r/javahelp on Reddit: A better way to find the index of an element in an array?
August 27, 2022 -
Currently the best way I have found is to typecast the array into an ArrayList, and using ArrayList's method .indexOf to get the index, for e.g:
import java.util.Arrays;
int[] arr = {2,3,4,5};
Arrays.asList(arr).indexOf(4); // returns 2Is this the best way? Or unnecessary wrapping?
Top answer 1 of 4
7
That way doesn't work. int[] can't be wrapped up as a List via List Arrays.asList(T...) - you'll get a List back (with a single element). If the array is sorted you can use Arrays.binarySearch(arr, 4) - if it is not sorted then you've got fewer choices. Most efficient is probably... int index = -1; for (int i = 0; i < arr.length; i++) { if (arr[i] == x) { index = i; break; } } But you could also do int index = IntStream.range(0, arr.length) .filter(i -> arr[i] == x) .findFirst() .orElse(-1); If the array was spectacularly large, this might pay off in parallel (but the findFirst limits this as it forces some sequencing to the parallel operation - findAny would help). Generally I'd just make a method to do the for-loop (or try to avoid needing to scan in the first place - not always possible).
2 of 4
3
If you're doing this, why not just use Lists instead of arrays?
Codekru
codekru.com › home › arraylist indexof() method in java
ArrayList indexOf() method in Java - Codekru
August 12, 2022 - What does it do? It will return the index of the first occurrence of the specified element. So, if an element is present multiple times in an ArrayList, then indexOf() will return the index of the first occurrence of that element in the list.
W3Schools
w3schools.com › java › java_arraylist.asp
Java ArrayList
It is part of the java.util package and implements the List interface. The difference between a built-in array and an ArrayList in Java, is that the size of an array cannot be modified (if you want to add or remove elements to/from an array, you have to create a new one).
Reddit
reddit.com › r/learnprogramming › java while loop to find index of elements in arraylist
r/learnprogramming on Reddit: Java while loop to find index of elements in ArrayList
May 4, 2017 -
I'm sorry for the possibly bad title! Didn't know what else to put there.
So my problem is, I am doing the MOOC course for java from University of Helsinki, and my challenge was to create a program which reads input from user and puts it on an ArrayList. If the user enters -1, the reading will end. After the list has been created (user enters -1), the program will ask the user for number and print out all the indexes where that number exists in the ArrayList. I have blasted my way through all of the earlier challenges but just can't manage to wrap my head around this. I do know how to solve this with for-loop and arrays, but I'm supposed to solve it using a while loop and without the use of arrays.
My current code is this:
if(list.contains(number)) {
while(i < list.size()) {
spot = list.indexOf(number);
i++;
}
System.out.println("Number " + number + " is in the index " + spot);
} and it works just fine when I have a list that contains only 1 instance of a number. But when I have a list that has for example two times the number eight in different indexes, it will only return the first one. Any suggestions?
Top answer 1 of 4
2
ArrayList.indexOf(Object o) will always return the index of the first occurrence of the given object, so what you observe is happening by design; to get the indices of the others, you would have to remove the item from the list once you find it so that when you call indexOf again, the next occurrence is seen. Alternatively, you could scan through the list and compare elements directly instead of using indexOf. That way, you wouldn't have to modify the list at all, since you'd simply be looking at each item and comparing with a reference value.
2 of 4
2
u/0x706272 already pointed out the biggest problem in the code but there are a couple more things to look out for. Currently you are assigning the index to your spot variable which is (presumably) an integer and so can only hold one number. During your while loop you keep overwriting that one value with the new index which discards the previous values. So even if you change indexOf to something that actually iterates through all the indices for your search number you would still only get the last index in the end. To fix that you will either have to output the spot directly within the while loop or store your results in something like an array or a list.