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.
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
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.
Java 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, 2017A better way to find the index of an element in an array?
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August 27, 2022Videos
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Learn Java arraylists in 9 minutes! 📃 - YouTube
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#121 Java ArrayList indexOf() – Find Element Position with Examples ...
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Does ArrayList has indexing in Java (Core Java Interview Question ...
Traversing a Java ArrayList (Printing an ArrayList) - 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.
Oracle
docs.oracle.com › javase › 8 › docs › api › java › util › ArrayList.html
ArrayList (Java Platform SE 8 )
April 21, 2026 - the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element ... Returns a shallow copy of this ArrayList instance.
Microsoft Learn
learn.microsoft.com › en-us › dotnet › api › system.collections.arraylist.indexof
ArrayList.IndexOf Method (System.Collections) | Microsoft Learn
This method performs a linear search; therefore, this method is an O(n) operation, where n is the number of elements from startIndex to the end of the ArrayList. This method determines equality by calling Object.Equals. This method uses the collection's objects' Equals and CompareTo methods on item to determine whether item exists. ... Searches for the specified Object and returns the zero-based index of the first occurrence within the range of elements in the ArrayList that starts at the specified index and contains the specified number of elements.
W3Schools
w3schools.com › java › java_arraylist.asp
Java ArrayList
An ArrayList keeps elements in the same order you add them, so the first item you add will be at index 0, the next at index 1, and so on.
W3Schools
w3schools.com › java › ref_arraylist_indexof.asp
Java ArrayList indexOf() Method
The indexOf() method returns the position of the first occurrence of a value in the list.
Medium
medium.com › @AlexanderObregon › javas-arraylist-indexof-method-explained-6bb47ce4c894
Java’s ArrayList.indexOf() Method Explained | Medium
August 19, 2024 - The indexOf() method is a fundamental part of the ArrayList class, allowing developers to search for elements within the list efficiently. Its primary function is to locate the first occurrence of a given element and return its index.
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 - Note that string “alex” is present in the list 3 times, but the method returns the index of the first occurrence. Please note that list indices start from 0. ArrayList<String> list = new ArrayList<>(Arrays.asList("alex", "brian", "charles","alex",","alex","harry")); int index = list.indexOf("alex"); System.out.println(index);
Vultr
docs.vultr.com › java › standard-library › java › util › ArrayList › indexOf
Java ArrayList indexOf() - Find Element Index | Vultr Docs
September 27, 2024 - The indexOf() function in Java's ArrayList is a powerful tool for finding the index of elements within the list. Mastery of this function allows you to manipulate lists more efficiently, especially when combined with custom object types or when ...
TutorialsPoint
tutorialspoint.com › java › util › arraylist_indexof.htm
Java ArrayList indexOf() Method
The Java ArrayList indexOf(Object) method returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. This method is used to search an element within arraylist and in case of a
BeginnersBook
beginnersbook.com › 2013 › 12 › java-arraylist-indexof-method-example
Java ArrayList indexOf() Method example
In the following example, the element 90 is present twice in ArrayList, at the index 0 and index 3. When we used the indexOf() method to find the index of 90, we got the index of first occurrence of 90, this is because this method returns the index of first occurrence of the specified element.
GeeksforGeeks
geeksforgeeks.org › java › java-util-arraylist-indexof-java
Java Arraylist indexOf() Method - GeeksforGeeks
December 10, 2024 - The indexOf() method in Java is used to find the index of the first occurrence of a specified element in an ArrayList.
How to do in Java
howtodoinjava.com › home › collections framework › java arraylist › arraylist get() – get element at index
ArrayList get() - Get Element at Index - HowToDoInJava
January 12, 2023 - The ArrayList.get(int index) method returns the element at the specified position 'index' in the list.
JavaGoal
javagoal.com › home › how to get index of object in arraylist java
java arraylist indexof and get index of object in arraylist java - JavaGoal
July 26, 2020 - The index(Object o) method is used to get the index of the specified element. Suppose if you have duplicate elements in ArrayList then it returns the index value of first occurrence of the element.
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.
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.
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.
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?