size() is a method specified in java.util.Collection, which is then inherited by every data structure in the standard library. length is a field on any array (arrays are objects, you just don't see the class normally), and length() is a method on java.lang.String, which is just a thin wrapper on a char[] anyway.
Perhaps by design, Strings are immutable, and all of the top-level Collection subclasses are mutable. So where you see "length" you know that's constant, and where you see "size" it isn't.
Answer from MattPutnam on Stack Overflow Top answer 1 of 7
103
size() is a method specified in java.util.Collection, which is then inherited by every data structure in the standard library. length is a field on any array (arrays are objects, you just don't see the class normally), and length() is a method on java.lang.String, which is just a thin wrapper on a char[] anyway.
Perhaps by design, Strings are immutable, and all of the top-level Collection subclasses are mutable. So where you see "length" you know that's constant, and where you see "size" it isn't.
2 of 7
48
length is constant which is used to find out the array storing capacity not the number of elements in the array
Example:
int[] a = new int[5]
a.length always returns 5, which is called the capacity of an array. But
number of elements in the array is called size
Example:
int[] a = new int[5]
a[0] = 10
Here the size would be 1, but a.length is still 5. Mind that there is no actual property or method called size on an array so you can't just call a.size or a.size() to get the value 1.
The size() method is available for collections, length works with arrays in Java.
Top answer 1 of 4
80
Just use
int listCount = data.size();
That tells you how many lists there are (assuming none are null). If you want to find out how many strings there are, you'll need to iterate:
int total = 0;
for (List<String> sublist : data) {
// TODO: Null checking
total += sublist.size();
}
// total is now the total number of strings
2 of 4
7
Java 8
import java.util.Arrays;
import java.util.List;
import java.util.ArrayList;
public class HelloWorld{
public static void main(String []args){
List<List<String>> stringListList = new ArrayList<>();
stringListList.add(Arrays.asList(new String[] {"(0,0)", "(0,1)"} ));
stringListList.add(Arrays.asList(new String[] {"(1,0)", "(1,1)", "(1,2)"} ));
stringListList.add(Arrays.asList(new String[] {"(2,0)", "(2,1)"} ));
int count=stringListList.stream().mapToInt(i -> i.size()).sum();
System.out.println("stringListList count: "+count);
}
}
Videos
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How does the size() method work in Java Lists?
When you call the size() method on a Java List, it does a little bit of internal magic. Rather than iterating through all elements, it directly accesses a variable or field that keeps track of the number of elements in the List, returning that value to you swiftly.
upgrad.com
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Java List Size: Understanding and Utilizing Dynamic Data Structures
Why is the size of a List different from its capacity?
Size and capacity may seem interchangeable, but in the context of a Java List, they convey distinct concepts. The size refers to the actual count of elements in the List, while capacity indicates how many elements it can house before needing to resize. Understanding this distinction helps manage lists more effectively.
upgrad.com
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Java List Size: Understanding and Utilizing Dynamic Data Structures
How often should I call the size() method?
The size() method is there for your use anytime you need to know the number of elements in the List. However, using it excessively or unnecessarily could have performance implications, so employ this tool wisely and only when needed.
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Java List Size: Understanding and Utilizing Dynamic Data Structures
TheServerSide
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Java array size, length and loop examples
However, Java arrays do not have a size() method, nor do they have a length() method. Instead, the property length provides a Java array’s size. To further confuse matters, every Java collection class that implements the List interface does have a size() method.
GeeksforGeeks
geeksforgeeks.org › java › list-size-method-in-java-with-examples
List size() method in Java with Examples - GeeksforGeeks
July 11, 2025 - That is, the list size() method returns the count of elements present in this list container. ... // Java Program to demonstrate // List size() Method import java.util.*; class GFG { public static void main (String[] args) { // Declared List ...
Upgrad
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Java List Size: Understanding and Utilizing Dynamic Data Structures
3 weeks ago - In Java, the List interface provides the size() method to retrieve the number of elements in a List, while the length property is used to determine the array size Java. The size() method is specific to List implementations and provides a dynamic ...
W3Schools
w3schools.com › java › ref_arraylist_size.asp
Java ArrayList size() Method
import java.util.ArrayList; public ... cars.add("Mazda"); System.out.println(cars.size()); } } ... The size() method indicates how many elements are in the list....
Scaler
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Java List Size() - Scaler Topics
April 28, 2024 - Java List size() method returns the count of elements present in the ArrayList. For example, if an ArrayList has five string objects stored in it, the size method will return five.
TutorialsPoint
tutorialspoint.com › get-the-size-of-an-arraylist-in-java
Get the Size of an ArrayList in Java
Finally, use the size() method to check and retrieve the total number of elements present in the list. A program that demonstrates this is given as follows ? import java.util.ArrayList; import java.util.List; public class Demo { public static void main(String[] args) { List aList = new ArrayList(); ...
Reddit
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r/learnprogramming on Reddit: [Java] size vs. length
July 16, 2013 -
What is the reasoning behind why some objects use size() and some use length(). Why don't they all use one or the other?
Top answer 1 of 3
2
Arrays are fixed length so they use a field/variable that gets set on initialization. Lists are variable length so they need a method to figure out their own size.
2 of 3
2
I think it's only arrays that use length (not length()). Everything else that I can think of uses "size()". Arrays are weird. They are objects, but they are implemented directly in the language and not via a library. The have members (like clone() and length), but you can't access them via reflection. Personally, I think the authors of Java just made a mistake. They probably should have made it be size(), for consistency with the other containers. I'm not sure if there is a good reason why they didn't. It's just something you have to get used to.
Java67
java67.com › 2016 › 07 › how-to-find-length-size-of-arraylist-in-java.html
How to find length/size of ArrayList in Java? Example | Java67
You can use the size() method of java.util.ArrayList to find the length or size of ArrayList in Java. The size() method returns an integer equal to a number of elements present in the array list.
BeginnersBook
beginnersbook.com › 2013 › 12 › how-to-find-length-of-arraylist-in-java
How to find length of ArrayList in Java
You can find the length (or size) of an ArrayList in Java using size() method.
Vultr
docs.vultr.com › java › standard-library › java › util › ArrayList › size
Java ArrayList size() - Get List Size | Vultr Docs
November 13, 2024 - Key to this approach is using the list's size to dictate the loop's boundaries, ensuring that the iteration does not exceed the list's current size and avoids IndexOutOfBoundsException. Understand that size() dynamically updates as elements are added or removed. Clear an ArrayList using clear() and check the size again to confirm it reflects the removals. ... import java.util.ArrayList; public class Main { public static void main(String[] args) { ArrayList<String> list = new ArrayList<>(); list.add("Apple"); list.add("Banana"); list.add("Cherry"); System.out.println("Initial size: " + list.size()); // Outputs 3 list.clear(); System.out.println("Size after clear: " + list.size()); // Outputs 0 } } Explain Code
GeeksforGeeks
geeksforgeeks.org › java › java-program-to-find-the-length-size-of-an-arraylist
Java Program to Find the Length/Size of an ArrayList - GeeksforGeeks
July 11, 2025 - // Java program to find the size // of an ArrayList import java.util.*; public class GFG { public static void main(String[] args) throws Exception { // Creating object of ArrayList<Integer> ArrayList<Integer> arrlist = new ArrayList<Integer>(); // Populating arrlist arrlist.add(1); arrlist.add(2); arrlist.add(3); arrlist.add(4); arrlist.add(5); // print arrlist System.out.println("ArrayList: " + arrlist); // getting total size of arrlist // using size() method int size = arrlist.size(); // print the size of arrlist System.out.println("Size of ArrayList = " + size); } }
iO Flood
ioflood.com › blog › length-of-arraylist-java
Java ArrayList Length: How to Get the Size of an ArrayList
February 27, 2024 - The capacity of an ArrayList refers to the amount of memory allocated for storing elements, regardless of whether those positions are currently holding elements or not. If you’re interested in the capacity, you’ll need to use different techniques, which we’ll cover in the advanced section. In Java, you can have an ArrayList of ArrayLists, also known as a multi-dimensional ArrayList. In such cases, finding the length of an individual ArrayList within this structure might be a bit tricky. Let’s look at an example: ArrayList<ArrayList<String>> multiList = new ArrayList<>(); ArrayList<String> list1 = new ArrayList<>(Arrays.asList('Hello', 'World')); ArrayList<String> list2 = new ArrayList<>(Arrays.asList('Java', 'Programming')); multiList.add(list1); multiList.add(list2); for (ArrayList<String> list : multiList) { System.out.println(list.size()); } # Output: # 2 # 2
Baeldung
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The Capacity of an ArrayList vs the Size of an Array in Java | Baeldung
November 11, 2025 - Integer[] array = new Integer[100]; System.out.println("Size of an array:" + array.length); Here, we created an Integer array of size 100, which resulted in the below output ... Technically, the default capacity (DEFAULT_CAPACITY) of a newly created ArrayList is 10. However, Java 8 changed how this initial capacity is used for performance reasons. It’s not used immediately and is guaranteed lazily once a new item is added to the list.
Tabnine
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java.util.List.size java code examples | Tabnine
Headers=" + javaResponseHeaders); } return values.get(0); } ... public static String getVerticalFormattedOutput(List<Map<String, Object>> content, String[] titleFields) { List<String[]> printContents = new ArrayList<String[]>(); int maxCol = content.size() > MAX_COL ?
Java67
java67.com › 2014 › 04 › array-length-vs-arraylist-size-java.html
Array length vs ArrayList Size in Java [Example] | Java67
Java native arrays have built-in length attribute but no size() method while the Java library containers, known as Collection classes like ArrayList<>, Vector<>, etc, all have a size() method.
TutorialsPoint
tutorialspoint.com › how-do-i-set-the-size-of-a-list-in-java
How do I set the size of a list in Java?
If we use this method to initialize a list, it won't allow us to add or remove elements from the list, as it creates an immutable list. ... In the below example, we will create a list of integers with a specific size using the Collections.nCopies() method. If you try to add an element to this list, it will throw an UnsupportedOperationException because the list is immutable. import java.util.Collections; import java.util.List; public class SetSizeOfList { public static void main(String[] args) { List<Integer> list = Collections.nCopies(5, 0); System.out.println("List of size 5: " + list); // list.add(1); } }
Javatpoint
javatpoint.com › difference-between-length-of-array-and-size-of-arraylist-in-java
Difference between Length of Array and Size of ArrayList in Java - javatpoint
Difference between Length of Array and Size of ArrayList in Java with list, set, map, queue, arraylist, linkedlist, hashset, linkedhashset, treeset, hashmap, linkedhashmap, storing and fetching data etc.