Java generic list as parameter

The Java List interface, java.util.List, represents an ordered sequence of objects. The elements contained in a Java List can be inserted, accessed, iterated and removed according to the order in which they appear internally in the Java List. The ordering of the elements is why this data structure is called a List.

Each element in a Java List has an index. The first element in the List has index 0, the second element has index 1 etc. The index means "how many elements away from the beginning of the list". The first element is thus 0 elements away from the beginning of the list - because it is at the beginning of the list.

You can add any Java object to a List. If the List is not typed, using Java Generics, then you can even mix objects of different types (classes) in the same List. Mixing objects of different types in the same List is not often done in practice, however.

The Java List interface is a standard Java interface, and it is a subtype of the Java Collection interface, meaning List inherits from Collection.

Java List Tutorial Video

If you prefer to watch a video instead of reading text, here is a video version of this Java List tutorial:

List vs. Set

The Java List and Java Set interfaces are quite similar in that they both represents a collection of elements. However, there are some significant differences. These differences are reflected in the methods the List and Set interfaces offer.

The first difference between the Java List and Java Set interface is, that the same element can occur more than once in a Java List. This is different from a Java Set where each element can occur only once.

The second difference between a Java List and Java Set interfaces is, that the elements in a List has an order, and the elements can be iterated in that order. A Java Set does not make any promises about the order of the elements kept internally.

List Implementations

Being a Collection subtype all methods in the Collection interface are also available in the List interface.

Since List is an interface you need to instantiate a concrete implementation of the interface in order to use it. You can choose between the following List implementations in the Java Collections API:

java.util.ArrayList
java.util.LinkedList
java.util.Vector
java.util.Stack

Of these implementations, the ArrayList is the most commonly used.

There are also concurrent List implementations in the java.util.concurrent package. These List implementations are explained in more detail in my java.util.concurrent tutorial .

Create a List

You create a List instance by creating an instance of one of the classes that implements the List interface. Here are a few examples of how to create a List instance:

List listA = new ArrayList(); List listB = new LinkedList(); List listC = new Vector(); List listD = new Stack();

Remember, most often you will use the ArrayList class, but there can be cases where using one of the other implementations might make sense.

Generic Lists

By default you can put any Object into a List, but from Java 5, Java Generics makes it possible to limit the types of object you can insert into a List. Here is an example:

List<MyObject> list = new ArrayList<MyObject>();

This List can now only have MyObject instances inserted into it. You can then access and iterate its elements without casting them. Here is how it looks:

List<MyObject> list = new ArrayList<MyObject>(); list.add(new MyObject("First MyObject")); MyObject myObject = list.get(0); for(MyObject anObject : list){ //do someting to anObject... }

Without generics the example above would have looked like this:

List list = new ArrayList(); //no generic type specified list.add(new MyObject("First MyObject")); MyObject myObject = (MyObject) list.get(0); //cast needed for(Object anObject : list){ //cast needed MyObject theMyObject = (MyObject) anObject; //do someting to anObject... }

Notice how it is necessary to cast the MyObject instances retrieved from the List to MyObject. Without a generic type set on the List variable declaration the Java compiler only knows that the List contains Object instances. Thus, you need to cast them to the concrete class (or interface) that you know the object to be of.

It is a good practice to specify a generic type for your List variables whenever you can. It helps you avoid inserting the wrong types of objects into the List. It enables you to retrieve the objects from the List without having to cast them their real type. And - it helps the reader of your code see what type of objects the List is supposed to contain. You should only omit a generic type if you have very good reasons to do so.

Throughout the rest of this Java List tutorial I will be using generic List examples as much as possible.

For more information about Java Generics, see the Java Generics Tutorial.

Insert Elements in a Java List

You insert elements (objects) into a Java List using its add() method. Here is an example of adding elements to a Java List using the add() method:

List<String> listA = new ArrayList<>(); listA.add("element 1"); listA.add("element 2"); listA.add("element 3");

The first three add() calls add a String instance to the end of the list.

Insert null Values

It is actually possible to insert null values into a Java List. Here is an example of inserting a null value into a Java List:

Object element = null; List<Object> list = new ArrayList<>(); list.add(element);

Insert Elements at Specific Index

It is possible to insert an element into a Java List at a specific index. The List interface has a version of the add() method that takes an index as first parameter, and the element to insert as the second parameter. Here is an example of inserting an element at index 0 into a Java List:

list.add(0, "element 4");

If the List already contained elements, these elements will now get pushed further down in the List's internal sequence. The element that had index 0 before the new element was inserted at index 0, will get pushed to index 1 etc.

Insert All Elements From One List Into Another

It is possible to add all elements from one Java List into another List. You do so using the List addAll() method. The resulting List is the union of the two lists. Here is an example of adding all elements from one List into another:

List<String> listSource = new ArrayList<>(); listSource.add("123"); listSource.add("456"); List<String> listDest = new ArrayList<>(); listDest.addAll(listSource);

This example adds all elements from listSource into listDest.

The addAll() method takes a Collection as parameter, so you can pass either a List or Java Set as parameter. In other words, you can add all elements from a List or Set into a List with addAll() .

Get Elements From a Java List

You can get the elements from a Java List using the index of the elements. You can do so using either the get(int index) method. Here is an example of accessing the elements of a Java List using the element indexes:

List<String> listA = new ArrayList<>(); listA.add("element 0"); listA.add("element 1"); listA.add("element 2"); //access via index String element0 = listA.get(0); String element1 = listA.get(1); String element3 = listA.get(2);

It is also possible to iterate the elements of a Java List in the order they are stored in internally. I will show you how to do that later in this Java List tutorial.

Find Elements in a List

You can find elements in a Java List using one of these two methods:

The indexOf() method finds the index of the first occurrence in the List of the given element. Here is an example finding the index of two elements in a Java List:

List<String> list = new ArrayList<>(); String element1 = "element 1"; String element2 = "element 2"; list.add(element1); list.add(element2); int index1 = list.indexOf(element1); int index2 = list.indexOf(element2); System.out.println("index1 = " + index1); System.out.println("index2 = " + index2);

Running this code will result in this output:

index1 = 0 index2 = 1

Find Last Occurrence of Element in a List

The lastIndexOf() method finds the index of the last occurrence in the List of a given element. Here is an example showing how to find the index of the last occurrence of a given element in a Java List:

List<String> list = new ArrayList<>(); String element1 = "element 1"; String element2 = "element 2"; list.add(element1); list.add(element2); list.add(element1); int lastIndex = list.lastIndexOf(element1); System.out.println("lastIndex = " + lastIndex);

The output printed from running the above Java example will be:

lastIndex = 2

The element element 1 occurs 2 times in the List. The index of the last occurrence is 2.

Checking if List Contains Element

You can check if a Java List contains a given element using the List contains() method. Here is an example of checking if a Java List contains an element using the contains() method:

List<String> list = new ArrayList<>(); String element1 = "element 1"; list.add(element1); boolean containsElement = list.contains("element 1"); System.out.println(containsElement);

The output from running this Java List example will be:

true

... because the List does actually contain the element.

To determine if the List contains the element, the List will internally iterate its elements and compare each element to the object passed as parameter. The comparison uses the Java equals method of the element to check if the element is equal to the parameter.

Since it is possible to add null values to a List, it is actually possible to check if the List contains a null value. Here is how you check if a List contains a null value:

list.add(null); containsElement = list.contains(null); System.out.println(containsElement);

Obviously, if the input parameter to contains() is null, the contains() method will not use the equals() method to compare against each element, but rather use the == operator.

Remove Elements From a Java List

You can remove elements from a Java List via these two methods:

remove(Object element)
remove(int index)

remove(Object element) removes that element in the list, if it is present. All subsequent elements in the list are then moved up in the list. Their index thus decreases by 1. Here is an example of removing an element from a Java List based on the element itself:

List<String> list = new ArrayList<>(); String element = "first element"; list.add(element); list.remove(element);

This example first adds an element to a List and then removes it again.

The List remove(int index) method removes the element at the given index. All subsequent elements in the list are then moved up in the list. Their index thus decreases by 1. Here is an example of removing an element from a Java List by its index:

List<String> list = new ArrayList<>(); list.add("element 0"); list.add("element 1"); list.add("element 2"); list.remove(0);

After running this Java example code the List would contain the Java String elements element 1 and element 2 at index 0 and 1. The first element (element 0) has been removed from the List.

Remove All Elements From a Java List

The Java List interface contains a clear() method which removes all elements from the list when called. Removing all elements from a List is also referrred to as clearing a List. Here is simple example of removing all elements from a List (clearing) with the clear() method:

List<String> list = new ArrayList<>(); list.add("object 1"); list.add("object 2"); //etc. list.clear();

First, a new List is created. Second, two elements are added to the List. Third, the clear() method is called. After the clear() method has been called, the List will be completely empty.

Retain All Elements From One List in Another

The Java List interface has a method called retainAll() which is capable of retaining all elements from one List which are also present in another List. In other words, the retain() method removes all the elements from the target List which are not found in the other List. The resulting List is the intersection of the two lists. Here is a Java example of calling the List retainAll() method:

List<String> list = new ArrayList<>(); List<String> otherList = new ArrayList<>(); String element1 = "element 1"; String element2 = "element 2"; String element3 = "element 3"; String element4 = "element 4"; list.add(element1); list.add(element2); list.add(element3); otherList.add(element1); otherList.add(element3); otherList.add(element4); list.retainAll(otherList);

First two lists are created. Second, 3 elements are added to list and 3 elements are added to otherList. Third, the retainAll() method is called on list, passing otherList as parameter. After list.retainAll(otherList) has finished executing, the list will only contain those elements which were both present in list and otherList before retainAll() was called. More specifically, that is element1 and element3 .

List Size

You can obtain the number of elements in the List by calling the size() method. Here is an example:

List<String> list = new ArrayList<>(); list.add("object 1"); list.add("object 2"); int size = list.size();

Sublist of List

The Java List interface has a method called subList() which can create a new List with a subset of the elements from the original List.

The subList() method takes 2 parameters: A start index and and end index. The start index is the index of the first element from the original List to include in the sublist. The end index is the last index of the sublist, but the element at the last index is not included in the sublist. This is similar to how the Java String substring method works.

Here is a Java example of creating a sublist of elements from another List using the subList() method:

List<String> list = new ArrayList<>(); list.add("element 1"); list.add("element 2"); list.add("element 3"); list.add("element 4"); List<String> sublist = list.subList(1, 3);

After executing the list.subList(1,3) instruction the sublist will contain the elements at index 1 and 2. Remember, the original list has 4 elements with indexes from 0 to 3. The list.subList(1,3) call will include index 1, but exclude index 3, thereby keeping the elements at index 1 and 2.

Convert List to Set

You can convert a Java List to a Java Set by creating a new Set and adding all elements from the List to it. The Set will remove all duplicates. Thus, the resulting Set will contain all the elements from the List, but only once. Here is an example of converting a Java List to a Set:

List<String> list = new ArrayList<>(); list.add("element 1"); list.add("element 2"); list.add("element 3"); list.add("element 3"); Set<String> set = new HashSet<>(); set.addAll(list);

Notice that the List contains the String element 3 two times. The Set will only contain that String once. Thus, the resulting set will contain the Strings element 1, element 2 and element 3 .

Convert List to Array

You can convert a Java List to a Java Array using the List toArray() method. Here is an example of converting a Java List to a Java array:

List<String> list = new ArrayList<>(); list.add("element 1"); list.add("element 2"); list.add("element 3"); list.add("element 3"); Object[] objects = list.toArray();

It is also possible to convert a List to an array of a specific type. Here is an example of converting a Java List to an array of a specific type:

List<String> list = new ArrayList<>(); list.add("element 1"); list.add("element 2"); list.add("element 3"); list.add("element 3"); String[] objects1 = list.toArray(new String[0]);

Note that even if we pass a String array of size 0 to the toArray(), the array returned will have all the elements in the List in it. It will have the same number of elements as the List.

Convert Array to List

It is also possible to convert a Java List to an array. Here is an example of converting a Java array to a List:

String[] values = new String[]{ "one", "two", "three" }; List<String> list = (List<String>) Arrays.asList(values);

It is the Arrays.asList() method that converts the array to a List.

Sort List

You can sort a Java List using the Collections sort() method. I have explained how that works in my sorting Java collections tutorial, but I will just show you a few ways to sort a Java List in the following sections.

Sort List of Comparable Objects

If the List contains objects that implement the Comparable interface (java.lang.Comparable), then the objects can compare themselves to each other. In that case you can sort the List like this:

List<String> list = new ArrayList<>(); list.add("c"); list.add("b"); list.add("a"); Collections.sort(list);

The Java String class implements the Comparable interface, you can sort them in their natural order, using the Collections sort() method.

Sort List Using Comparator

If the objects in the Java List do not implement the Comparable interface, or if you want to sort the objects in another order than their compare() implementation, then you need to use a Comparator implementation (java.util.Comparator). Here is an example of sorting a list of Car objects using a Comparator. Here is first the Car class:

public class Car{ public String brand; public String numberPlate; public int noOfDoors; public Car(String brand, String numberPlate, int noOfDoors) { this.brand = brand; this.numberPlate = numberPlate; this.noOfDoors = noOfDoors; } }

Here is the code that sorts a Java List of the above Car objects:

List<Car> list = new ArrayList<>(); list.add(new Car("Volvo V40" , "XYZ 201845", 5)); list.add(new Car("Citroen C1", "ABC 164521", 4)); list.add(new Car("Dodge Ram" , "KLM 845990", 2)); Comparator<Car> carBrandComparator = new Comparator<Car>() { @Override public int compare(Car car1, Car car2) { return car1.brand.compareTo(car2.brand); } }; Collections.sort(list, carBrandComparator);

Notice the Comparator implementation in the example above. This implementation only compares the brand field of the Car objects. It is possible to create another Comparator implementation which compares the number plates, or even the number of doors in the cars.

Note too, that it is possible to implement a Comparator using a Java Lambda. Here is an example that sorts a List of Car objects using three different Java lambda implementations of the Comparator interface, each of which compares Car instances by a different field:

List<Car> list = new ArrayList<>(); list.add(new Car("Volvo V40" , "XYZ 201845", 5)); list.add(new Car("Citroen C1", "ABC 164521", 4)); list.add(new Car("Dodge Ram" , "KLM 845990", 2)); Comparator<Car> carBrandComparatorLambda = (car1, car2) -> car1.brand.compareTo(car2.brand); Comparator<Car> carNumberPlatComparatorLambda = (car1, car2) -> car1.numberPlate.compareTo(car2.numberPlate); Comparator<Car> carNoOfDoorsComparatorLambda = (car1, car2) -> car1.noOfDoors - car2.noOfDoors; Collections.sort(list, carBrandComparatorLambda); Collections.sort(list, carNumberPlatComparatorLambda); Collections.sort(list, carNoOfDoorsComparatorLambda);

Iterate List

You can iterate a Java List in several different ways. The three most common ways are:

Using an Iterator
Using a for-each loop
Using a for loop
Using the Java Stream API

I will explain each of these methods of iterating a Java List in the following sections.

Iterate List Using Iterator

The first way to iterate a List is to use a Java Iterator. Here is an example of iterating a List with an Iterator:

List<String> list = new ArrayList<>(); list.add("first"); list.add("second"); list.add("third"); Iterator<String> iterator = list.iterator(); while(iterator.hasNext()) { String next = iterator.next(); }

You obtain an Iterator by calling the iterator() method of the List interface.

Once you have obtained an Iterator you can keep calling its hasNext() method until it returns false. Calling hasNext() is done inside a while loop as you can see.

Inside the while loop you call the Iterator next() method of the Iterator interface to obtain the next element pointed to by the Iterator.

If the List is typed using Java Generics you can save some object casting inside the while loop. Here is an example:

Iterate List Using For-Each Loop

The second way to iterate a List is to use the for loop added in Java 5 (also called a "for each" loop). Here is an example of iterating a List using the for loop:

List list = new ArrayList(); list.add("first"); list.add("second"); list.add("third"); for(Object element : list) { System.out.println(element); }

The for loop is executed once per element in the List. Inside the for loop each element is in turn bound to the obj variable.

If the list is typed (a generic List) you can change the type of the variable inside the for loop. Here is typed List iteration example:

List<String> list = new ArrayList<String>(); //add elements to list for(String element : list) { System.out.println(element); }

Notice how the List is typed to String. Therefore you can set the type of the variable inside the for loop to String.

Iterate List Using For Loop

The third way to iterate a List is to use a standard for loop like this:

List list = new ArrayList(); list.add("first"); list.add("second"); list.add("third"); for(int i=0; i < list.size(); i++) { Object element = list.get(i); }

The for loop creates an int variable and initializes it to 0. Then it loops as long as the int variable i is less than the size of the list. For each iteration the variable i is incremented.

Inside the for loop the example accesses the elements in the List via its get() method, passing the incrementing variable i as parameter.

Again, if the List is typed using Java Generics to e.g. to a String, then you can use the generic type of the List as type for the local variable that is assigned each element in the List during iteration. An example will make this more clear:

List<String> list = new ArrayList<String>(); list.add("first"); list.add("second"); list.add("third"); for(int i=0; i < list.size(); i++) { String element = list.get(i); }

Notice the type of the local variable inside the for loop is now String. Because the List is generically typed to String, it can only contain String objects. Hence, the compiler knows that only a String can be returned from the get() method. Therefore you do not need to cast the element returned by get() to String.

Iterate List Using Java Stream API

The fourth way to iterate a Java List is via the Java Stream API. To iterate a Java List you must first obtain a Stream from the List. Obtaining a Stream from a List in Java is done by calling the List stream() method. Here is an example of obtaining a Java Stream from a Java List:

List<String> stringList = new ArrayList<String>(); stringList.add("abc"); stringList.add("def"); Stream<String> stream = stringList.stream();

It is the last line of this example that calls the List stream() method to obtain the Stream representing the elements in the List.

Once you have obtained a Stream from a List you can iterate the Stream by calling its forEach() method. Here is an example of iterating the elements of a List using the Stream forEach() method:

List<String> stringList = new ArrayList<String>(); stringList.add("one"); stringList.add("two"); stringList.add("three"); Stream<String> stream = stringList.stream(); stream .forEach( element -> { System.out.println(element); });

Calling the forEach() method will make the Stream iterate all the element of the Stream internally, and call the Consumer passed as parameter to the forEach() method for each element in the Stream.

You can read more about the many different options available for processing the elements of a List via the Java Stream API in my Java Stream API Tutorial.

More Details in the Java List JavaDoc

There is a lot more you can do with a Java List, but you will have to check out the JavaDoc for more details. This text focused on the two most common operations: Adding / removing elements, and iterating the elements.