Java List Interface

The List interface represents an ordered collection accessed by numeric index, and is one of the three core interfaces -- alongside Set and Map -- in Java's Collections Framework.

In this page:

What is the List Interface?
List Implementations
Common List Methods
Ordered and Indexed Access
Allowing Duplicates

Syntax

java

List<Type> list = new ArrayList<>();
list.add(item);
list.get(index);

What is the List Interface?

The List interface represents an ordered collection of elements that can be accessed by their numeric index, and is one of the three core interfaces -- alongside Set and Map -- that make up Java's Collections Framework.

Example: What is the List Interface?

java

import java.util.List;
import java.util.ArrayList;
public class Main {
	public static void main(String[] args) {
		List<String> names = new ArrayList<>();
		names.add("Alice");
		System.out.println(names.get(0)); // ordered, accessible by index
	}
}

List Implementations

ArrayList and LinkedList are the two most common implementations of the List interface: ArrayList stores elements in a resizable array for fast index access, while LinkedList stores them as a chain of linked nodes for fast insertion and removal.

Example: List Implementations

java

// Import java.util.List so it can be used by its short name
import java.util.List;
// Import java.util.ArrayList so it can be used by its short name
import java.util.ArrayList;
import java.util.LinkedList;
// Define the class Main
public class Main {
	// Program entry point: the JVM starts running here
	public static void main(String[] args) {
		// Create a new ArrayList object and store it in arrayList
		List<String> arrayList = new ArrayList<>();
		// Create a new LinkedList object and store it in linkedList
		List<String> linkedList = new LinkedList<>();
		arrayList.add("fast index access");
		linkedList.add("chain of nodes");
		// Print a line to the console
		System.out.println(arrayList);
		// Print a line to the console
		System.out.println(linkedList);
	}
}

Common List Methods

The List interface defines common methods like add, remove, get, set, size, and contains, all of which work identically regardless of which concrete List implementation is actually being used underneath.

Example: Common List Methods

java

// Import java.util.List so it can be used by its short name
import java.util.List;
// Import java.util.ArrayList so it can be used by its short name
import java.util.ArrayList;
// Define the class Main
public class Main {
	// Program entry point: the JVM starts running here
	public static void main(String[] args) {
		// Create a new ArrayList object and store it in names
		List<String> names = new ArrayList<>();
		names.add("Alice");
		names.add("Bob");
		// Print a line to the console
		System.out.println(names.get(0));
		// Print a line to the console
		System.out.println(names.contains("Bob"));
		System.out.println(names.size());
		names.remove("Alice");
		System.out.println(names);
	}
}

Ordered and Indexed Access

A List always preserves the order elements were inserted in and lets any element be retrieved or replaced directly by its zero-based index using get and set, unlike a Set which has no concept of position.

Example: Ordered and Indexed Access

java

import java.util.List;
import java.util.ArrayList;
public class Main {
	public static void main(String[] args) {
		List<String> names = new ArrayList<>();
		names.add("Alice");
		names.add("Bob");
		names.set(0, "Charlie"); // replace by index
		System.out.println(names.get(0));
	}
}

Allowing Duplicates

A List explicitly allows duplicate values to be stored, with each occurrence kept as its own separate entry at its own index, which is the key behavioral difference that distinguishes it from the Set interface.

Example: Allowing Duplicates

java

import java.util.List;
import java.util.ArrayList;
public class Main {
	public static void main(String[] args) {
		List<String> names = new ArrayList<>();
		names.add("Alice");
		names.add("Alice"); // duplicates allowed, kept as separate entries
		System.out.println(names);
	}
}

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Java List Interface

What is the List Interface?

List Implementations

Common List Methods

Ordered and Indexed Access

Allowing Duplicates

Chapter Quiz — Complete all 17 topics to unlock

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