Java ArrayList clear() Method

The clear() method is a fundamental operation available in the Java ArrayList class. It allows you to remove all elements from an ArrayList with a single command, resetting the list to an empty state while maintaining its original capacity. This makes it an essential tool for managing collections that need to be reused or reset during a program’s execution.

What Is the clear() Method in ArrayList?

The clear() method removes all elements from the list.
After calling clear(), the size of the list becomes zero, but the underlying array capacity is not decreased unless trimToSize() is explicitly called.
The method does not return a value (its return type is void).

Syntax

public void clear()

Parameters

The method does not accept any parameters.

Return Value

void – There is no return value. The ArrayList is simply emptied.

Exceptions

The clear() method may throw an UnsupportedOperationException if the list does not support the clear operation (this is rare and only for custom, immutable list implementations).
For standard ArrayList, there are no typical exceptions.

How Does clear() Work Internally?

Internally, clear() sets the size of the ArrayList to zero and dereferences all elements to allow for garbage collection.
The capacity of the underlying array does not change; it will only shrink if you call trimToSize() after clearing.

Examples of the clear() Method

1. Clear All Elements

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> colors = new ArrayList<>();
        colors.add("Red");
        colors.add("Green");
        colors.add("Blue");

        System.out.println("Before clear: " + colors); // [Red, Green, Blue]
        colors.clear();
        System.out.println("After clear: " + colors); // []
    }
}

Output:

Before clear: [Red, Green, Blue]
After clear: []

2. Using clear() to Reuse the Same List

import java.util.ArrayList;

public class Demo {
    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(10);
        numbers.add(20);

        System.out.println("First version: " + numbers); // [10, 20]
        numbers.clear();

        numbers.add(30);
        numbers.add(40);
        System.out.println("Reused list: " + numbers); // [30, 40]
    }
}