Java .containsKey()

The .containsKey() method is declared in the Map interface and is primarily implemented in the HashMap class. It is used to determine if a Map object contains a specific key. The method returns true if the key exists and false otherwise.

Syntax

The .containsKey() method can be called on a HashMap instance like this:

hashMap.containsKey(key);

Parameters:

• key: The key to be checked. This can take on the form of any type of Object.

Return value:

The .containsKey() method returns true if the map contains the given key and false otherwise.

Example 1: Basic Usage of .containsKey()

In this example, the .containsKey() method is used to check for the presence of the keys "Apples" and "Bananas" within a HashMap:

import java.util.HashMap;

public class Groceries {
  public static void main(String[] args) {
    // Create a HashMap
    HashMap<String, Integer> basket = new HashMap<String, Integer>();

    // Add items to the HashMap
    basket.put("Apples", 50);
    basket.put("Oranges", 30);

    // Check if the key "Apples" exists in the HashMap
    System.out.println("Basket contains Apples: " + basket.containsKey("Apples"));

    // Check if the key "Bananas" exists in the HashMap
    System.out.println("Basket contains Bananas: " + basket.containsKey("Bananas"));
  }
}

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Here is the output:

Basket contains Apples: true
Basket contains Bananas: false

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Example 2: Conditional Logic with .containsKey()

In this example, the .containsKey() method is used to check if the key "Germany" exists in the map before adding it. This prevents overwriting existing data unintentionally:

import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    // Create a HashMap
    HashMap<String, String> capitals = new HashMap<>();

    // Add items to the HashMap
    capitals.put("USA", "Washington, D.C.");
    capitals.put("France", "Paris");

    // Create a string
    String country = "Germany";

    // Check if the key "Germany" exists in the HashMap
    if (!capitals.containsKey(country)) {
      // Add the key "Germany" to the HashMap
      capitals.put(country, "Berlin");

      System.out.println(country + " is added to the map.");
    } else {
      System.out.println(country + " is already in the map.");
    }
  }
}

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Here is the output:

Germany is added to the map.

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Example 3: Using .containsKey() in Loops

In this example, the .containsKey() method is used in a loop to validate the presence of multiple keys in a map before attempting to retrieve their values:

import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    // Create a HashMap
    HashMap<Integer, String> idMap = new HashMap<>();

    // Add items to the HashMap
    idMap.put(1, "Apple");
    idMap.put(2, "Banana");
    idMap.put(3, "Cherry");

    // Create an array
    int[] searchKeys = {2, 4};

    // Check if the keys in the array exist in the HashMap
    for (int key : searchKeys) {
      if (idMap.containsKey(key)) {
        System.out.println("Key " + key + " maps to value: " + idMap.get(key));
      } else {
        System.out.println("Key " + key + " not found.");
      }
    }
  }
}

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In this example, the .get() method is used to get the value of a specific key.

Here is the output:

Key 2 maps to value: Banana
Key 4 not found.

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Frequently Asked Questions

1. Can .containsKey() be used with all map types?

Yes, all classes implementing the Map interface (e.g., HashMap, TreeMap, LinkedHashMap) support the .containsKey() method.

2. What happens if I pass null as a key to .containsKey()?

If the map allows null keys (like HashMap), .containsKey() will work. Otherwise, a NullPointerException is thrown.

3. Is .containsKey() time-efficient?

Yes. In a HashMap, .containsKey() has average time complexity of O(1), making it very efficient for key lookups.