Guava:Maps.newHashMapWithExpectedSize

In Guava, Maps.newHashMapWithExpectedSize(int expectedSize) creates a new HashMap with an initial capacity that is sufficient to hold the specified number of entries without needing to resize. This can improve performance when you know in advance how many entries the map will contain.

Here's a brief example of how to use it:

import com.google.common.collect.Maps;
import java.util.Map;

public class Example {
    public static void main(String[] args) {
        int expectedSize = 100;
        Map<String, String> map = Maps.newHashMapWithExpectedSize(expectedSize);

        // Add entries to the map
        map.put("key1", "value1");
        map.put("key2", "value2");
        
        // Print the map
        System.out.println(map);
    }
}

Benefits

• Performance: Reduces the number of resizing operations.
• Memory Efficiency: Allocates just enough space for the expected entries.

Usage

Use this method when you have a good estimate of the number of entries to optimize performance.

Differences and Purpose:

1. Initial Capacity:

   • When you use newHashMapWithExpectedSize(size), the HashMap is initialized with a capacity that can accommodate the specified number of entries without needing to resize. This can lead to performance improvements since resizing a HashMap can be costly.

2. Load Factor:

   • The default load factor for a HashMap is 0.75. This means that when the HashMap is 75% full, it will resize to accommodate more entries. By specifying an expected size, you can avoid unnecessary resizing operations if you know the approximate size of your data.

3. Performance:

   • Using this method can improve performance in scenarios where you have a good estimate of the number of entries that will be stored in the map. It reduces the overhead associated with resizing and rehashing.

Summary:

Using Maps.newHashMapWithExpectedSize(int expectedSize) is beneficial for performance optimization when the number of entries is predictable, as it helps avoid the overhead of resizing during insertion operations.

Similarly, guava also has Lists.newArrayListWithExpectedSize(). However, it does not give significant difference against native java Arraylist.

Key Differences

1. Convenience: Guava's method is more explicit about the intent of initializing the list with a specific size, which can enhance code readability.

2. Dependency: Using Guava adds a dependency to your project, while ArrayList is part of the standard Java Collections Framework.

3. Flexibility: Native ArrayList allows more versatility with its constructors, including creating empty lists or specifying sizes.

Conclusion

If you're already using Guava in your project, Lists.newArrayListWithExpectedSize is a convenient and optimized option for initializing lists when the size is known. If you prefer not to add external dependencies or if you're simply using standard Java collections, the native ArrayList with a specified initial capacity is sufficient.