Java Creating WeakHashMap from Other Maps

In Java, you can create a WeakHashMap from other maps using the new WeakHashMap(Map m) constructor.

Here is an example of how to create a WeakHashMap from a HashMap:

import java.util.HashMap;
import java.util.Map;
import java.util.WeakHashMap;

public class Main {
  public static void main(String[] args) {
    // Create a HashMap
    Map<String, Integer> hashMap = new HashMap<>();
    hashMap.put("Apple", 1);
    hashMap.put("Banana", 2);
    hashMap.put("Cherry", 3);

    // Create a WeakHashMap from the HashMap
    WeakHashMap<String, Integer> weakHashMap = new WeakHashMap<>(hashMap);

    // Print the WeakHashMap
    System.out.println(weakHashMap);  // Output: {Apple=1, Banana=2, Cherry=3}
  }
}

In this example, we create a HashMap and add some key-value pairs to it. We then create a WeakHashMap from the HashMap using the new WeakHashMap(Map m) constructor. This creates a new WeakHashMap with the same key-value pairs as the HashMap.

You can also create a WeakHashMap from other maps, such as a TreeMap or a LinkedHashMap. Here is an example:

import java.util.LinkedHashMap;
import java.util.Map;
import java.util.WeakHashMap;

public class Main {
  public static void main(String[] args) {
    // Create a LinkedHashMap
    Map<String, Integer> linkedHashMap = new LinkedHashMap<>();
    linkedHashMap.put("Apple", 1);
    linkedHashMap.put("Banana", 2);
    linkedHashMap.put("Cherry", 3);

    // Create a WeakHashMap from the LinkedHashMap
    WeakHashMap<String, Integer> weakHashMap = new WeakHashMap<>(linkedHashMap);

    // Print the WeakHashMap
    System.out.println(weakHashMap);  // Output: {Apple=1, Banana=2, Cherry=3}
  }
}

In this example, we create a LinkedHashMap and add some key-value pairs to it. We then create a WeakHashMap from the LinkedHashMap using the new WeakHashMap(Map m) constructor. This creates a new WeakHashMap with the same key-value pairs as the LinkedHashMap.

Note that when you create a WeakHashMap from another map, the keys in the WeakHashMap will be weak references. This means that the keys in the WeakHashMap may be garbage collected if there are no strong references to them. This can be useful if you want to store keys that are only weakly reachable, as it allows the keys to be garbage collected if they are no longer needed.