Collections.checkedCollection in Java
February 04, 2022
Collections.checkedCollection method.
static <E> Collection<E> checkedCollection(Collection<E> c, Class<E> type)
checkedCollection returns a dynamically typesafe view of the specified collection.
2. Any attempt to insert an element of the wrong type will result in an immediate
ClassCastException.
3. The collection returned by
checkedCollection guarantees that the collection cannot contain an incorrectly typed element.
4. Generic mechanism alone cannot guarantee type checking, because it is compile-time type checking. Generic mechanism will fail when we pass our collection to third party library of older version which does not follow type checking.
Example-1
ThecheckedCollection is used as following.
Collection<String> items = Collections.checkedCollection(new ArrayList<>(), String.class);
items.add("Item1");
items.add("Item2");
System.out.println(items);
[Item1, Item2]
Collection<String> items = new ArrayList<>();
Example-2
In this example we will show that a generic collection will fail to keep type safe and we will not get any error.For this, create a generic collection and pass to a method that is not type safe.
CheckedCollection2.java
package com.concretepage;
import java.util.ArrayList;
import java.util.Collection;
public class CheckedCollection2 {
public static void main(String[] args) {
Collection<String> items = new ArrayList<>();
items.add("Item1");
CheckedCollection2.thirdPartyMethod(items);
items.add("Item2");
System.out.println(items);
}
public static void thirdPartyMethod(Collection list) {
list.add(10);
}
}
[Item1, 10, Item2]
Collections.checkedCollection method.
CheckedCollection3.java
package com.concretepage;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
public class CheckedCollection3 {
public static void main(String[] args) {
Collection<String> items = Collections.checkedCollection(new ArrayList<>(), String.class);
items.add("Item1");
CheckedCollection3.thirdPartyMethod(items);
items.add("Item2");
System.out.println(items);
}
public static void thirdPartyMethod(Collection list) {
list.add(10);
}
}
Exception in thread "main" java.lang.ClassCastException: Attempt to insert class java.lang.Integer element into collection with element type class java.lang.String at java.base/java.util.Collections$CheckedCollection.typeCheck(Collections.java:3097)
