Source of Java ClassCastException sometimes can be hard to find. In such situation type safe List may be really helpful. In this tutorial you will learn how to find ClassCastException using Java type safe List!

The following, simplified, Java example shows the ClassCastException issue and how Collections.checkedList(List, type) helps to solve it:

package com.farenda.java;

import java.util.Collections;
import java.util.LinkedList;
import java.util.List;

public class CollectionsCheckedList {

    public static void main(String[] args) {
        List<String> items = new LinkedList<>();
        Collections.addAll(items, "java", "clojure", "python", "scala");
        System.out.println("Items: " + items);

        addItemInsecurely(items);
        System.out.println("Insecure items: " + items);

        List<String> typeSafe = Collections.checkedList(items, String.class);
        addItemInsecurely(typeSafe);
        System.out.println("No new insecure items!");
    }

    // Add by reflection, 3rd party lib or other insecure way:
    private static void addItemInsecurely(List items) {
        int wrongTypeItem = items.size();
        System.out.println("Adding item: " + wrongTypeItem);
        items.add(wrongTypeItem);
    }
}

In the example we expect a list to be of type String, and then add an integer there is some insecure way. It often happens when a bunch of frameworks are used. This may result in adding an element of different type and ClassCastException may occur much more later in execution flow and be hard to diagnose. Using Collections.checkedList() we can trigger ClassCastException as soon as item of wrong type is inserted into the collection!

As you can seed, when run, the above Java code immediately prevents addition of unsafe element:

Items: [[java, clojure, python, scala]]
Adding item: 4
Insecure items: [[java, clojure, python, scala, 4]]
Adding item: 5
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.util.Collections$CheckedCollection.typeCheck(Collections.java:3037)
      at java.util.Collections$CheckedCollection.add(Collections.java:3080)
      at com.farenda.java.CollectionsCheckedList.addItemInsecurely(CollectionsCheckedList.java:26)
      at com.farenda.java.CollectionsCheckedList.main(CollectionsCheckedList.java:18)

java.util.Collections has checked methods for different types of Java collections:

• Collection<E> checkedCollection(Collection<E> c, Class<E> type)
• Queue<E> checkedQueue(Queue<E> queue, Class<E> type) since Java 8
• Set<E> checkedSet(Set<E> s, Class<E> type)
• SortedSet<E> checkedSortedSet(SortedSet<E> s, Class<E> type)
• NavigableSet<E> checkedNavigableSet(NavigableSet<E> s, Class<E> type) since Java 8
• List<E> checkedList(List<E> list, Class<E> type)
• Map<K,V> checkedMap(Map<K,V> m, Class<K> keyType, Class<V> valueType)
• SortedMap<K,V> checkedSortedMap(SortedMap<K,V> m, Class<K> keyType, Class<V> valueType)
• NavigableMap<K,V> checkedNavigableMap(NavigableMap<K,V> m, Class<K> keyType, Class<V> valueType) since Java 8

They all work in the same way and can be really helpful. Simple and may save the day! ;-)