Home › Java exceptions › ClassCastException

What it looks like

java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String

Extends RuntimeException in java.lang.

What it means

A cast such as (String) obj is checked at run time. If the object is not an instance of the target class or one of its subclasses, the JVM throws ClassCastException. The message names both classes, and for classes in different modules or class loaders it also tells you which loader defined each, which is the key to class-loader problems in application servers.

Common causes

• Raw types and unchecked generics: a List<String> that actually holds integers because of heap pollution.
• Deserialising JSON into Object and casting, when Jackson produced a LinkedHashMap instead of your class.
• The same class loaded by two class loaders (a duplicate jar, or hot reload), so the "same" class is not assignable to itself.
• Casting down a hierarchy without checking the type first.

How to fix it

1. Check with instanceof first, and prefer pattern matching: if (o instanceof String s).
2. Avoid raw types and fix unchecked warnings; use typed generics and a TypeReference with Jackson.
3. If the message mentions two loaders, look for duplicate jars on the classpath or in the shared library folder.
4. Redesign with polymorphism so you do not need the cast at all.

Broken and fixed

Object value = cache.get("count");
String text = (String) value; // value is an Integer

if (cache.get("count") instanceof Integer n) {
    String text = String.valueOf(n);
}

🔎 Unravel the failing code 🔎 Unravel the fix

In Spring and enterprise code

A "cannot be cast to class X (X is in unnamed module of loader ...)" message with two different loaders points to a packaging problem, typical with Spring Boot devtools restarts and with shared libraries in Tomcat.

Debug it faster

Paste the whole stack trace into the stack trace beautifier to hide framework frames, jump to the root cause and find the first line of your own code. It runs in your browser, so the trace never leaves your machine.

Related exceptions