Java Downcasting: When Does It Succeed and When Does It Fail?

Downcasting in Java: Why and When

In Java, upcasting is freely allowed, but downcasting may result in a compile error. This error can be resolved by adding a cast, but it might still trigger a runtime exception. While this seems counterintuitive, downcasting is still permitted in Java for specific scenarios.

Why Java Allows Downcasting

Downcasting is permissible when it has the potential to succeed during runtime. For instance:

Object o = getSomeObject();
String s = (String) o; // Allowed because o may reference a String

When Downcasting May Fail

However, downcasting may fail at runtime when the cast is invalid:

Object o = new Object();
String s = (String) o; // Will fail at runtime as o doesn't reference a String

In this case, the cast will cause a ClassCastException.

When Downcasting Succeeds

On the other hand, downcasting will succeed in some instances:

Object o = "a String";
String s = (String) o; // Will work because o references a String

Compile-Time Errors in Downcasting

Certain casts are forbidden during compilation as they can never succeed:

Integer i = getSomeInteger();
String s = (String) i; // Compiler error as i can never reference a String

In conclusion, downcasting in Java is allowed when it might potentially succeed at runtime. By understanding these scenarios, developers can harness this concept and avoid runtime errors.

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