Type Casting Compile and Runtime Error

Java Tutorials - Herong's Tutorial Examples

∟Type Casting Compile and Runtime Error

This section provides a tutorial example on compilation time casting error and runtime casting exception.

If you are trying to convert reference type to another type that is not compatible, you will get a compiler error. Here is a simple example:

/* TypeCastingCompileError.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.util.*;
class TypeCastingCompileError {
   public static void main(String[] a) {

      // Compiler error: Casting from Integer to Long is not allowed
      Integer myInteger = new Integer(777);
      Long myLong = (Long) myInteger;
   }
}

Try to compile it. You will see a compilation error:

TypeCastingCompileError.java:10: error: incompatible types: Integer
cannot be converted to Long
      Long myLong = (Long) myInteger;
                           ^
Note: TypeCastingCompileError.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
1 error

If you want, you can avoid the compiler error by casting Integer to Object, before casting it to Long. But you will get a runtime error. Here is a simple example:

/* TypeCastingRuntimeError.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
class TypeCastingRuntimeError {
   public static void main(String[] a) {

      try {
         // Runtime error: Casting from Integer to Long is not allowed
         Integer myInteger = Integer.valueOf(777);
         Object myObject = (Object) myInteger;
         Long myLong = (Long) myObject;
      } catch (Exception e) {
         e.printStackTrace();
      }

      try {
         // Runtime error: Casting from Integer to Long is not allowed
         Integer hisInteger = Integer.valueOf(888);
         Number hisNumber = (Number) hisInteger;
         Long hisLong = (Long) hisNumber;
      } catch (Exception e) {
         e.printStackTrace();
      }
   }
}

Try to compile and run it with JDK 13:

java.lang.ClassCastException: java.base/java.lang.Integer cannot be cast
to java.base/java.lang.Long
        at TypeCastingRuntimeError.main(TypeCastingRuntimeError.java:11)
java.lang.ClassCastException: java.base/java.lang.Integer cannot be cast
to java.base/java.lang.Long
        at TypeCastingRuntimeError.main(TypeCastingRuntimeError.java:20)

Try to compile and run it with JDK 8:

java.lang.ClassCastException:
   java.lang.Integer cannot be cast to java.lang.Long
        at TypeCastingRuntimeError.main(TypeCastingRuntimeError.java:11)

java.lang.ClassCastException:
   java.lang.Integer cannot be cast to java.lang.Long
        at TypeCastingRuntimeError.main(TypeCastingRuntimeError.java:20)

As you can see from this example, the compiler fails to detect the type casting error, because down casting "(Long) Object" or "(Long) Number" is a valid casting if the reference object is a Long object.