Is it possible to tell the compiler that a method always throws an Exception

Question

Generally speaking, the Java compiler does not propagate the information that a method "always" throw an Exception, and therefore, does not detect that all code paths are complete.

(This is due to the fact that Java compiler compiles each class independently).

It's a problem when you want to write something like that.

public class ErrorContext {     public void fatalISE(String message) {         String context = "gather lots of information about the context of the error";         throw new IllegalStateException(context +": " + message);     } }  public class A {     public MyObject myMethod() {         if (allIsGood()) {             return new MyObject();         }         ErrorContext.fatalISE("all is not good");     } } 

(ie, a kind of "assertion helper" that gathers context information).

Because the compiler will complain that myMethod does not always return a MyObject.

To my knowledge, there is no specific annotation to indicate that a method always throws.

Answer 1

A simple workaround is to let your fatalISE method not throw the exception, but only create it:

public class ErrorContext {     public IllegalStateException fatalISE(String message) {         String context = "gather lots of information about the context of the error";         return new IllegalStateException(context +": " + message);     } }  public class A {     public MyObject myMethod() {         if (allIsGood()) {             return new MyObject();         }         throw ErrorContext.fatalISE("all is not good");     } } 

This way the compiler will know not to complain about a missing return. And forgetting to use the throw is unlikely, exactly because the compiler will usually complain.

Answer 2

A trick that I use is to replace

public void fatalISE(String message) {     String context = "gather lots of information about the context of the error";     throw new IllegalStateException(context +": " + message); } 

with

public <T> T fatalISE(String message) {     String context = "gather lots of information about the context of the error";     throw new IllegalStateException(context +": " + message); } 

Then, in myMethod, use:

public MyObject myMethod() {    if (allIsGood()) {         return new MyObject();     }     return ErrorContext.fatalISE("all is not good"); } 

It will work whatever the return type of myMethod, including primitive types. You can still use fatalISE in a void method, by just not using the returnkeyword.