The code below does not compile because the initializer block can throw an exception. This is explicitly allowed in the language spec (second edition), sec. 8.6, page 188:

An instance initializer of a named class may not throw a checked
exception unless that exception or one of its superclasses is
explicitly declared in the throws clause of each constructor of its
class and the class has a least one explicitly declared constructor.

The code meets these criteria, but the compiler does not let it pass.


import java.io.*;
 
class TestInitExcept {
 static boolean stuff;
 
 {
  if (stuff)
   throw new IOException("Junk");
 }
 
 TestInitExcept() throws IOException {
 }
 
 public static void main(String[] args) {
  try {
   TestInitExcept t = new TestInitExcept();
  } catch (IOException e) {
   e.printStackTrace();
  }
 }
}