According to the JVM spec, class files can have several fields with the same name. As long as they have different types, they will be distinguished since all field operations specify both the name and the field of the field. Sun's JVM accepts such classes, and correctly distinguishes the fields, as you can see in the attached testcase. Here is the disassembly of this class:
Access flags: 0x20 super This class: 2=DuplicateField, super: 4=java.lang.Object Interfaces (count: 0): Fields (count: 2): Field name:"foo" static Signature: 6=java.lang.String Field name:"foo" static Signature: 7=int Methods (count: 3): Method name:"<init>" Signature: 9=()void Attribute "Code", length:17, max_stack:1, max_locals:1, code_length:5 0: aload_0 1: invokespecial #11=<Method java.lang.Object.<init> ()void> 4: return Method name:"main" public static Signature: 14=(java.lang.String[])void Attribute "Code", length:31, max_stack:2, max_locals:1, code_length:19 0: getstatic #20=<Field java.lang.System.out java.io.PrintStream> 3: getstatic #22=<Field DuplicateField.foo java.lang.String> 6: invokevirtual #28=<Method java.io.PrintStream.println (java.lang.String)void> 9: getstatic #20=<Field java.lang.System.out java.io.PrintStream> 12: getstatic #30=<Field DuplicateField.foo int> 15: invokevirtual #33=<Method java.io.PrintStream.println (int)void> 18: return Method name:"<clinit>" static Signature: 9=()void Attribute "Code", length:24, max_stack:1, max_locals:0, code_length:12 0: ldc #36=<String "WOW"> 2: putstatic #22=<Field DuplicateField.foo java.lang.String> 5: sipush 42 8: putstatic #30=<Field DuplicateField.foo int> 11: return Sun's JVM correctly executes that program: $ java DuplicateField WOW 42 gcj fails during compilation with: $ gcj DuplicateField.class DuplicateField.java: In class 'DuplicateField': DuplicateField.java: In method 'DuplicateField.main(java.lang.String[])': DuplicateField.java:0: error: mismatching signature for field 'foo' in 'DuplicateField' DuplicateField.java: In method 'DuplicateField.<clinit>()': DuplicateField.java:0: error: mismatching signature for field 'foo' in 'DuplicateField' Surprisingly, gij also fails with: Exception in thread "main" java.lang.NullPointerException at java.io.PrintStream.println(java.lang.String) (/tmp/gcc/lib/libgcj.so.6.0.0) at DuplicateField.main(java.lang.String[]) (Unknown Source) at gnu.java.lang.MainThread.call_main() (/tmp/gcc/lib/libgcj.so.6.0.0) at gnu.java.lang.MainThread.run() (/tmp/gcc/lib/libgcj.so.6.0.0) gcj (GCC) 4.0.0 20050223 -- Summary: gcj does not accept classes with same name fields Product: gcc Version: 4.0.0 Status: UNCONFIRMED Severity: normal Priority: P2 Component: java AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: bonniot at users dot sf dot net CC: gcc-bugs at gcc dot gnu dot org,java-prs at gcc dot gnu dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20215