frog:~/workspaces/test/Generic9 $ cat -n T.java
     1 import java.io.*;
     2
     3 abstract class AChurchBoolean {
     4 public abstract <Return, Parameter, Throws extends Throwable>
     5 Return accept(IVisitor<Return, Parameter, Throws> visitor, Parameter parameter) throws Throws;
     6
     7 public interface IVisitor<Return, Parameter, Throws extends Throwable> {
     8 public Return caseTrue(Parameter parameter) throws Throws;
     9 public Return caseFalse(Parameter parameter) throws Throws;
    10 }
    11 }
    12
    13 class TrueChurchBoolean extends AChurchBoolean {
    14 private static TrueChurchBoolean instance = new TrueChurchBoolean();
    15 private TrueChurchBoolean() {}
    16 public static TrueChurchBoolean singleton() {
    17 return instance;
    18 }
    19 public <Return, Parameter, Throws extends Throwable>
    20 Return accept(IVisitor<Return, Parameter, Throws> visitor, Parameter parameter) throws Throws {
    21 return visitor.caseTrue(parameter);
    22 }
    23 }
    24
    25 class FalseChurchBoolean extends AChurchBoolean {
    26 private static FalseChurchBoolean instance = new FalseChurchBoolean();
    27 private FalseChurchBoolean() {}
    28 public static FalseChurchBoolean singleton() {
    29 return instance;
    30 }
    31 public <Return, Parameter, Throws extends Throwable>
    32 Return accept(IVisitor<Return, Parameter, Throws> visitor, Parameter parameter) throws Throws {
    33 return visitor.caseFalse(parameter);
    34 }
    35 }
    36
    37 class Pair<T,U> {
    38 private T first;
    39 private U second;
    40 Pair(T first, U second) {
    41 this.first = first;
    42 this.second = second;
    43 }
    44 T getFirst() {
    45 return first;
    46 }
    47 U getSecond() {
    48 return second;
    49 }
    50 }
    51
    52 // Perhaps a bit of a toy example, but relevant nonetheless.
    53 class ChurchBooleanTest {
    54 private AChurchBoolean bool;
    55 public ChurchBooleanTest(AChurchBoolean bool) {
    56 this.bool = bool;
    57 }
    58 public AChurchBoolean readIf(File file, byte[] output) throws IOException {
    59 return bool.accept(new AChurchBoolean.IVisitor<AChurchBoolean, Pair<File, byte[]>, IOException>() {
    60 public AChurchBoolean caseTrue(Pair<File, byte[]> parameter) throws IOException {
    61 FileInputStream input = new FileInputStream(parameter.getFirst()); // throws
    62 input.read(parameter.getSecond()); // throws
    63 input.close(); // throws
    64 return TrueChurchBoolean.singleton();
    65 }
    66 public AChurchBoolean caseFalse(Pair<File, byte[]> parameter) throws IOException {
    67 return FalseChurchBoolean.singleton();
    68 }
    69 }, new Pair<File, byte[]>(file, output));
    70 }
    71 }
frog:~/workspaces/test/Generic9 $ newjavac -gj T.java
T.java:20: accept(AChurchBoolean.IVisitor<Return,Parameter,Throws>,Parameter) in TrueChurchBoolean cannot override accept(AChurchBoolean.IVisitor<Return,Parameter,Throws>,Parameter) in AChurchBoolean; overridden method does not throw Throws
    Return accept(IVisitor<Return, Parameter, Throws> visitor, Parameter parameter) throws Throws {
           ^
T.java:32: accept(AChurchBoolean.IVisitor<Return,Parameter,Throws>,Parameter) in FalseChurchBoolean cannot override accept(AChurchBoolean.IVisitor<Return,Parameter,Throws>,Parameter) in AChurchBoolean; overridden method does not throw Throws
    Return accept(IVisitor<Return, Parameter, Throws> visitor, Parameter parameter) throws Throws {
           ^
2 errors
frog:~/workspaces/test/Generic9 $