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  1. import java.util.function.Function;
  2. import java.util.stream.Stream;
  3.  
  4. @FunctionalInterface
  5. interface Selector<TYPE> {
  6.   Object $get(Function<TYPE, Object> function);
  7.  
  8.   @SuppressWarnings("unchecked")
  9.   public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
  10.     return (VALUE) $get(
  11.       (Function<TYPE, Object>) function
  12.     );
  13.   }
  14.  
  15.   public default TYPE from(Selector<TYPE> selector) {
  16.     throw new InternalError("Selector::from is not implemented.");
  17.   }
  18.  
  19.   public default <VALUE> TYPE with(Function<TYPE, ? extends VALUE> function, VALUE value) {
  20.     return from(f -> (f == function)
  21.       ? value
  22.       : get(f)
  23.     );
  24.   }
  25.  
  26.   public static <TYPE> Object none(Function<TYPE, Object> function) {
  27.     throw new IllegalStateException("no value");
  28.   }
  29. }
  30.  
  31. @FunctionalInterface
  32. interface $Point extends Point, Selector<Point> {
  33.   public static final Function<Point, Integer> X = Point::getX;
  34.   public static final Function<Point, Integer> Y = Point::getY;
  35.  
  36.   @Override
  37.   public default int getX() {
  38.     return get(X);
  39.   }
  40.  
  41.   @Override
  42.   public default int getY() {
  43.     return get(Y);
  44.   }
  45.  
  46.   @Override
  47.   public default Point withX(int x) {
  48.     return with(X, x);
  49.   }
  50.  
  51.   @Override
  52.   public default Point withY(int y) {
  53.     return with(Y, y);
  54.   }
  55.  
  56.   @Override
  57.   public default Point from(Selector<Point> selector) {
  58.     return ($Point) selector::get;
  59.   }
  60.  
  61.   public static $Point none() {
  62.     return Selector::none;
  63.   }
  64. }
  65.  
  66. interface Point {
  67.   public int getX();
  68.   public int getY();
  69.  
  70.   public Point withX(int x);
  71.   public Point withY(int y);
  72.  
  73.   public static Point none() {
  74.     return $Point.none();
  75.   }
  76.  
  77.   public static void main(String[] args) {
  78.     Point point = none()
  79.       .withX(3)
  80.       .withY(4)
  81.     ;
  82.     Stream.of(
  83.       point.getX(),
  84.       point.getY()
  85.     )
  86.       .forEach(System.out::println)
  87.     ;
  88.   }
  89. }
  90.  
  91. public interface Main {
  92.   public static void main(String... arguments) {
  93.     Point.main(arguments);
  94.   }
  95. }
Compilation error #stdin compilation error #stdout 0s 0KB
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Standard input is empty
compilation info 
Main.java:9: error: illegal start of type
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
         ^
Main.java:9: error: = expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
                 ^
Main.java:9: error: ';' expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
                  ^
Main.java:9: error: illegal start of type
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
                       ^
Main.java:9: error: = expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
                               ^
Main.java:9: error: illegal start of type
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
                                  ^
Main.java:9: error: = expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
                                           ^
Main.java:9: error: <identifier> expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
                                                ^
Main.java:9: error: ';' expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
                                                 ^
Main.java:9: error: illegal start of type
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
                                                       ^
Main.java:9: error: = expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
                                                                 ^
Main.java:9: error: illegal start of type
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
                                                                   ^
Main.java:9: error: <identifier> expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> function) {
                                                                    ^
Main.java:10: error: <identifier> expected
    return (VALUE) $get(
                 ^
Main.java:10: error: ';' expected
    return (VALUE) $get(
                  ^
Main.java:11: error: illegal start of type
      (Function<TYPE, Object>) function
                             ^
Main.java:11: error: '(' expected
      (Function<TYPE, Object>) function
                               ^
Main.java:12: error: illegal start of type
    );
    ^
Main.java:12: error: <identifier> expected
    );
     ^
Main.java:13: error: = expected
  }
  ^
Main.java:13: error: ';' expected
  }
   ^
Main.java:15: error: illegal start of type
  public default TYPE from(Selector<TYPE> selector) {
         ^
Main.java:15: error: = expected
  public default TYPE from(Selector<TYPE> selector) {
                      ^
Main.java:15: error: illegal start of type
  public default TYPE from(Selector<TYPE> selector) {
                          ^
Main.java:15: error: = expected
  public default TYPE from(Selector<TYPE> selector) {
                                   ^
Main.java:15: error: <identifier> expected
  public default TYPE from(Selector<TYPE> selector) {
                                        ^
Main.java:15: error: ';' expected
  public default TYPE from(Selector<TYPE> selector) {
                                         ^
Main.java:15: error: illegal start of type
  public default TYPE from(Selector<TYPE> selector) {
                                                  ^
Main.java:15: error: <identifier> expected
  public default TYPE from(Selector<TYPE> selector) {
                                                   ^
Main.java:16: error: = expected
    throw new InternalError("Selector::from is not implemented.");
    ^
Main.java:16: error: ';' expected
    throw new InternalError("Selector::from is not implemented.");
         ^
Main.java:16: error: <identifier> expected
    throw new InternalError("Selector::from is not implemented.");
                           ^
Main.java:16: error: illegal start of type
    throw new InternalError("Selector::from is not implemented.");
                            ^
Main.java:19: error: class, interface, or enum expected
  public default <VALUE> TYPE with(Function<TYPE, ? extends VALUE> function, VALUE value) {
         ^
Main.java:24: error: class, interface, or enum expected
  }
  ^
Main.java:26: error: class, interface, or enum expected
  public static <TYPE> Object none(Function<TYPE, Object> function) {
                ^
Main.java:28: error: class, interface, or enum expected
  }
  ^
Main.java:33: error: ';' expected
  public static final Function<Point, Integer> X = Point::getX;
                                                        ^
Main.java:33: error: <identifier> expected
  public static final Function<Point, Integer> X = Point::getX;
                                                              ^
Main.java:33: error: ';' expected
  public static final Function<Point, Integer> X = Point::getX;
                                                               ^
Main.java:34: error: ';' expected
  public static final Function<Point, Integer> Y = Point::getY;
                                                        ^
Main.java:34: error: <identifier> expected
  public static final Function<Point, Integer> Y = Point::getY;
                                                              ^
Main.java:34: error: ';' expected
  public static final Function<Point, Integer> Y = Point::getY;
                                                               ^
Main.java:36: error: <identifier> expected
  @Override
           ^
Main.java:37: error: = expected
  public default int getX() {
  ^
Main.java:37: error: ';' expected
  public default int getX() {
        ^
Main.java:42: error: illegal start of type
  public default int getY() {
         ^
Main.java:42: error: = expected
  public default int getY() {
                 ^
Main.java:42: error: ';' expected
  public default int getY() {
                    ^
Main.java:42: error: illegal start of type
  public default int getY() {
                         ^
Main.java:42: error: <identifier> expected
  public default int getY() {
                          ^
Main.java:42: error: = expected
  public default int getY() {
                            ^
Main.java:42: error: ';' expected
  public default int getY() {
                             ^
Main.java:43: error: <identifier> expected
    return get(Y);
              ^
Main.java:43: error: <identifier> expected
    return get(Y);
                ^
Main.java:47: error: class, interface, or enum expected
  public default Point withX(int x) {
         ^
Main.java:49: error: class, interface, or enum expected
  }
  ^
Main.java:52: error: class, interface, or enum expected
  public default Point withY(int y) {
         ^
Main.java:54: error: class, interface, or enum expected
  }
  ^
Main.java:57: error: class, interface, or enum expected
  public default Point from(Selector<Point> selector) {
         ^
Main.java:59: error: class, interface, or enum expected
  }
  ^
Main.java:61: error: class, interface, or enum expected
  public static $Point none() {
                ^
Main.java:63: error: class, interface, or enum expected
  }
  ^
Main.java:86: error: ')' expected
      .forEach(System.out::println)
                         ^
Main.java:86: error: illegal start of expression
      .forEach(System.out::println)
                          ^
Main.java:86: error: ';' expected
      .forEach(System.out::println)
                                  ^
66 errors
stdout
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Standard output is empty
https://ideone.com/Rr9OWL
language:
Java (HotSpot 12)
created:
10 years ago
visibility:
public

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