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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> fun);
  7.  
  8.   @SuppressWarnings("unchecked")
  9.   public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
  10.     return (VALUE) $get(
  11.       (Function<TYPE, Object>) fun
  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> fun, VALUE value) {
  20.     return from(f -> (f == fun)
  21.       ? value
  22.       : get(f)
  23.     );
  24.   }
  25. }
  26.  
  27. @FunctionalInterface
  28. interface $Point extends Point, Selector<Point> {
  29.   public static final Function<Point, Integer> X = Point::getX;
  30.   public static final Function<Point, Integer> Y = Point::getY;
  31.  
  32.   @Override
  33.   public default int getX() {
  34.     return get(X);
  35.   }
  36.  
  37.   @Override
  38.   public default int getY() {
  39.     return get(Y);
  40.   }
  41.  
  42.   @Override
  43.   public default Point withX(int x) {
  44.     return with(X, x);
  45.   }
  46.  
  47.   @Override
  48.   public default Point withY(int y) {
  49.     return with(Y, y);
  50.   }
  51.  
  52.   @Override
  53.   public default Point from(Selector<Point> selector) {
  54.     return ($Point) selector::get;
  55.   }
  56.  
  57.   public static $Point none() {
  58.     return f -> {
  59.       throw new IllegalStateException("no value");
  60.     };
  61.   }
  62. }
  63.  
  64. interface Point {
  65.   public int getX();
  66.   public int getY();
  67.  
  68.   public Point withX(int x);
  69.   public Point withY(int y);
  70.  
  71.   public static Point none() {
  72.     return $Point.none();
  73.   }
  74.  
  75.   public static void main(String[] args) {
  76.     Point point = none()
  77.       .withX(3)
  78.       .withY(4)
  79.     ;
  80.     Stream.of(
  81.       point.getX(),
  82.       point.getY()
  83.     )
  84.       .forEach(System.out::println)
  85.     ;
  86.   }
  87. }
  88.  
  89. public interface Main {
  90.   public static void main(String... arguments) {
  91.     Point.main(arguments);
  92.   }
  93. }
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> fun) {
         ^
Main.java:9: error: = expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
                 ^
Main.java:9: error: ';' expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
                  ^
Main.java:9: error: illegal start of type
  public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
                       ^
Main.java:9: error: = expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
                               ^
Main.java:9: error: illegal start of type
  public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
                                  ^
Main.java:9: error: = expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
                                           ^
Main.java:9: error: <identifier> expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
                                                ^
Main.java:9: error: ';' expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
                                                 ^
Main.java:9: error: illegal start of type
  public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
                                                       ^
Main.java:9: error: = expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
                                                            ^
Main.java:9: error: illegal start of type
  public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
                                                              ^
Main.java:9: error: <identifier> expected
  public default <VALUE> VALUE get(Function<TYPE, VALUE> fun) {
                                                               ^
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>) fun
                             ^
Main.java:11: error: '(' expected
      (Function<TYPE, Object>) fun
                               ^
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> fun, VALUE value) {
         ^
Main.java:24: error: class, interface, or enum expected
  }
  ^
Main.java:29: error: ';' expected
  public static final Function<Point, Integer> X = Point::getX;
                                                        ^
Main.java:29: error: <identifier> expected
  public static final Function<Point, Integer> X = Point::getX;
                                                              ^
Main.java:29: error: ';' expected
  public static final Function<Point, Integer> X = Point::getX;
                                                               ^
Main.java:30: error: ';' expected
  public static final Function<Point, Integer> Y = Point::getY;
                                                        ^
Main.java:30: error: <identifier> expected
  public static final Function<Point, Integer> Y = Point::getY;
                                                              ^
Main.java:30: error: ';' expected
  public static final Function<Point, Integer> Y = Point::getY;
                                                               ^
Main.java:32: error: <identifier> expected
  @Override
           ^
Main.java:33: error: = expected
  public default int getX() {
  ^
Main.java:33: error: ';' expected
  public default int getX() {
        ^
Main.java:38: error: illegal start of type
  public default int getY() {
         ^
Main.java:38: error: = expected
  public default int getY() {
                 ^
Main.java:38: error: ';' expected
  public default int getY() {
                    ^
Main.java:38: error: illegal start of type
  public default int getY() {
                         ^
Main.java:38: error: <identifier> expected
  public default int getY() {
                          ^
Main.java:38: error: = expected
  public default int getY() {
                            ^
Main.java:38: error: ';' expected
  public default int getY() {
                             ^
Main.java:39: error: <identifier> expected
    return get(Y);
              ^
Main.java:39: error: <identifier> expected
    return get(Y);
                ^
Main.java:43: error: class, interface, or enum expected
  public default Point withX(int x) {
         ^
Main.java:45: error: class, interface, or enum expected
  }
  ^
Main.java:48: error: class, interface, or enum expected
  public default Point withY(int y) {
         ^
Main.java:50: error: class, interface, or enum expected
  }
  ^
Main.java:53: error: class, interface, or enum expected
  public default Point from(Selector<Point> selector) {
         ^
Main.java:55: error: class, interface, or enum expected
  }
  ^
Main.java:57: error: class, interface, or enum expected
  public static $Point none() {
                ^
Main.java:60: error: class, interface, or enum expected
    };
    ^
Main.java:61: error: class, interface, or enum expected
  }
  ^
Main.java:84: error: ')' expected
      .forEach(System.out::println)
                         ^
Main.java:84: error: illegal start of expression
      .forEach(System.out::println)
                          ^
Main.java:84: error: ';' expected
      .forEach(System.out::println)
                                  ^
65 errors
stdout
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Standard output is empty
https://ideone.com/GseqZX
language:
Java (HotSpot 12)
created:
10 years ago
visibility:
public

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