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  1. import java.lang.reflect.Constructor;
  2. import java.util.Arrays;
  3. import java.util.List;
  4. import java.util.Objects;
  5. import java.util.Optional;
  6. import java.util.stream.Collectors;
  7.  
  8. class Ideone {
  9.   public static void main(String[] args) {
  10.     System.out.println(
  11.         getConstructorsFromTypeFittingActualParameters(Bar.class, Baz.class).size());
  12.     System.out.println(
  13.         getConstructorsFromTypeFittingActualParameters(Bar.class).size());
  14.     System.out.println(
  15.         getConstructorsFromTypeFittingActualParameters(Bar.class, String.class).size());
  16.     System.out.println(
  17.         getConstructorsFromTypeFittingActualParameters(Bar.class, Object.class).size());
  18.   }
  19.  
  20.   public static List<Constructor<?>> getConstructorsFromTypeFittingActualParameters(
  21.       Class<?> type,
  22.       Class<?>... actualParameters) {
  23.     final Class<?>[] actualParametersNotNull = 
  24.         Optional.ofNullable(actualParameters).orElseGet(() -> new Class<?>[0]);
  25.     return Arrays.stream(type.getConstructors())
  26.         .filter(constructor -> constructor.getParameterCount() == actualParametersNotNull.length)
  27.         .filter(constructor -> formalParameterTypesAcceptActualParameterTypes(
  28.             constructor.getParameterTypes(),
  29.             actualParametersNotNull))
  30.         .collect(Collectors.toList());
  31.   }
  32.  
  33.   private static boolean formalParameterTypesAcceptActualParameterTypes(
  34.       Class<?>[] formalParameterTypes,
  35.       Class<?>[] actualParameterTypes) {
  36.     Objects.requireNonNull(formalParameterTypes);
  37.     Objects.requireNonNull(actualParameterTypes);
  38.     if (formalParameterTypes.length != actualParameterTypes.length) {
  39.       throw new IllegalArgumentException();
  40.     }
  41.  
  42.     for (int index = 0; index < formalParameterTypes.length; ++index) {
  43.       if (!formalParameterTypes[index].isAssignableFrom(actualParameterTypes[index])) {
  44.         return false;
  45.       }
  46.     }
  47.     return true;
  48.   }
  49. }
  50.  
  51. interface Foo {}
  52.  
  53. class Bar {
  54.   public Bar() {}
  55.  
  56.   public Bar(Bar other) {}
  57.  
  58.   public Bar(Foo other) {}
  59.  
  60.   public Bar(String other) {}
  61. }
  62.  
  63. class Baz extends Bar implements Foo {}
Success #stdin #stdout 0.09s 48508KB
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https://ideone.com/uVfwQ1
language:
Java (HotSpot 12)
created:
4 years ago
visibility:
public

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