Using reflection to get information about the constructors of a class.

• https://github.com/mtumilowicz/java11-reflection-executables
• java.lang.reflect.Constructor is used to represent method
• Class provides us with four methods to gather information concerning constructors:
  • all constructors:
    • Constructor[] getConstructors() - returns all public constructors
    • Constructor[] getDeclaredConstructors() - returns all declared constructors
  • constructor by parameters
    • Constructor getConstructor() - you can think of this method as a way of trying to find a constructor in getConstructors() by parameters or throwing NoSuchMethodException (if the constructor cannot be found)
    • Constructor getDeclaredConstructor() - you can think of this method as a way of trying to find a constructor in getDeclaredConstructors() by parameters or throwing NoSuchMethodException (if the constructor cannot be found)

Class structure:

class Child extends Parent {
    private Child() {

    }

    Child(String name) {

    }

    protected Child(int count) {

    }

    public Child(String name, int count) {

    }
}


class Parent {
    public Parent() {

    }
    
    public Parent(String s1, String s2, String s3) {
        
    }
}

All tests are in ConstructorReflectionTest

• getConstructors

  var constructors = Child.class.getConstructors();
  
  assertThat(constructors.length, is(1));
  
  var constructorsAsString = Arrays.toString(constructors);
  assertThat(constructorsAsString, containsString("public Child(java.lang.String,int)"));
  

• getDeclaredConstructors

  var constructors = Child.class.getDeclaredConstructors();
  
  assertThat(constructors.length, is(4));
  
  var constructorsAsString = Arrays.toString(constructors);
  assertThat(constructorsAsString, containsString("public Child(java.lang.String,int)"));
  assertThat(constructorsAsString, containsString("protected Child(int)"));
  assertThat(constructorsAsString, containsString("Child(java.lang.String)"));
  assertThat(constructorsAsString, containsString("private Child()"));
  

• public constructor by params
  • not found - NoSuchMethodException

    @Test(expected = NoSuchMethodException.class)
    public void getConstructor_notFound() throws NoSuchMethodException {
        Child.class.getConstructor(int.class, int.class);
    }
    

  • not public - NoSuchMethodException

    @Test(expected = NoSuchMethodException.class)
    public void getConstructor_private() throws NoSuchMethodException {
        Child.class.getConstructor();
    }
    

  • from parent - NoSuchMethodException

    @Test(expected = NoSuchMethodException.class)
    public void getConstructor_fromParent() throws NoSuchMethodException {
        Child.class.getConstructor(String.class, String.class, String.class);
    }
    

  • public

    assertThat(Child.class.getConstructor(String.class, int.class).toGenericString(),
            is("public Child(java.lang.String,int)"));
    

• declared constructor by params
  • not found - NoSuchMethodException

    @Test(expected = NoSuchMethodException.class)
    public void getDeclaredConstructor_notFound() throws NoSuchMethodException {
        Child.class.getDeclaredConstructor(int.class, int.class);
    }
    

  • private

    assertThat(Child.class.getDeclaredConstructor().toGenericString(),
            is("private Child()"));
    

  • public

    assertThat(Child.class.getDeclaredConstructor(String.class, int.class).toGenericString(),
            is("public Child(java.lang.String,int)"));