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

• https://github.com/mtumilowicz/java11-reflection-executables
• java.lang.reflect.Method is used to represent method
  • Class<?> getReturnType()
• Class provides us with four methods to gather information concerning methods:
  • all methods:

    • Method[] getMethods() - returns all public methods of the class or interface (including public methods from superclasses and superinterfaces)

      algorithm: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Class.html#getMethods()

    • Method[] getDeclaredMethods() - returns all methods explicitly declared by the class or interface (not inherited)

  • if we know method's name and parameter types:

    • Method getMethod(String name, Class<?>... parameterTypes) - you can think of this method as a way of trying to find a method in getMethods() by its name and parameters or throwing NoSuchMethodException (if the method cannot be found)

      note that argument type has to be exact

      the algorithm of finding method is quite complex: more info: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Class.html#getMethod(java.lang.String,java.lang.Class...)

      there may be more than one method with matching name and parameter types in a class because while the Java language forbids a class to declare multiple methods with the same signature but different return types, the Java virtual machine does not.

    • Method getDeclaredMethod(String name, Class<?>... parameterTypes) - you can think of this method as a way of trying to find a method in getDeclaredMethods() by its name and parameters or throwing NoSuchMethodException (if the method cannot be found)

We will show how to obtain info about declared methods.

Class structure is as simple as it can be:

• parent

  class Parent implements ParentInterface {
      private long privateParentMethod(String name) {
          return 1;
      }
  
      void packagePrivateParentMethod() {
  
      }
  
      protected String protectedParentMethod(int x1, int x2) {
          return "";
      }
  
      public void publicParentMethod() {
  
      }
      
      public CharSequence override(int count) {
          return "";
      }
  }
  
  interface ParentInterface {
      default void defaultParentInterfaceMethod() {
      }
  }
  

• child

  class Child extends Parent implements ChildInterface {
      private int privateChildMethod(String name) {
          return 1;
      }
      
      void packagePrivateChildMethod() {
          
      }
      
      protected String protectedChildMethod(int x1, int x2) {
          return "";    
      }
      
      public void publicChildMethod() {
          
      }
  
      @Override
      public String override(int count) {
          return "";
      }
  }
  
  interface ChildInterface {
      default void defaultChildInterfaceMethod() {
      }
  }
  

All tests are in MethodReflectionTest class

• getMethods

  var methods = Child.class.getMethods();
  
  assertThat(methods.length, is(15));
  
  var methodsAsString = Arrays.toString(methods);
  
  // Child
  assertThat(methodsAsString, containsString("public java.lang.String Child.override(int)"));
  assertThat(methodsAsString, containsString("public java.lang.CharSequence Child.override(int)"));
  assertThat(methodsAsString, containsString("public void Child.publicChildMethod()"));
  assertThat(methodsAsString, containsString("public default void ChildInterface.defaultChildInterfaceMethod()"));
  
  // Parent
  assertThat(methodsAsString, containsString("public void Parent.publicParentMethod()"));
  assertThat(methodsAsString, containsString("public default void ParentInterface.defaultParentInterfaceMethod()"));
  
  // Object
  assertThat(methodsAsString, containsString("public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException"));
  assertThat(methodsAsString, containsString("public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException"));
  assertThat(methodsAsString, containsString("public final void java.lang.Object.wait() throws java.lang.InterruptedException"));
  assertThat(methodsAsString, containsString("public boolean java.lang.Object.equals(java.lang.Object)"));
  assertThat(methodsAsString, containsString("public java.lang.String java.lang.Object.toString()"));
  assertThat(methodsAsString, containsString("public native int java.lang.Object.hashCode()"));
  assertThat(methodsAsString, containsString("public final native java.lang.Class java.lang.Object.getClass()"));
  assertThat(methodsAsString, containsString("public final native void java.lang.Object.notify()"));
  assertThat(methodsAsString, containsString("public final native void java.lang.Object.notifyAll()"));
  

• getDeclaredMethods

  var methods = Child.class.getDeclaredMethods();
  
  assertThat(methods.length, is(6));
  
  var methodsAsString = Arrays.toString(methods);
  
  assertThat(methodsAsString, containsString("public java.lang.String Child.override(int)"));
  assertThat(methodsAsString, containsString("public java.lang.CharSequence Child.override(int)"));
  assertThat(methodsAsString, containsString("public void Child.publicChildMethod()"));
  assertThat(methodsAsString, containsString("private int Child.privateChildMethod(java.lang.String)"));
  assertThat(methodsAsString, containsString("void Child.packagePrivateChildMethod()"));
  assertThat(methodsAsString, containsString("protected java.lang.String Child.protectedChildMethod(int,int)"));
  

• get method by name and params
  • not found - NoSuchMethodException

    @Test(expected = NoSuchMethodException.class)
    public void getMethod_notFound() throws NoSuchMethodException {
        Child.class.getMethod("not exists");
    }
    

  • private - NoSuchMethodException

    @Test(expected = NoSuchMethodException.class)
    public void getMethod_private() throws NoSuchMethodException {
        Child.class.getMethod("privateChildMethod", String.class);
    }
    

  • public

    assertThat(Child.class.getMethod("publicChildMethod").toGenericString(),
            is("public void Child.publicChildMethod()"));
    

  • from parent

    assertThat(Child.class.getMethod("publicParentMethod").toGenericString(),
            is("public void Parent.publicParentMethod()"));
    

• get declared method by name and params
  • not found - NoSuchMethodException

    @Test(expected = NoSuchMethodException.class)
    public void getDeclaredMethod_notFound() throws NoSuchMethodException {
        Child.class.getDeclaredMethod("not exists");
    }
    

  • private

    assertThat(Child.class.getDeclaredMethod("privateChildMethod", String.class).toGenericString(),
            is("private int Child.privateChildMethod(java.lang.String)"));
    

  • argument must be exact

    @Test(expected = NoSuchMethodException.class)
    public void getMethod_public_byObjectParam() throws NoSuchMethodException {
        assertThat(Child.class.getDeclaredMethod("privateChildMethod", Object.class).toGenericString(),
                is("private int Child.privateChildMethod(java.lang.String)"));
    }
    

  • from parent

    @Test(expected = NoSuchMethodException.class)
    public void getDeclaredMethod_parent() throws NoSuchMethodException {
        Child.class.getDeclaredMethod("publicParentMethod");
    }