Get List of Classes from an object in java

Question

Iam currently trying to create a distinct List<Class> classList which contains all Classes of an object for example

DemoObject.java

public class DemoObject {

    private Integer id;
    private String name;
    private BigDecimal price;
    private Boolean isActive;
    private List<NestedDemoObject> nested;
}

NestedDemoObject.java

public class NestedDemoObject {

    private Integer id;
    private String nameNest;
    private Boolean isActive;
}

What i want to create is a method public List<Class> getDistinctClasses(Class cl); which you give as input for example DemoObject.class and returns a list with

[DemoObject.class, Integer.class, String.class, BigDecimal.class, Boolean.class, List.class, NestedDemoObject.class]

Another example for NestedDemoObject.class would be

[NestedDemoObject.class, Integer.class, String.class, Boolean.class]

I tried to use the .getDeclaredClasses() from Class without any luck. There is any way to get all nested classes from an object with Reflection API?

Any help or direction appreciated.

Answer 1

The solution provided by Mark is partially correct. You're on the right way trying to retrieve the classes from declared fields. However getType() method does not reveal the generic types.

In order to access the generic types you should use Field.getGenericType() instead. It returns the classes as Type objects. The Field objects DO KNOW their own types (they are not erased as one may believe mistakenly). This is a java 1.8+ example printing the types with generics:

Arrays.stream(DemoObject.class.getDeclaredFields())
            .map(Field::getGenericType)
            .map(Type::getTypeName)
            .distinct()
            .forEach(System.out::println);

It will print the following result:

java.lang.Integer
java.lang.String
java.math.BigDecimal
java.lang.Boolean
java.util.List<com.eto.sandbox.NestedDemoObject>

If you want to play with generic types or parse them for any reason then you could use this example:

 Arrays.stream(DemoObject.class.getDeclaredFields())
            .map(Field::getGenericType)
            .distinct()
            .forEach(type -> {
                if (type instanceof Class) {
                    // This is a simple class
                } else if (type instanceof ParameterizedType) {
                    // This is a generic type. You can parse its parameters recursively.
                }
            });