CODEWITHSUNDEEP
javagenericsinstanceoftype-erasure
lviggiani
lviggiani

Reputation: 6086

Check if an object is instance of List of given class name

Given an Object o and a String className = "org.foo.Foo", I want to check if o is instance of List<className>

I tried this but won't compile:

Class<?> cls = Class.forName(className);
if (o instanceof List<cls>){ // this gives error: cls cannot be resolved to a type
  doSomething();
}

Please note that my inputs are Object o and String className (please mind types).

Upvotes: 17

Views: 71571

Answers (6)

崔乐天
崔乐天

Reputation: 41

if what you want to do is like this:

class XXX implements InterfaceX {
    @Override
    public void methedX() { }
}

interface InterfaceX {
    void methedX();
}
List<XXX> x = new ArrayList<>();

if(x instanceOf List<InterfaceX>) {
    data.get(0).methedX();
}

you may try to use template like:

class TypeHelper <T extends InterfaceX> {
    List<T> data;
    TypeHelper (List<T> data) {
        this.data = data;
    }

    public void func() {
        data.get(0).methedX();
    }
}

TypeHelper helper = new TypeHelper<XXX>(x);
helper.func();

Upvotes: 0

Vasanth
Vasanth

Reputation: 6385

Solution i used in my project.

/**
 * Used to get List of String From Object.
 *
 * 1. used to convert object to list of String.
 * 
 * @param objectList Object List.
 * @return List Of String.
 */
private List<String> getListOfStringFromObject(final Object objectList) {
    List<String> stringList = new ArrayList<>();
    if (objectList instanceof List<?>) {
        for (Object object : (List<?>) objectList) {
            if (object instanceof String) {
                stringList.add((String) object);
            }
        }
    }
    return stringList;
}

Upvotes: 6

lviggiani
lviggiani

Reputation: 6086

Ok, I found a method... it's a little dirty code but it works:

if (o instanceof List<?>){
  ParameterizedType pt = (ParameterizedType)o.getClass().getGenericSuperclass();
  String innerClass = pt.getActualTypeArguments()[0].toString().replace("class ", "");
  System.out.println(innerClass.equals(className)); // true
}

Upvotes: 6

Mansueli
Mansueli

Reputation: 7034

public class Main{

    public static void main(String[] args){
        String str = "";
        int oi = 0;
        char c = 'a';
        Main af = new Main();
        checkType(str);
        checkType(oi);
        checkType(c);
        checkType(af);
    }

    public static void checkType(Object o){
        String str = o.getClass().getName();
        switch(str){
            case "java.lang.String":
                System.out.println("String");
                break;  
            case "java.lang.Integer":
                System.out.println("Integer");
                break;
            case "java.lang.Character":
                System.out.println("Char");
                break;
            case "Main":
                System.out.println("Main");
                break;
            default :
                System.out.println("Something else:" + str);
                break;
        }
    }

}

Note> just change to public static boolean checkType(Object o) and then return true in each case that isn't default.

Upvotes: 2

Fabien Fleureau
Fabien Fleureau

Reputation: 693

I think you can do it in two steps: First, you check it's a List.

if (o instanceof List)

Then, you check that one (each?) member of the list has the given type.

for (Object obj : (List) o) {
    if (obj instanceof cls) {
        doSomething();
    }
}

Upvotes: 11

Konstantin Yovkov
Konstantin Yovkov

Reputation: 62884

It's because of Type Erasure. The statement

if (o instanceof List<cls>) {
  doSomething();
}

will be executed at runtime, when the generic type of the list will be erased. Therefore, there's no point of checking for instanceof generic-type.

Upvotes: 11

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