如何使用通用类检查类的有效性

Question

I'm working on code where I have to cast base class onto derived one where I have an array of generic types that are derived by the base on.

For example, I have Base and Derived1, Derived2 and I put them into Class[]{Derived1.class, Derived2.class} and I pass this array to the constructor of the class.

In this constructor, I have to create instances of these derived classes and I don't know how to do that because I get the info that Class and Base are incompatible.

Here is my code example

public abstract class Base {
public abstract Base create(String s);
}

public class Derived extends Base {
java.lang.Integer value;
private static Derived integer = new Derived();

public static Derived getInstance(){
    return integer;
}

public Base create(String s) {
    value = java.lang.Integer.parseInt(s);
    return this;
}
}

public class Clazz {
Class<? extends Base> type;
ArrayList<Base> arrayList;

public Class<? extends Base> getType() {
    return type;
}
}


public class AnotherClazz{

ArrayList<Clazz> clazzArrayList;
Class<? extends Base>[] types;

AnotherClazz(Class<? extends Base>[] args){
    clazzArrayList = new ArrayList<>();
    types = args; // assuming I pass 2 elements in array
    String[] strings = new String[]{"1","2"};
    for (int i=0; i<args.length; ++i){
        if (types[i] instanceof Base){
            // here i want to check validity of class
        }
    }

    for (int i=0; i<strings.length; ++i){
        clazzArrayList.get(i).arrayList.add(((types[i]) Base).getInstance().create(strings[i])); 
    //here i want to create instance of object from type assigned to specific column
    }
}

Thanks for the help.

Answer 1

要检查有效性，请尝试此

if (types[i].getClass().isAssignableFrom(Base.class))

Answer 2

If I read the question correctly, you want to create a few instances of derived classes that all have the same constructor arguments. If that is the case, then you need to give each derived class the same constructor (it does not need to be in the base class) and use Constructor.newInstance(parameters) to create the instances. Further, since you want to ensure that each derived class extents the base class then you will want to use Class.isAssignableFrom(class). For example,

import java.lang.reflect.Constructor;

public class SO52930530 {

    public abstract static class Base {

        public abstract <T> T getValue();
    }

    public static class Derived1 extends Base {

        String value;

        public Derived1(String value) {
            this.value = value;
        }

        public <T> T getValue() {
            return (T) value;
        }
    }

    public static class Derived2 extends Base {

        Integer value;

        public Derived2(String value) {
            this.value = new Integer(value);
        }

        public <T> T getValue() {
            return (T) value;
        }
    }

    public static void main(String... args) throws Exception {

        Class<? extends Base>[] extensions = new Class[]{Derived1.class, Derived2.class};
        String[] values = new String[]{"a", "1"};
        Base[] instances = new Base[values.length];

        for (int i = 0; i < instances.length; i++) {
            Class extension = extensions[i];
            if (Base.class.isAssignableFrom(extension)) {
                Constructor constructor = extension.getConstructor(String.class);
                instances[i] = (Base) constructor.newInstance(values[i]);
            }
        }

        for (int i = 0; i < instances.length; i++) {
            System.out.printf("%d %s %s\n", i, instances[i].getClass(), instances[i].getValue());
        }
    }
}

I hope this helps.

Answer 3

Thanks for helping with checking validity (it works!) but I still don't get this newInstance creation because I have to read data from .csv file and my derived classes are in fact "wrappers" for primitive types like int, float, etc. and I am supposed to create new object using methods getInstance() and create(string s), so it looks like this:

public static class Derived1 extends Base { //Integer wrapper

    Integer value;

    public Derived1(Integer value) {
        this.value = value;
    }

    private static Integer integer = new Integer();

    public static Integer getInstance(){
        return integer;
    }

    private Integer(){};

    public Base create(String s) {
        value = java.lang.Integer.parseInt(s);
        return this;
    }

}

and I don't know how to use Class to cast to appriopriate type.