Java中的参数类

Question

I'm a little bit confused about this code, it is a test to understand the parametric class.

I have 2 classes, one is the Main class and the other is the parametric class. When I create a new Object with Integer as a parameter and I think into the constructor will call the print(Integer x) method, but it is not like this, instead java will call the print(Object o) method.

Someone of you know why it happened?

package classi.parametriche;

public class Contenitore <E> {
public E variabile;

public Contenitore(E value){
    variabile = value;
    System.out.println("Variabilie : "+variabile.getClass().toString());
    System.out.println("Variabile : "+value.getClass().toString());
    println(variabile);
}
public void println(String s){
    System.out.println("Stringa : "+ s);
}
public void println(Integer x){
    System.out.println("Int : " + x);
}
public void println(short x){
    System.out.println("short : " + x);
}
public void println(byte x){
    System.out.println("byte : " + x);
}
public void println(long x){
    System.out.println("long : " + x);
}
public void println(char x){
    System.out.println("char : " + x);
}
public void println(float x){
    System.out.println("float : " + x);
}
public void println(double x){
    System.out.println("double : " + x);
}
public void println(Object o){
    if (o != null){
        System.out.println("Object : " + o.toString());
    }else{
        System.out.println("null");
    }
}

}

public class ClassiParametriche {
    public static void main(String[] args) {
        Contenitore<Integer> c = new Contenitore <Integer>(42);

    }
}

this is the result :

 run:
 Variabilie : class java.lang.Integer
 Variabile : class java.lang.Integer
 Object : 42

Answer 1

This is because of type erasure and compile time binding.

The compiler decides at compile time which of the println methods is called. Because the field of your class is a generic with no defined super class the remaining type after type erasure is Object. This means the field variable is internally handled as an Object.

You can also check this post for details.

Answer 2

The answer is type-erasure. At run-time the class has no parameter type. Generics in Java is purely compile-time sugar. Once the code is compiled, it loses any knowledge of the associated parameter type. So when you pass the Integer into the constructor, the constructor is actually expecting a variable of type java.lang.Object