type parameter with restriction

? extends Xは、?そのものか、サブクラスを意味する
T extends Aは、AかAのサブクラス（以下の例ではB)

class A{
    void method(){
        System.out.println("A");
    };
}
class B extends A{
    @Override
    void method(){
        System.out.println("B");
    };
}
class C {
    void method(){};
}
class X <T extends A>{
    void method(T t) {
        t.method();
    }
}
public class Outer {
    public static void main(String args[]) {
        X<A> x1 = new X();
        x1.method(new A());
        x1.method(new B());
        X<B> x2 = new X();
        x2.method(new B());
//        X<C> x3 = new X();
    }
}

A
B
B

AのサブクラスではないCをtype parameterに指定するとcompile error

        X<C> x3 = new X();

-------------------------------------------------------------
COMPILATION ERROR : 
-------------------------------------------------------------
com/mycompany/mavenproject1/Outer.java:[37,11] 型引数com.mycompany.mavenproject1.Cは型変数Tの境界内にありません
1 error

List,ArrayListを使ったサンプル
declare side, set sideのgenericsは一致しなければならない

class Sample extends Number{

    @Override
    public int intValue() {
        throw new UnsupportedOperationException("Not supported yet."); // Generated from nbfs://nbhost/SystemFileSystem/Templates/Classes/Code/GeneratedMethodBody
    }

    @Override
    public long longValue() {
        throw new UnsupportedOperationException("Not supported yet."); // Generated from nbfs://nbhost/SystemFileSystem/Templates/Classes/Code/GeneratedMethodBody
    }

    @Override
    public float floatValue() {
        throw new UnsupportedOperationException("Not supported yet."); // Generated from nbfs://nbhost/SystemFileSystem/Templates/Classes/Code/GeneratedMethodBody
    }

    @Override
    public double doubleValue() {
        throw new UnsupportedOperationException("Not supported yet."); // Generated from nbfs://nbhost/SystemFileSystem/Templates/Classes/Code/GeneratedMethodBody
    }
}
public class SampleProject {
    public static void main(String[] args) {
        List<Number> l = new ArrayList<Number>();
        List<? extends Number> l2 = new ArrayList<Number>();
        List<? super Number> l3 = new ArrayList<Number>();
        List<Sample> l4 = new ArrayList<Number>();
        List<Object> l5 = new ArrayList<Number>();
    }
}

C:\Users\java\Documents\NetBeansProjects\SampleProject\src\sampleproject\SampleProject.java:36: エラー: 不適合な型: ArrayList<Number>をList<Sample>に変換できません:
        List<Sample> l4 = new ArrayList<Number>();
C:\Users\java\Documents\NetBeansProjects\SampleProject\src\sampleproject\SampleProject.java:37: エラー: 不適合な型: ArrayList<Number>をList<Object>に変換できません:
        List<Object> l5 = new ArrayList<Number>();