java - “实例化”和“子类”之间的区别

Question

我正在阅读 Java 教程，它说：

抽象类不能被实例化，但它们可以被子类化。

这是什么意思？我认为必须实例化才能创建子类？这条线真的让我很困惑，非常感谢任何和所有的帮助。

Answer 1

实例化：

AbstractClass a = new AbstractClass(); //illegal

子类：

class ConcreteClass extends AbstractClass { ... }
ConcreteClass c = new ConcreteClass(); //legal

您必须创建一个扩展抽象类的新类，实现所有抽象方法，然后使用该新类。

请注意，您也可以这样做：

class ConcreteClass extends AbstractClass { ... }
AbstractClass a = new ConcreteClass(); //legal

Answer 2

子类可以获取其父类具有的所有属性/方法，而实例化类是在您在内存中创建该父类的实例时。

Answer 3

当你说实例化时，它意味着你想创建类的对象。子类是继承的孩子。例如，

abstract class A{
  //Class A is abstract hence, can not be instantiated. The reason being abstract class provides the layout of how the concrete sub-class should behave. 

   pubic abstract void doSomething();
   //This abstract method doSomething is required to be implemented via all the sub-classes. The sub-class B and C implement this method as required.

}

class B extends A{
   //Class B is subclass of A
  public void doSomething(){ System.out.println("I am class B"); }
}
class C extends A{
   //Class C is subclass of A
  public void doSomething(){ System.out.println("I am class C"); }
}

if you try to do this, it would generate an exception

A a = new A();

But this would work fine.
B b = new B();
or 
A a = new B(); //Note you are not instantiating A, here class A variable is referencing the instance of class B