A supclass of a nonabstract superclass cannot be abstract
时间: 2024-04-04 22:34:16 浏览: 8
这是一个Java编程语言中的规则,指的是一个非抽象的父类的子类不能被声明为抽象类。因为一个抽象类中必须包含至少一个抽象方法,而子类会继承其父类的所有非私有方法,如果一个子类被声明为抽象类,但是没有重写其父类的所有抽象方法,那么这个子类就不能被实例化,这与Java的面向对象编程的思想不符。因此,如果一个子类想要被声明为抽象类,那么它的父类必须也是一个抽象类。
相关问题
if no superclass is specified when a class is define
If no superclass is specified when a class is defined in Java, it implicitly becomes a subclass of the `Object` class. This means that every class in Java is either a direct or an indirect subclass of the `Object` class, which is the root of the class hierarchy.
The `Object` class provides some common methods that are inherited by all classes, such as `equals()`, `hashCode()`, and `toString()`. For example, if a class does not explicitly override the `toString()` method, it will inherit the implementation of the `toString()` method from the `Object` class, which returns a string representation of the object's memory address.
Here is an example of a class that does not specify a superclass:
```java
public class MyClass {
// class code
}
```
In this example, `MyClass` does not extend any other class, so it implicitly extends the `Object` class. Therefore, it inherits the methods of the `Object` class and can be treated as an object in Java.
Can we override abstract method?
Yes, we can override an abstract method in its concrete subclass, by providing the implementation for the abstract method.
When a class extends an abstract class, it must provide the implementation for all the abstract methods declared in the abstract class. If a subclass does not provide an implementation for any of the abstract methods, it must also be declared as abstract.
When a subclass overrides an abstract method, it must provide an implementation that adheres to the method signature declared in the abstract class. The implementation in the subclass can use the same or a more permissive access modifier than the abstract method in the superclass. However, it cannot use a less permissive access modifier.
For example, consider the following abstract class that declares an abstract method:
```
public abstract class Shape {
public abstract double area();
}
```
A concrete subclass of Shape, such as Square, must provide an implementation for the area() method:
```
public class Square extends Shape {
private double side;
public Square(double side) {
this.side = side;
}
public double area() {
return side * side;
}
}
```
In this example, the Square class extends the abstract class Shape and provides an implementation for the area() method by calculating the area of a square.