Interfaces may have member variables, but these are implicitly public, static, and final- in other words, interfaces can declare only constants, not instance variables that are available to all implementations and may be used as key references for method arguments for example.
Yes, it is always necessary to create an object implementation for an interface. Interfaces cannot be instantiated in their own right, so you must write a class that implements the interface and fulfill all the methods defined in it.
You can't instantiate an interface directly, but you can instantiate a class that implements an interface.
An interface is a description of a set of methods that conforming implementing classes must have.
Note:
You can't mark an interface as final.
Interface methods must be static.
An Interface cannot extend anything but another interfaces.
To prevent a specific method from being overridden in a subclass, use the final modifier on the method declaration, which means "this is the final implementation of this method", the end of its inheritance hierarchy.
public final void exampleMethod() {
// Method statements
}
Super is a keyword which is used to access the method or member variables from the superclass. If a method hides one of the member variables in its superclass, the method can refer to the hidden variable through the use of the super keyword. In the same way, if a method overrides one of the methods in its superclass, the method can invoke the overridden method through the use of the super keyword.
Note:
You can only go back one level.
In the constructor, if you use super(), it must be the very first code, and you cannot access any this.xxx variables or methods to compute its parameters.
To invoke a superclass method that has been overridden in a subclass, you must either call the method directly through a superclass instance, or use the super prefix in the subclass itself. From the point of the view of the subclass, the super prefix provides an explicit reference to the superclass' implementation of the method.
// from subclass
super.overriddenMethod ();
NO, because main is a static method. A static method can't be overridden in Java.
Yes, derived classes still can override the overloaded methods. Polymorphism can still happen. Compiler will not binding the method calls since it is overloaded, because it might be overridden now or in the future.
|
| Overloaded Method | Overridden Method |
| Arguments | Must change | Must not change |
| Return type | Can change | Can't change except for covariant returns |
| Exceptions | Can change | Can reduce or eliminate. Must not throw new or broader checked exceptions |
| Access | Can change | Must not make more restrictive (can be less restrictive) |
| Invocation | Reference type determines which overloaded version is selected. Happens at compile time. | Object type determines which method is selected. Happens at runtime. |
Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior that's specific to a particular subclass type.
Note:
method marked public and make it protected).
You cannot override a method marked final
You cannot override a method marked static