This is my java code:
class A {
interface That {
void show();
}
}
class B implements A.That {
public void show() {
System.out.println("Hi");
}
}
public class MainClass {
public static void main(String args[]) {
A obj = new A();
obj.That object = new B();
object.show();
}
}
Since A is a class (not abstract) we can create its instance and than we can use members of that instance. Now interface is member so obj.That should work but javac says that obj.That is not package. Why?
Interfaces are always static when nested in a class. You should therefore access your interface declaration as A.That, not obj.That.
Related
This question already has answers here:
Java superclass calls subclass method
(5 answers)
Closed 1 year ago.
Why does method() call the overridden subclass method2 instead of method2 in the BaseClass?
public class BaseClass {
public void method(){
System.out.println("method() called");
method2();
}
public void method2(){
System.out.println("method2() called");
}
}
public class ChildClass extends BaseClass {
public void method2(){
System.out.println("method2() from BaseClass");
}
}
public class Main {
public static void main(String[] args) {
ChildClass obj = new ChildClass();
obj.method();
}
}
This is the concept of Runtime polymorphism (Dynamic Method Dispatch). Because you are assigning the object (instance) of ChildClass to obj reference variable, it will call the method of child class.
Always the method of the class whose instance is created gets called first. If that method is not present in that particular child class, then the parent's inherited method gets called.
If you come from the C++ corner:
all instance methods (non-static) in Java are virtual.
All class methods (static) are NOT.
This is why your case happens.
This is also, why the Java compiler will complain (warn) that if you access a static method via an object, that you should call via the distinct class, because calls to the "static method of a object" could be ambiguous, because it could be two static methods with the same signature that get called.
Extending your Example:
package stackoverflow.staticcalls;
public class BaseClass {
public void method() {
System.out.println("method() called");
method2();
}
public void method2() {
System.out.println("method2() called");
}
static public void sayHello() {
System.out.println("BaseClass.sayHello()");
}
}
and
package stackoverflow.staticcalls;
public class ChildClass extends BaseClass {
public void method2() { // compiler warning: The method method2() of type ChildClass should be tagged with #Override since it actually overrides a superclass method
System.out.println("method2() from BaseClass");
}
public void originalCallToBaseMethod2() {
super.method2(); // will run BaseClass.method2()
}
static public void sayHello() {
System.out.println("ChildClass.sayHello()");
}
}
and
package stackoverflow.staticcalls;
public class Main {
public static void main(final String[] args) {
final ChildClass obj = new ChildClass();
System.out.println("\nCalling obj.method(); ...");
obj.method();
System.out.println("\nCalling obj.sayHello(); ...");
obj.sayHello(); // compiler warning: The static method sayHello() from the type ChildClass should be accessed in a static way
System.out.println("\nCalling ChildClass.sayHello(); ...");
ChildClass.sayHello(); // the proper call
System.out.println("\nCalling BaseClass.sayHello(); ...");
BaseClass.sayHello(); // but you can also explpicitly call the other method
System.out.println("\nCalling obj.originalCallToBaseMethod2(); ...");
obj.originalCallToBaseMethod2(); //
}
}
Here you see the examples to what I said.
Note: In the last call in Main.main() we still can call BaseClass.method2(), but not directly. We have to be within ChildClass to do that, and it's done via the super keyword/reference.
A little off-topic note, to complete addressing patterns:
If you're inside an inner class and need to call to a overshadowed name in the outer class, you can use Outer.this.method():
package stackoverflow.staticcalls;
import stackoverflow.staticcalls.OuterInner.Outer.Inner;
public class OuterInner {
class Outer {
void method() {
System.out.println("OuterInner.Outer.method()");
}
class Inner {
void method() {
System.out.println("OuterInner.Outer.Inner.method()");
}
void callOuter() {
Outer.this.method();
}
}
}
public static void main(final String[] args) {
final OuterInner oi = new OuterInner();
final Outer outer = oi.new Outer();
final Inner inner = outer.new Inner();
System.out.println("\nCalling inner.method(); ...");
inner.method();
System.out.println("\nCalling inner.callOuter(); ...");
inner.callOuter();
}
}
I am told there are three different contexts in which a class can be declared in Java.
It has to do with the location within a program, but I can't think of what they are.
Obviously a class can be declared at the top of a page, the only other example I can think of is like a nested class?
I feel I may be going about this the wrong way.
In a package
package com.example.mypackage;
public class TheClass {
}
In a class
package com.example.mypackage;
public class OuterClass {
class InnerClass {
}
}
Anonymously
public class MainClass {
public static void main(String[] args) {
AbstractClass myObject = new AbstractClass() {
// overrides and other fields of the
// anonymous class goes in this block
};
}
}
EDIT: As #daniu stated in the comment, a class can also be created in a method:
public class MainClass {
public static void main(String[] args) {
class MethodClass {
}
MethodClass myObject = new MethodClass();
}
}
There are actually 4 distinct syntactic contexts:
In the "compilation-unit" context; i.e. a top-level class declaration.
package foo;
public class Bar{}
As a nested class declaration in a class declaration.
package moo;
public class Cow {
public class Inner {}
}
As a nested class declaration in a method declaration.
package too;
public class Far {
public void test() {
class MethodInner{}
}
}
As an anonymous class declaration in a new expression. The expression could appear in a variety of contexts. For example:
package goo;
public class Tar {
public void test() {
Runnable r = new Runnable() {
public run() { }
};
}
}
This may be what you are looking for:
public class TopLevel {
public static void main(String[] args) {
TopLevel topLevel = new TopLevel();
Nested nested = new TopLevel.Nested();
Inner inner = topLevel.new Inner();
}
public static class Nested {
}
public class Inner {
}
}
How to access the anonymous inner class object in main method. It is giving compile time error saying that "cannot make static reference to non static method". If I am making anonymous inner class as static then I can access ut I want to access without making it static.
How to do that. Please help.
AnonymousInnerClass2.java
abstract class AnonymousInnerClass21
{
abstract void m();
}
public class AnonymousInnerClass2
{
AnonymousInnerClass21 a=new AnonymousInnerClass21()
{
#Override
void m() {
System.out.println("Hello");
}
};
public static void main(String[] args)
{
a.m();
}
}
this is because for accessing inner class (whether its is normal/named class or Anonymous class), you must create object of class in which inner class is defined, you can try below
abstract class AnonymousInnerClass21
{
abstract void m();
}
public class AnonymousInnerClass2
{
AnonymousInnerClass21 a=new AnonymousInnerClass21()
{
#Override
void m() {
System.out.println("Hello");
}
};
public static void main(String[] args)
{
AnonymousInnerClass2 anonymousInnerClass2= new AnonymousInnerClass2 ();//create outer class object
anonymousInnerClass2.a.m(); // access inner class object through outer class object
}
}
As I have gone through some of the programming sites which explains anonymous inner class in java.
But, I still have doubts that how it really works and on the conclusions which I made below.
Here is code :
public class Implementing {
public static void main(String[] args) {
SuperClass a = new SuperClass() { // ... 1st
public void call () { // ... 2nd
System.out.println("Method call");
}
public void call2 () { // creating new method in anonymous inner class
System.out.println ("New call2 Method in Anonymous Class");
}
};
a.call(); // ... 3rd
try {
a.getClass().getMethod("call2",null).invoke(a,null); // ... 4th
} catch (Exeception e) {
System.out.println(e);
}
}
}
class SuperClass {
public void call() {
System.out.println("Super Class");
}
}
What I understand is this :
At 1st :-
Anonymous inner class is sub-class.
we are creating instance of anonymous inner class as well as we
are extending the class SuperClass.
we are running constructor of SuperClass, but we created object
of anonymous class of type SuperClass.
polymorphism at work.
Actually, we are doing this at 1st :
* 1. anonymousClass extends SuperClass
* 2. anonymousClass a = new SuperClass()
* 3. { }; // our anonymous class field
At 2nd : - overrides SuperClass call Method().
At 3rd : - calling call() Method by object 'a'.
At 4th : - accessing new method call2() Method in anonymous inner class.
So, my these conclusions are right or wrong ? If wrong why they are wrong ? Please Explain.
Your code creating an Anonymous Class:
public static void main(String[] args) {
SuperClass a = new SuperClass() { // ... 1st
public void call() {
System.out.println("Method call");
}
public void call2() {
System.out.println("New call2 Method in Anonymous Class");
}
};
...
}
is the same as this code creating a Local Class:
public static void main(String[] args) {
class LocalClass extends SuperClass {
public void call() {
System.out.println("Method call");
}
public void call2() {
System.out.println("New call2 Method in Anonymous Class");
}
}
SuperClass b = new LocalClass();
...
}
except that the class is unnamed, aka anonymous.
Since you're doing this from a static method, and you're not using any local variables, they are both the same as this Static Nested Class:
static class NestedClass extends SuperClass {
public void call() {
System.out.println("Method call");
}
public void call2() {
System.out.println("New call2 Method in Anonymous Class");
}
}
If you look at the compiled .class files, you'll see they are all just classes:
SuperClass.class <-- Your superclass
Implementing.class <-- Your main class
Implementing$1.class <-- Anonymous class
Implementing$1LocalClass.class <-- Local class
Implementing$NestedClass.class <-- Nested class
I'm new to Java and is trying to learn the concept of anonymous class. Could someone please tell me how I can invoke the 'awesomeMethod' from the main method of the LocallClassExample?
public class LocalClassExample {
interface Awesome {
public void awesomeMethod();
}
class AwesomeClass {
public int finalInt= 10;
Awesome a1 = new Awesome() {
#Override
public void awesomeMethod() {
System.out.println(finalInt);
}
};
}
public static void main(String[] args) {
}
}
Consider this:
new AwesomeClass().a1.awesomeMethod();
will invoke the method awesomeMethod() on the member variable a1 (which is something Awesome) of the newly created instance of AwesomeClass.
It will get more tricky once your main is outside of your AwesomeClass - and more so once it's outside of the package. In these cases you'd have to provide a getter like
public Awesome getAwesome() {
return a1;
}
Which would when invoked still execute the method as defined in your anonymous class.
Try to use this to create inner class object as:
public static void main(String[] args) {
LocalClassExample.AwesomeClass oi = new LocalClassExample().new AwesomeClass();
oi.awesomeMethod();
}