Abstract Method With No Implementation
Mar 6, 2014An abstract method is a method with no implementation. So would like to know what is the purpose of calling it if there is no implementation?
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How To Call Abstract Method In Java
Feb 18, 2014I am new to Java and have been learning it. I have a question here. I came across the following Java class and trying to understand it thoroughly but got confused how it is able to call an abstract method. Here is the code I am referring to :
package sampleapps.gui;
import javax.swing.*;
import java.awt.*;
public class InnerClassAnimationExample {
int x=70, y=70;
public static void main(String[] args) {
[Code] ....
So, in the code above, there is an inner class NewMyDrawPanel which has a paintComponent(Graphics g) method. I have highlighted 2 lines of code above.
Line 1 : Graphics2D g2d = (Graphics2D) g;
Line 2 : g2d.fillOval(x,y,40,40);
I understand we are type casting reference g to Graphics2D reference g2d and we are calling fillOval() method on g2d. I don't see a fillOval() method in Graphics2D class but it is there in Graphics class and fillOval method is an abstract method.
So, my question here is :
1. If we are not able to instantiate an abstract class(Graphics2D and Graphics classes), how are we able to access the fillOval() abstract method,
2. Secondly, since the fillOval() method is an abstract method, it does not have any implementation for the method.
However, when I call the method fillOval() on Graphics2D reference, I was able to draw and fill an oval of the specified co-ordinates. So, where would the actual implementation code be?
Stuck On IsBorrowed Method Implementation
Jul 19, 2014Basically I have a class called Book. Its supposed to have a few simple methods that set the title of the book, mark it as returned or borrowed. So specifically my problem lies in the implementation of the method that decides if the book isBorrowed. I was thinking i'd use a simple if/else statement but the way i've been trying hasn't been successful. This is what i've tried so far.
public class Book {
String title;
boolean borrowed;
boolean returned;
boolean isBorrowed;
// Creates a new Book
public Book(String bookTitle) {
[Code] .....
Creating Applet With Dropdown Lists - Method Not Abstract
Oct 21, 2011i am trying to create an applet with drop-down lists. When I compile the program the following error message appears '. . .is not abstract and doesn't override abstract method action Performed. . . Here is my code . . .
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
public class DavidApplet extends Applet implements ActionListener //class header {
Label fName=new Label("First Name");
[Code] .....
Performing Method On Instance Inside Abstract Class
Mar 12, 2015How do I create an instance of a class in a method?
I am a bit rusty whenever I think of instances. I always think of main method and objects when I see instance which gets me confused on what to do when I am not in a main method. The example:
I have a abstract class, School, and inside School I have some methods that must preform some action on an instance. For example, there is a move() method that must move the instance of School. Another method named, personOld(), which returns whether or not an instance of School surpassed some determined age.
How do I do this and create this instance?
Method Declaration - Partial Interface Implementation
Sep 3, 2014I have one interface with three(more than one) method declaration. In the subclass that implements it I want to define only one method not all three not even blank definition of them.Is there any keyword or method for that. How to do it? Is it possible to do it? In GUI we use adapter classes to achieve it. What for console application?
View Replies View RelatedMethod Implementation Declaring Exception Other Than That Declared In Interface
Aug 31, 2014The 2 minute drill from page 69 SCJP kathy and bert book, says regarding Interfaces, that - "A legal nonabstract implementing class must not declare any new checked exceptions for an implementation method."
When I try the below given code in eclipse , it does not throw any errors . (Here I have tried to throw NullPointerException from testFunc whereas the interface function throws IllegalStateExc)
package abstracttesting;
public class StaticCheck implements check{
public void testFunc() throws NullPointerException{
// TODO Auto-generated method stub
} public static void main(String[] args) {
// TODO Auto-generated method stub
}
}
interface check{
void testFunc() throws IllegalStateException;
}
Swing/AWT/SWT :: Error At Class Name - Type JTextField Must Implement Inherited Abstract Method
Oct 27, 2014This code is directly from Swing: I'm using Eclipse and keep getting an error on line 10 saying :
"The type JTextField must implement the inherited abstract method ActionListener.actionPerformed(ActionEvent)."
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
[Code] ......
What Are Benefits Of Using Interface Plus Abstract Class Over Just Abstract Class
Oct 8, 2014What are the benefits of using an Interface plus an abstract class, over just an abstract class?
View Replies View RelatedWhat Is Abstract Class
Jun 1, 2014I am just started to learn java and i am facing trouble learning abstract class.
View Replies View RelatedWhat Programs Use Abstract Classes
Sep 15, 2014What programs use abstract classes?
View Replies View RelatedConstructor In Abstract Class?
Jun 23, 2014Do we have constructor in abstract class? If we have then what is the use of it?
View Replies View RelatedWhen To Use Interface And Abstract Class
Apr 17, 2014I know whats the interfaces and abstract class and also know that difference between interface and abstract class,but here my doubt is eventhough abstract class more advantage than the interface,then why should we use interfaces and when?
View Replies View RelatedDifference Between Abstract And Inheritance
Dec 16, 2014I am new to java i dont understand the difference between the abstract and inheritance i mean we use the abstract class with extends with other class name ,even we do that same in the inheritance pls tell me main difference between abstract and inheritance...
View Replies View RelatedPolymorphism Possible Only When Superclass Is Abstract?
Jan 23, 2014I've a question regarding polymorphism.
Is polymorphism possible only when the superclass is abstract?
Reference Of Abstract Class
Aug 29, 2014I passed my abstract class private final reference to another concrete class and I used abstract class reference as parameter to that concrete class constructor and in my main method and null to that parameter then only that program executes correctly...i placing my code below ..if there is any error tell me where is error occurring then i will check my code...i think my code is right but little bit doubt abstract class concept.
{
}
class concept1 extends concept
{
private final concept parent;
public concept1(concept aparent)
{
parent=aparent;
System.out.println(parent);
}
public static void main(String args[])
{
//concept p=new concept1(null);
concept c=new concept1(null);
}}
Abstract Classes And Interfaces
Mar 29, 2014 I did research again....
interface:
methods - abstract, default, static ONLY(abstract methods have no body, while static and defaults do, right?)
fields - public, static, final ONLY
abstract class: a normal class, but has at least one abstract method
methods - all
i.e., static, non-static, abstract (can it have a default method?)
fields - all
i.e., public, protected, private / final, non-final / static, non-static
Interface And Abstract Classes In Java?
Jun 4, 2014why don't I define my methods in a class, rather than going a level up and declaring it first in an abstract class/interface? If the point is to have different implementations for different needs, then we have the option to override the methods.
View Replies View RelatedWhy Are Final And Abstract Called As Modifiers
Feb 8, 2015I just wanted to know that why are final and abstract called as modifiers ,what is the essence of calling them as modifiers since there are two types of modifiers access modifiers and non-access modifiers so final and abstract come under the second category ,so why are these called as modifiers?
View Replies View RelatedDifference Between Abstract Class And Interface?
May 18, 2011Difference between Abstract class and Interface??
View Replies View RelatedWhy Can't Create Object Of Abstract Class
Jul 18, 2014Why we can't create object of abstract class ,when we can create its constructor?
View Replies View RelatedPlot Functions Using Abstract Methods
May 1, 2014Assignment 13 – Plot functions using abstract methods (Plot functions using abstract methods) Write an abstract class that draws the diagram for a function. The class is defined as follows:
Java Code:
public abstract class AbstractDrawFunction extends JPanel {
/** Polygon to hold the points */
private Polygon p = new Polygon();
protected AbstractDrawFunction() {
drawFunction();
[code].....
For each function, create a class that extends the AbstractDrawFunction class and implement the f method. Figure below displays the drawing for the first three functions.how to make the jframe to work so that I displays the panel
Java Code:
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Container;
import java.awt.Graphics;
import java.awt.LayoutManager;
import java.awt.Polygon;
[code]....
How Does Inheritance Work With Abstract Classes
Nov 19, 2014I am writing small pieces of code to make sure I understand Java basics and I have the following.
package teams1;
public abstract class Team1{
private String sport = new String();
public abstract String getSport();
public abstract void setSport();
}
import teams1.*;
[Code] .....
It doesn't compile because sport is private in the super class, but I thought FootballTeam1 would inherit it's own copy of sport because it is extending Team1.
Abstract Classes And Access Modifiers
Jan 21, 2015I'm having difficulty understanding this small piece of code:
package food;
public abstract class Fruit {
private String color;
private String tasteType;
public void setColor(String color) {
this.color = color;
}
public abstract void setTasteType(String taste);
}
The above is an abstract class which describes the basic structure that every fruit should "extend".
The below is a concrete subclass of the Fruit class called Apple.
import food.Fruit;
class Apple extends Fruit {
public void setTasteType(String taste) {
tasteType = taste;
}
}
Also do note that the two pieces of code are in different packages!
Upon compiling the Apple class I get the following error:
Apple.java:4: error: tasteType has private access in Fruit
tasteType = taste;
^
What I don't understand is this: I've given a non-abstract implementation to the "setTasteType" method in the Apple class and clearly setTasteType should have the authority to modify the private instance variables of Fruit. But it turns out I'm wrong.
Temporary Object Of Abstract Class
Feb 21, 2015I am under the assumption that In the return statement of getReciprocal() method(of the following code), a temporary Number object is created to hold the result of the calculation.
My question is, Number is an abstract class and we are only able to create reference of an abstract class not an object. But then how a temp Number object is created and returned?
class Gen<T extends Number>{
T ob;
Gen(T ob){
this.ob = ob;
}
Number getReciprocal(){ // Number is abstract class
[Code] .....
Abstract Class Constructors Are Not Recommended
Aug 28, 2014in abstract class constructors are not recommended since we don't call it directly ...my doubt is below code is right or wrong...
3public abstract class Concept
4{
5 private String id;
6
7 protected Concept( String anId )
8 {
9 if ( anId == null )
10 {
11 throw new NullPointerException( "id must not be null" );
12 }
13
14 id = anId;
15 }