class Dad
{
protected static String me = "dad";
public void printMe()
{
System.out.println(me);
}
}
class Son extends Dad
{
protected static String me = "son";
}
public void doIt()
{
new Son().printMe();
}
The function doIt will print "dad". Is there a way to make it print "son"?
In short, no, there is no way to override a class variable.
You do not override class variables in Java you hide them. Overriding is for instance methods. Hiding is different from overriding.
In the example you've given, by declaring the class variable with the name 'me' in class Son you hide the class variable it would have inherited from its superclass Dad with the same name 'me'. Hiding a variable in this way does not affect the value of the class variable 'me' in the superclass Dad.
For the second part of your question, of how to make it print "son", I'd set the value via the constructor. Although the code below departs from your original question quite a lot, I would write it something like this;
public class Person {
private String name;
public Person(String name) {
this.name = name;
}
public void printName() {
System.out.println(name);
}
}
The JLS gives a lot more detail on hiding in section 8.3 - Field Declarations
Yes. But as the variable is concerned it is overwrite (Giving new value to variable. Giving new definition to the function is Override). Just don't declare the variable but initialize (change) in the constructor or static block.
The value will get reflected when using in the blocks of parent class
if the variable is static then change the value during initialization itself with static block,
class Son extends Dad {
static {
me = "son";
}
}
or else change in constructor.
You can also change the value later in any blocks. It will get reflected in super class
Yes, just override the printMe() method:
class Son extends Dad {
public static final String me = "son";
#Override
public void printMe() {
System.out.println(me);
}
}
You can create a getter and then override that getter. It's particularly useful if the variable you are overriding is a sub-class of itself. Imagine your super class has an Object member but in your sub-class this is now more defined to be an Integer.
class Dad
{
private static final String me = "dad";
protected String getMe() {
return me;
}
public void printMe()
{
System.out.println(getMe());
}
}
class Son extends Dad
{
private static final String me = "son";
#Override
protected String getMe() {
return me;
}
}
public void doIt()
{
new Son().printMe(); //Prints "son"
}
If you are going to override it I don't see a valid reason to keep this static. I would suggest the use of abstraction (see example code). :
public interface Person {
public abstract String getName();
//this will be different for each person, so no need to make it concrete
public abstract void setName(String name);
}
Now we can add the Dad:
public class Dad implements Person {
private String name;
public Dad(String name) {
setName(name);
}
#Override
public final String getName() {
return name;
}
#Override
public final void setName(String name) {
this.name = name;
}
}
the son:
public class Son implements Person {
private String name;
public Son(String name) {
setName(name);
}
#Override
public final String getName() {
return name;
}
#Override
public final void setName(String name) {
this.name = name;
}
}
and Dad met a nice lady:
public class StepMom implements Person {
private String name;
public StepMom(String name) {
setName(name);
}
#Override
public final String getName() {
return name;
}
#Override
public final void setName(String name) {
this.name = name;
}
}
Looks like we have a family, lets tell the world their names:
public class ConsoleGUI {
public static void main(String[] args) {
List<Person> family = new ArrayList<Person>();
family.add(new Son("Tommy"));
family.add(new StepMom("Nancy"));
family.add(new Dad("Dad"));
for (Person person : family) {
//using the getName vs printName lets the caller, in this case the
//ConsoleGUI determine versus being forced to output through the console.
System.out.print(person.getName() + " ");
System.err.print(person.getName() + " ");
JOptionPane.showMessageDialog(null, person.getName());
}
}
}
System.out Output : Tommy Nancy Dad
System.err is the same as above(just has red font)
JOption Output: Tommy then Nancy then Dad
This looks like a design flaw.
Remove the static keyword and set the variable for example in the constructor. This way Son just sets the variable to a different value in his constructor.
Though it is true that class variables may only be hidden in subclasses, and not overridden, it is still possible to do what you want without overriding printMe () in subclasses, and reflection is your friend. In the code below I omit exception handling for clarity. Please note that declaring me as protected does not seem to have much sense in this context, as it is going to be hidden in subclasses...
class Dad
{
static String me = "dad";
public void printMe ()
{
java.lang.reflect.Field field = this.getClass ().getDeclaredField ("me");
System.out.println (field.get (null));
}
}
https://docs.oracle.com/javase/tutorial/java/IandI/hidevariables.html
It's called Hiding Fields
From the link above
Within a class, a field that has the same name as a field in the superclass hides the superclass's field, even if their types are different. Within the subclass, the field in the superclass cannot be referenced by its simple name. Instead, the field must be accessed through super, which is covered in the next section. Generally speaking, we don't recommend hiding fields as it makes code difficult to read.
class Dad
{
protected static String me = "dad";
public void printMe()
{
System.out.println(me);
}
}
class Son extends Dad
{
protected static String _me = me = "son";
}
public void doIt()
{
new Son().printMe();
}
... will print "son".
It indeed prints 'dad', since the field is not overridden but hidden. There are three approaches to make it print 'son':
Approach 1: override printMe
class Dad
{
protected static String me = "dad";
public void printMe()
{
System.out.println(me);
}
}
class Son extends Dad
{
protected static String me = "son";
#override
public void printMe()
{
System.out.println(me);
}
}
public void doIt()
{
new Son().printMe();
}
Approach 2: don't hide the field and initialize it in the constructor
class Dad
{
protected static String me = "dad";
public void printMe()
{
System.out.println(me);
}
}
class Son extends Dad
{
public Son()
{
me = "son";
}
}
public void doIt()
{
new Son().printMe();
}
Approach 3: use the static value to initialize a field in the constructor
class Dad
{
private static String meInit = "Dad";
protected String me;
public Dad()
{
me = meInit;
}
public void printMe()
{
System.out.println(me);
}
}
class Son extends Dad
{
private static String meInit = "son";
public Son()
{
me = meInit;
}
}
public void doIt()
{
new Son().printMe();
}
Variables don't take part in overrinding. Only methods do. A method call is resolved at runtime, that is, the decision to call a method is taken at runtime, but the variables are decided at compile time only. Hence that variable is called whose reference is used for calling and not of the runtime object.
Take a look at following snippet:
package com.demo;
class Bike {
int max_speed = 90;
public void disp_speed() {
System.out.println("Inside bike");
}
}
public class Honda_bikes extends Bike {
int max_speed = 150;
public void disp_speed() {
System.out.println("Inside Honda");
}
public static void main(String[] args) {
Honda_bikes obj1 = new Honda_bikes();
Bike obj2 = new Honda_bikes();
Bike obj3 = new Bike();
obj1.disp_speed();
obj2.disp_speed();
obj3.disp_speed();
System.out.println("Max_Speed = " + obj1.max_speed);
System.out.println("Max_Speed = " + obj2.max_speed);
System.out.println("Max_Speed = " + obj3.max_speed);
}
}
When you run the code, console will show:
Inside Honda
Inside Honda
Inside bike
Max_Speed = 150
Max_Speed = 90
Max_Speed = 90
only by overriding printMe():
class Son extends Dad
{
public void printMe()
{
System.out.println("son");
}
}
the reference to me in the Dad.printMe method implicitly points to the static field Dad.me, so one way or another you're changing what printMe does in Son...
You cannot override variables in a class. You can override only methods. You should keep the variables private otherwise you can get a lot of problems.
No. Class variables(Also applicable to instance variables) don't exhibit overriding feature in Java as class variables are invoked on the basis of the type of calling object. Added one more class(Human) in the hierarchy to make it more clear. So now we have
Son extends Dad extends Human
In the below code, we try to iterate over an array of Human, Dad and Son objects, but it prints Human Class’s values in all cases as the type of calling object was Human.
class Human
{
static String me = "human";
public void printMe()
{
System.out.println(me);
}
}
class Dad extends Human
{
static String me = "dad";
}
class Son extends Dad
{
static String me = "son";
}
public class ClassVariables {
public static void main(String[] abc) {
Human[] humans = new Human[3];
humans[0] = new Human();
humans[1] = new Dad();
humans[2] = new Son();
for(Human human: humans) {
System.out.println(human.me); // prints human for all objects
}
}
}
Will print
human
human
human
So no overriding of Class variables.
If we want to access the class variable of actual object from a reference variable of its parent class, we need to explicitly tell this to compiler by casting parent reference (Human object) to its type.
System.out.println(((Dad)humans[1]).me); // prints dad
System.out.println(((Son)humans[2]).me); // prints son
Will print
dad
son
On how part of this question:- As already suggested override the printMe() method in Son class, then on calling
Son().printMe();
Dad's Class variable "me" will be hidden because the nearest declaration(from Son class printme() method) of the "me"(in Son class) will get the precedence.
Just Call super.variable in sub class constructor
public abstract class Beverage {
int cost;
int getCost() {
return cost;
}
}`
public class Coffee extends Beverage {
int cost = 10;
Coffee(){
super.cost = cost;
}
}`
public class Driver {
public static void main(String[] args) {
Beverage coffee = new Coffee();
System.out.println(coffee.getCost());
}
}
Output is 10.
Of course using private attributes, and getters and setters would be the recommended thing to do, but I tested the following, and it works... See the comment in the code
class Dad
{
protected static String me = "dad";
public void printMe()
{
System.out.println(me);
}
}
class Son extends Dad
{
protected static String me = "son";
/*
Adding Method printMe() to this class, outputs son
even though Attribute me from class Dad can apparently not be overridden
*/
public void printMe()
{
System.out.println(me);
}
}
class Tester
{
public static void main(String[] arg)
{
new Son().printMe();
}
}
Sooo ... did I just redefine the rules of inheritance or did I put Oracle into a tricky situation ?
To me, protected static String me is clearly overridden, as you can see when you execute this program. Also, it does not make any sense to me why attributes should not be overridable.
Why would you want to override variables when you could easily reassign them in the subClasses.
I follow this pattern to work around the language design. Assume a case where you have a weighty service class in your framework which needs be used in different flavours in multiple derived applications.In that case , the best way to configure the super class logic is by reassigning its 'defining' variables.
public interface ExtensibleService{
void init();
}
public class WeightyLogicService implements ExtensibleService{
private String directoryPath="c:\hello";
public void doLogic(){
//never forget to call init() before invocation or build safeguards
init();
//some logic goes here
}
public void init(){}
}
public class WeightyLogicService_myAdaptation extends WeightyLogicService {
#Override
public void init(){
directoryPath="c:\my_hello";
}
}
Related
I would like to use a method using a constant according to class she is called.
Sorry for this bad explanation, here is an example :
public class Mom{
public void execute(parameters){
// Some actions
String nf = String.format(C_CARS)
}
}
public class Son1 extends Mom{
private static final String C_CARS=" myFormat "
}
public class Son2 extends Mom{
private static final String C_CARS=" myFormat2 "
}
public static void main(String[] args){
Son1 son1=new Son1();
Son2 son2=new Son2();
son1().execute(myparameters);
son2().execute(myparameters);
}
I would like to do something like that, is there a way ? The problem here is C_CARS is unknown in Mom Class.
This is just not how inheritance works in Java.
You need a guarantee that all instances of Mom have a value. Do this via an instance method, e.g.
public abstract class Mom{
public void execute(parameters){
String nf = String.format(getCCars());
}
protected abstract String getCCars();
}
And then implement in the child classes:
class Son1 extends Mom {
#Override protected String getCCars() {
return "something";
}
}
There is something up with your object oriented design.
Remember: every instance of a Son1 class is also an instance of Mom. But in the real world, most sons aren't mothers.
extends is not the right thing to use here.
Don't use static, it brings troubles.
public class Mom {
private final String cCars;
protected Mom(String cCars) {
this.cCars = cCars;
}
public void execute(parameters){
// Some actions
String nf = String.format(cCars)
}
}
public class Son1 extends Mom {
public Son1() {
super(" myFormat ");
}
}
public class Son2 extends Mom {
public Son2() {
super(" myFormat2 ");
}
}
This can't work because a superclass cannot access the members of it's subclass. Also, you can't override a variable like you can override methods. Try it with the following code instead:
public static abstract class Mom {
public void execute() {
System.out.println(getCCars());
}
abstract String getCCars();
}
public static class Son1 extends Mom {
#Override
String getCCars() {
return " myFormat ";
}
}
public static class Son2 extends Mom {
#Override
String getCCars() {
return " myFormat2 ";
}
}
public static void main(String[] args) {
Son1 son1 = new Son1();
Son2 son2 = new Son2();
son1.execute();
son2.execute();
}
This is the way you would to it in java.
I've tested the code above. It does compile and it produces the desired result.
You cannot override a variable in this way. Use a method instead:
abstract public class Mom {
abstract protected String cCars();
public void execute(parameters){
// Some actions
String nf = String.format(cCars())
}
}
public class Son1 extends Mom{
private static final String C_CARS=" myFormat ";
#Override
protected String cCars() {
return C_CARS;
}
}
Alternatively, you could provide a default implementation in Mom, then there is no need to make the class and method abstract.
A field marked static belongs to the class rather than an instance. Also, C_CARS in Son1 is not related in any way to C_CARS in Son2.
A way to achieve such thing is this:
class Mom {
public abstract String getCCars();
}
Then each ascendant of Mom must override the getCCars() method.
You could also accept a string cCars in the constructor of Mom. Each ascendant then must call the super constructor defined in Mom:
class Mom {
final String cCars;
Mom(String cCars) {
this.cCars = cCars;
}
void execute(String parameters) {
System.out.println(this.cCars);
}
}
class Son1 {
Son1() {
super("MyFormat"); // We have to call the super constructor
}
}
I want to access and use 3 private variables in an Abstract Class(MainAbstract.java) from another class that has extended (SubAbstract.java) from the previously mentioned Abstract Class.
From the sub class I want to access the getters() and setters() of the main class's.
In the main class (this is an abstract class) there is an abstract method called ShowInfo().
This ShowInfo() abstract method should do something to view the each instance of the subclass's.
Below is the source code for the MainClass(Abstract) and the Sub Class SubAbstract. Please refer them.
MainAbstract.java
package abstractionexample;
public abstract class MainAbstract {
private String sName;
private String sType;
private int iQty;
public String getsName() {
return sName;
}
public void setsName(String sName) {
this.sName = sName;
}
public String getsType() {
return sType;
}
public void setsType(String sType) {
this.sType = sType;
}
public int getiQty() {
return iQty;
}
public void setiQty(int iQty) {
this.iQty = iQty;
}
public abstract void showInfo();
public static void main(String[] args) {
}
}
SubAbstract.java
package abstractionexample;
public class SubAbstract extends MainAbstract{
#Override
public void showInfo() {
}
//This is an instance and the getters() and setters() should use each instance of this kind of to get values and set values.
SubAbstract nSubAbs = new SubAbstract();
}
If I understand correctly, you want to use the setter methods to set properties of the instance nSubAbs and show these properties using the showInfo() method.
The getters and setters are readily available to you in your subclass SubAbstract because it has inherited from the parent class MainAbstract
Here's a code sample:
class SubAbstract extends MainAbstract{
SubAbstract nSubAbs;
SubAbstract(int iQty, String name, String type) {
nSubAbs = new SubAbstract();
this.nSubAbs.setiQty(iQty);
this.nSubAbs.setsName(name);
this.nSubAbs.setsType(type);
}
private SubAbstract() {
//no arg constructor
}
#Override
public void showInfo() {
System.out.println("iQty:" + nSubAbs.getiQty());
System.out.println("name:" + nSubAbs.getsName());
System.out.println("type:" + nSubAbs.getsType());
}
}
And the main method of your MainAbstract class would look something like this (although this is a very bad place for the main method, but I suppose, you are trying to experiment)
public abstract class MainAbstract {
//..existing code..
public static void main(String[] args) {
SubAbstract subAbstract = new SubAbstract(10, "someName", "someType");
subAbstract.showInfo();
}
}
there I'm pretty new to Java and have german class and method titles. This Code is meant to give a string output for every class extending "Musiker". I have already looked on SO but my problem is that changing it to static gives an error on the class itself. The main reason why I open a new Question is, that every other class is working as planned. And please don't wonder why the Strings look weird, the Book I copied this from is meant to be humoristic.
public class Proberaum {
public static void main(String[] args) {
try {
Musiker saenger = new Saenger();
Musiker gitarrist = new Gitarrist();
Musiker bassist = new Bassist();
Musiker trompeter = new Trompeter();
Musiker backgroundSaengerin = new BackgroundSaengerin();
machtMusik(saenger, gitarrist, bassist, trompeter, backgroundSaengerin);
} catch(Exception e) {
new Exception().printStackTrace();
}
}
public static void machtMusik(Musiker... gruppe) {
for(Musiker musiker : gruppe) {
musiker.musizieren();
}
}
public class Musiker {
private String name;
private int alter;
private Band band;
public void musizieren() {
System.out.println("OO Mmmmmmmmh, OO Mmmmmmmmh");
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAlter() {
return alter;
}
public void setAlter(int alter) {
this.alter = alter;
}
public Band getBand() {
return band;
}
public void setBand(Band band) {
this.band = band;
}
}
public class Band {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
public class Saenger extends Musiker {
#Override
public void musizieren() {
this.singen();
}
public void singen() {
System.out.println("Oh, bäbi, juuuu a mei sannnnscheiiiiin");
}
}
public class BackgroundSaengerin extends Saenger {
}
public class Bassist extends Musiker {
}
public class Gitarrist extends Musiker {
public void musizieren() {
System.out.println("Tschiiiiiingzäääängggggg");
}
}
public class Trompeter extends Musiker {
}
}
Your Saenger class is actually a non-static member of the Proberaum class. Because it's non-static, you actually need to create an instance of Proberaum before you can use any of these classes:
Proberaum proberaumObject = new Proberaum();
Musiker saenger = new proberaumObject.Saenger();
In your case, classes inside classes is probably not what you want to do. If you extract each of your classes into its own file, you should find your problem going away. (If that's not possible for whatever reason, declaring your subclasses as static should work too.)
Like Joe C also mentioned in his answer: the core of the problem is that your classes Saenger, Musiker, etc etc. are all nested classes (nested inside Proberaum), but they are defined as non-static.
In Java, non-static nested classes are called "inner classes". Inner classes have implicit access to their enclosing class members (even private ones), but of course the flipside of this is that there first needs to be an object of that enclosing class for the inner class to reference. That is why the compiler is complaining in your example: you're trying to create an object of class Saenger, which is an inner class of Proberaum, so to create that object it needs to have a reference to an object of type Proberaum. Since you're doing the object creation in the (static) main method, no such object exists.
So, to fix, you have to change your inner classes. Easiest is to declare them all static. Note that you can do this is in addition to be making them public:
public static class Seanger extends Musiker { ...
As also remarked elsewhere however, you really should not put every class in the same file. Learn to work with one class per file, it's the Java Way™.
Instead of declaring the nested classes as static, one can alternatively create objects of nested classes like mentioned below.
Proberaum proberaumObject = new Proberaum();
Musiker saenger = proberaumObject.new Saenger();
I am a little confused on how to set up the TestHomework method so that it prints properly when using the toString() method. Right now when I run the main method it prints "null - 0" but what I would like it to say is "Math - 6". This program is supposed to extend an abstract class. It is supposed to say how many pages there are for homework and for what subject.
public abstract class Homework {
private int pagesToRead;
private String typeHomework;
{
// initialise instance variables
pagesToRead = 0;
typeHomework = "none";
}
public Homework(int pages, String hw) {
this.pagesToRead = pages;
this.typeHomework = hw;
}
public abstract void createAssignment(int p);
public int getPages() {
return pagesToRead;
}
public void setPagesToRead(int p) {
pagesToRead = p;
}
public String getTypeHomework() {
return typeHomework;
}
public void setTypeHomework(String hw) {
typeHomework = hw;
}
}
public class MyMath extends Homework {
private int pagesRead;
private String typeHomework;
public MyMath(int pages, String hw) {
super(pages,hw);
}
public void createAssignment(int p) {
setTypeHomework("Math");
setPagesToRead(p);
}
public String toString() {
return typeHomework + " - " + pagesRead;
}
}
public class TestHomework {
public static void main(String[] args) {
MyMath one = new MyMath(6, "Math");
one.createAssignment(6);
System.out.println(one);
}
}
That's because you are defining the 2 properties (that one of them happen to have the same name as one of the abstract class's) but you are not initializing them, you are initializing those of the abstract class. (So their values is always set to their type's default)
You need to drop those from the MyMath class, & define the toString method in your abstract class: it's the one to be used by default by its inheriting classes.
public abstract class Homework {
private int pagesToRead;
private String typeHomework;
// Same code
// Define the toString here
#Override
public String toString() {
return typeHomework + " - " + pagesToRead;
}
}
public class MyMath extends Homework {
// You don't need to define any extra attributes
public MyMath(int pages, String hw) {
super(pages,hw);
}
public void createAssignment(int p) {
setTypeHomework("Math");
setPagesToRead(p);
}
}
public static void main(String[] args) {
// Calls the constructor of the MyMath class, which in turn
// invokes the constructor of its superclass, the 'Homework' class
MyMath one = new MyMath(6, "Math");
one.createAssignment(6);
// Invokes the toString of the MyMath class. Since it does not have one,
// The toString method of its superclass (Homework) is called.
System.out.println(one);
}
Your derived class has its own typeHomework and pagesRead fields, which are never set (even though the base class happens to have fields with the same names). Therefore, they stay null and 0.
You should delete those fields and use the data from the base class, via the public getter methods.
Why it doesn't work:
Be careful you redeclared the attribute typeHomework of you parent class. Attributes are automatically added to your extending class so you don't have to write them again.
By redeclaring it you confused the compiler, viewing your code in debug shows, that your one object contains your typeHomework twice:
typeHomework = null // The one from the super class
typeHomework = "Math" // The one from your child class
Your method now uses the typeHomework from your super-class therefor the output is null!
pagesRead is 0 because you are setting the pagesToRead of your super-class to 6(not pagesRead!) when calling setPagesToRead(p);.
Some style tips
Use the #Override annotation when overriding methods like this:
#Override
public void createAssignment(int p) {
setTypeHomework("Math");
setPagesToRead(p);
}
It's not really needed but it's good practice (readers of your code know that it overrides something).
When referring to attributes of your class it's also good practice to use the this statement so it's clear, that you're referring to an attribute and not a local variable:
#Override
public String toString() {
return this.typeHomework + " - " + this.pagesRead;
}
public abstract class Human{
public String name;
public int number
public void getInfo(){
Name = JOptionPane.showInputDialog("Please enter your name: ");
money = Double.parseDouble(JOptionPane.showInputDialog("Please enter amount of money .00: "));
}
public void displayInfo(){
JOptionPane.showMessageDialog(null,"Name: "+name+"\n"+
"Number: "+number);
}
}
public class Student extends Human {
}
public class Teacher extends Human{
}
public class Janitor extends Human{
{
I need help if calling the methods getInfo() and displayInfo() in all 3 classes below. I have tried:
public class Student extends Human{
public Student(){
getInfo();
displayInfo();
}
it works, but it generates a warning saying "problematic call in constructor" I guess it is not the best way to do it.
I also tried:
#Override
public void getInfo() {
}
but if I leave it empty nothing happens. Basically I am trying to call the method in the abstract class in a simple way without needing to type it up in every class.
As already mentioned, you shouldn't call overridable methods in constructors, because if another class overrides this method and invokes the constructor of the superclass, it may try to use values that are not initialized yet, since the overriden method will be invoked. Example:
public class Superclass {
protected int id;
protected void foo() {
System.out.println("Foo in superclass");
}
public Superclass() {
foo();
}
}
public class Subclass extends Superclass {
public Subclass(int id) {
super();
this.id = id;
}
#Override
protected void foo() {
System.out.println("Id is " + id);
}
}
This will print the unitialized value of id, since you first call the constructor of the superclass which invokes the foo method of the subclass.
You can fix this by making your methods final if this suits your case.
You get the warning because it's a good practice not to call overridables in the constructor; since these overridables could try to access member variables that are not initialized yet (== null) .
You shouldn't call overridable functions inside a constructor. check this link