I have two classes that extend the same abstract class. They both need the same constant, but with different values. How can I do this? Some example code to show what I want to do.
abstract class A {
public static int CONST;
}
public class B extends A {
public static int CONST = 1;
}
public class C extends A {
public static int CONST = 2;
}
public static void main(String[] args){
A a = new B();
System.out.println(a.CONST); // should print 1
}
The above code does not compile because CONST is not initialized int class A. How can I make it work? The value of CONST should be 1 for all instances of B, and 2 for all instances of C, and 1 or 2 for all instances of A. Is there any way to use statics for this?
You can't do that.
You can do this, however:
abstract class A {
public abstract int getConst();
}
public class B extends A {
#Override
public int getConst() { return 1; }
}
public class C extends A {
#Override
public int getConst() { return 2; }
}
public static void main(String[] args){
A a = new B();
System.out.println(a.getConst());
}
If a constant has a variable value, it's not a constant anymore. Static fields and methods are not polymorphic. You need to use a public method to do what you want.
Related
I have two classes A and B and they both have a common field in them, and I want to create a function in which if I pass Class A object then I want to set that common field value to the passed value and if I pass Class B object then I want to set that common field value to the passed value. Can anyone please tell me how can I do this, I am new to Java Generic Classes.
Otherwise I would have to make two different functions OR I would have to make an if and else which would decide that passed object belongs to which class ??
Class A
public class A{
int footer;
public void setFooter(int fo) {
footer = fo;
}
}
Class B
public class B{
int footer;
public void setFooter(int fo) {
footer = fo;
}
}
Class D
public class D{
public void change_footer(T generic_param, int value) {
generic_param.setFooter(value);
}
}
Class HelloWorld
public class HelloWorld{
public static void main(String []args){
Here I want to call
A a = new A();
new D().change_footer(a, 5);
B b = new B();
new D().change_footer(b, 5)
}
}
Thank You
And if I got all of the question wrong, and nor A nor B are generic, AND the type of field is fixed.
then you mean something like:
class D {
/*public <T extends Super> would be muuuch nicer here as well!*/
public /*static*/ <T> void change_footer(T obj, int data) {
//otherwise, you could just cast to Super...and set dat field.
if (obj instanceof A) {
((A) obj).setField(data);
} else if (obj instanceof B) {
((B) obj).setField(data);
} // else ... ?
}
}
Original answer:
Easy peasy (the "straight forward" implementation produces the desired results.):
class A<T> {
T daField;
public void setField(T pField) {
daField = pField;
}
public T getField() {
return daField;
}
}
class B<T> extends A {//empty
}
class Test {
public static void main(String... args) {
B<Object> testB1 = new B<>(); //
testB1.setField(new Object());
System.out.println(testB1.getField());
B<String> testB2 = new B<>();
testB2.setField("blah blah");
System.out.println(testB2.getField());
B<Integer> testB3 = new B<>();
testB3.setField(42);
System.out.println(testB3.getField());
}
}
System.out:
java.lang.Object#6d06d69c
blah blah
42
It get's (little) more complicated, when you want to instantiate Ts ...but still possible/other question. :)
Edit to your comment:
If there's only one common field, then why not:
/*abstract */class Super<T> {
T daField;
public void setField(T pField) {
daField = pField;
}
public T getField() {
return daField;
}
}
? ...and:
class A<T> extends Super { ... }
class B<T> extends Super { ... }
public class MyTest {
public static void main(final String[] args) {
B b = new B();
b.print();
}
}
class A {
private final int x = 5;
protected int getX() {
return x;
}
public void print() {
System.out.println(getX());
}
}
class B extends A {
private final int x = 10;
#Override
protected int getX() {
return x;
}
}
In this example, I need to print subclass value in the parent class.
It is working fine. No issue.
Now it is printing 10.
But I do not want to define that property in the parent class A.
Because in this example this x datatype is very simple. So no issue.
But in real-time I want to use other datatype which may be another Class variable or List<something> which have huge data.
So ultimately I do not wish to store that value in Class A.
Because it is redundant data. It will slow down in my Hibernate thing.
Please let me know, how to achieve this without declaring variable in parent class. But I still need to use subclass variable in parent class.
make abstract your class A and the getX(); method.
public class Test {
public static void main(final String[] args) {
B b = new B();
b.print();
}
}
abstract class A {
protected abstract int getX();
public void print() {
System.out.println(getX());
}
}
class B extends A {
private final int x = 10;
#Override
protected int getX() {
return x;
}
}
and override the toString method in place of your print method
#Override
public String toString() {
return String.valueOf(getX());
}
the final code
public class Test {
public static void main(final String[] args) {
B b = new B();
System.out.println(b);
}
}
abstract class A {
protected abstract int getX();
#Override
public String toString() {
return String.valueOf(getX());
}
}
class B extends A {
private static final int X = 10;
#Override
protected int getX() {
return X;
}
}
you could also define as static your x variable
But as say Andrew Tobilko you can consider also to use an interface if A doesn't represent a stateful entity.
It's certainly the best solution for your case, mix the use of an interface and an abstract class
public class Test {
public static void main(final String[] args) {
B b = new B();
System.out.println(b);
}
}
interface MyInterface {
int getX();
}
abstract class A implements MyInterface{
#Override
public String toString() {
return String.valueOf(getX());
}
}
class B extends A {
private static final int X = 10;
#Override
public int getX() {
return X;
}
}
You need the getX within the parent class, but you don't have information enough to implement this method there.
You can declare this class as abstract and mark the method with abstract as well. Doing that, you are handing the responsibility of method implementation over its subclasses and preventing from parent field declaration.
If the A doesn't describe any state (only actions/methods), you should consider replacing it with an interface. At the current state, it is the case.
You could make the parent class abstract, eliminate the property in the parent class, make getX() abstract, and then leave print() as concrete. Then just use the concrete implementation of getX() in the child class.
Structure
-ClassA
|---|
|---ClassAImplA
|---ClassAImplB
-Main
Class A:
public interface ClassA {
public void execute();
}
Implementaion A:
public class ClassAImplA implements ClassA
{
private int a = 5;
public ClassAImplA (int a){setA(a);}
#Override
public void execute() {
System.out.println(a);
}
public int getA() {
return a;
}
public void setA(int a) {
this.a = a;
}
Implementaion B:
public class ClassAImplB implements ClassA
{
private boolean b = false;
public ClassAImplB (int a){setB(b);}
#Override
public void execute() {
System.out.println(b);
}
public booelan getB() {
return b;
}
public void setA(boolean b) {
this.b = b;
}
main:
public class main {
/**
* #param args
*/
public static void main(String[] args) {
ClassAImplA param1 = new ClassAImplA(10);
ClassA = param1;
}
}
By doing this I make ClassA interchangeable,
but I lose the capability to access the parameter int a.
Is there a way to still make it interchangeable, and still have access to int a,
or in case of ClassAImplB, the field boolean b ?
There is a way, but it's not a good idea to do, as it defeats the purpose:
ClassAImplA param1 = new ClassAImplA(10);
ClassA = param1;
if (param1 instanceof ClassAImplA) {
param1x = (ClassAImplA) param1;
System.out.println(param1x.getA());
}
But don't do this. It defeats the purpose of the pattern.
The purpose of the pattern is to use objects of type ClassA,
without having to know how they work.
The getA method is only defined in ClassAImplA,
it's an implementation detail that should not be relevant to users of the ClassA type.
They shouldn't have to know. It's hidden.
This is called good encapsulation and information hiding.
you need one more class using composition to decide which implementation is needed.
public ClassHelper{
private A a;
public ClassHelper(A a){
this.a = a;
}
public void execute() {
this.a.execute();
}
}
public class main {
/**
* #param args
*/
public static void main(String[] args) {
ClassHelper param1 = new ClassHelper(new ClassAImplA(10));
param1.execute();
//or when you need classBIMpl
param1 = new ClassHelper(new ClassAImplB(true));
param1.execute();
}
}
And about the ability to access member of implA or implB , no you cannot have that flexibilty with this patter, whole point of this pattern is that caller need not be aware of implementation details.
Define an interface for the strategy and a Factory with different overloaded methods to create the concrete instances of the classes. Of course the methods are typed to the interface instead of the concrete classes.
The interface.
public interface Strategy {
void execute();
}
The first implementation.
public class ConcreteStrategy implements Strategy {
private boolean a;
public ConcreteStrategy(final boolean a) { this.a = a; }
public void execute() {}
}
The second implementation.
public class AnotherConcreteStrategy implements Strategy {
private int a;
public AnotherConcreteStrategy(final int a) { this.a = a; }
public void execute() {}
}
The factory.
public class Factory {
public static Strategy create(final boolean a) {
return new ConcreteStrategy(a);
}
public static Strategy create(final int a) {
return new AnotherConcreteStrategy(a);
}
}
I have the following problem. Am trying to make a polymorphic call and the result would depend on the variable that changes value depending on the underlying class. Tried different things however it doesn't work. Please let me know what should be changed. Problem is that although c.w reads both the local variable w, which is defaulted to 0 and reads the one from appropriate class it always defaults to 0. Here is the code:
class Cycle{
private int w = 0;
public void move(){
System.out.println("Cycle moving");
}
public int wheels(Cycle c){
switch (c.w){
case 1: return 1;
case 2: return 2;
case 3: return 3;
default: return 0;
}
}
}
class Unicycle extends Cycle{
public int w = 1;
public void go(){
System.out.println("Unicycle go");
}
}
class Bicycle extends Cycle{
public int w = 2;
public void go(){
System.out.println("Bicycle go");
}
}
class Tricycle extends Cycle{
public int w = 3;
public void go(){
System.out.println("Tricycle go");
}
}
public class TestCycle {
public static void ride(Cycle c){
c.move();
int now = c.wheels(c);
System.out.println(now);
}
public static void main(String[] args){
Bicycle b = new Bicycle();
ride(b);
Unicycle u = new Unicycle();
ride(u);
Tricycle t = new Tricycle();
ride(t);
}
}
Your problem (well one of them) is that you are redefining the class variable 'w' in each of your subclasses. Define it one as a member of 'Cycle' and have each subclass set it correctly in their constructors.
class Cycle{
protected int w;
public void move(){
System.out.println("Cycle moving");
}
public int wheels(){
return w;
}
}
class Unicycle extends Cycle{
public Unicycle() {
w = 1;
}
public void go(){
System.out.println("Unicycle go");
}
}
Or you can define an abstract method called 'wheels()' in the superclass and override it in the subclasses. It's a matter of taste.
the wheels method should be more like
public int getWheelCount(){
return this.w;
}
You invoke it on the instance itself, you don't need to pass an argument. If the current instance is a Tricycle, the method will return 3, etc...
Since Cycle.w is private, it's not visible from its inheritors. This means that for example Tricycle.w it's not the "same" variable, and it's not visible in Cycle (that's why you always get 0). You have to make Cycle.w at least protected, then remove w from all subclasses, and set its value in each subclass's constructor to what you want.
It's probably not the answer you are looking for, but the following works. Please give more details on what you are trying to do.
public abstract class Cycle {
protected int nWheels;
protected String goText;
// no constructor.
public void go() {
System.out.println(goText);
}
public int wheels() {
return nWheels;
}
}
...
public class Unicycle extends Cycle {
public Unicycle() {
nWheels = 1;
goText = "Unicycle go";
}
}
Note that I made Cycle abstract because I don't want it to ever be instantiated.
EDIT:
public static int getNumberOfWheels(Cycle cycle) {
return cycle.wheels();
}
which is obviously not very useful since a simple call to cycle.wheels() would do the same as calling this function.
I'm not sure why you want to avoid constructors. Maybe you should write the exact question you are trying to answer.
Consider the following code in Python:
class A(object):
CLASS_ATTRIBUTE = 42
def f(self):
return "CLASS_ATTRIBUTE: %d" % self.CLASS_ATTRIBUTE
class B(A):
CLASS_ATTRIBUTE = 44
Now A().f() and B().f() return "CLASS_ATTRIBUTE: 42" and "CLASS_ATTRIBUTE: 44" respectively.
How can I achieve a similar effect in Java? I want a CLASS_ATTRIBUTE field to be initialized statically and redefined in the inherited class but the f method should be only defined in the base class.
Is there a particular reason you want the attribute to be static? In Java the typical way you'd do this is to have A contain a protected variable that you then set in the constructors of the 2 classes:
public class A
{
protected int CLASS_ATTRIBUTE;
public A()
{
CLASS_ATTRIBUTE = 42;
}
public String f()
{
return "CLASS_ATTRIBUTE: " + CLASS_ATTRIBUTE;
}
}
public class B extends A
{
public B()
{
CLASS_ATTRIBUTE = 44;
}
}
Alternatively (and probably more consistent with Java design patterns) you'd declare a function that you can override to return the value instead of using a member variable.
Short answer: you cant solve it like this in Java. You'll have to solve it in another way.
In Java you can't override or "redeclare" fields in subclasses, and you can't override static methods.
It can be solved using an ugly reflection-hack (should be avoided though):
public class Main {
public static void main(String... args) {
A a = new A();
B b = new B();
System.out.println(a.f()); // Prints 42.
System.out.println(a.fReflection()); // Prints 42.
System.out.println(b.f()); // Prints 42.
System.out.println(b.fReflection()); // Prints 44.
}
}
class A {
static int CLASS_ATTRIBUTE = 42;
public int f() {
return CLASS_ATTRIBUTE;
}
public int fReflection() {
try {
return getClass().getDeclaredField("CLASS_ATTRIBUTE").getInt(null);
} catch (Exception wontHappen) {
return -1;
}
}
}
class B extends A {
// Compiles, but will not "override" A.CLASS_ATTRIBUTE.
static int CLASS_ATTRIBUTE = 44;
}
You can't do this directly with only a variable, because in Java variables cannot override (they only shadow the super classes variables).
You need to use a protected "getter" method, which can then be overridden by the subclass:
class A
{
private int attribute=42;
...
protected int getAttribute() {
return attribute;
}
}
class B
extends A
{
private int attribute=44;
...
protected int getAttribute() {
return attribute;
}
}
But note there's a special consideration to calling methods from an object's constructor, in that it allows object code to run before object construction is complete.
I'm not sure if you meant "statically" literally or not, but here's a brief example of how inheritance at it's most basic form looks in Java. Note that using a getter method to access the variable is a better idea for several reasons -- this is just an example.
public class Dog {
protected String whatISay = "Woof!";
public void speak(){
System.out.println(whatISay);
}
}
public class Poodle extends Dog {
public Poodle(){
whatISay = "Yap!";
}
}
public class Main {
public static void main(String[] args){
Poodle fluffy = new Poodle();
fluffy.speak();
Dog dog = new Dog();
dog.speak();
}
}
Yap!
Woof!
This way of doing it introduces as little intrusion as I could think of. setAttribute() could be named something like setDefaultValue() if that's clearer.
public class A
{
protected int attribute;
public A()
{
setAttribute();
}
public String f()
{
return "CLASS_ATTRIBUTE: " + attribute;
}
protected void setAttribute()
{
attribute = 42;
}
}
public class B extends A
{
#Override
protected void setAttribute()
{
attribute = 44;
}
}
public class Main
{
public static void main(String[] args)
{
A a = new A();
B b = new B();
System.out.println("A: " + a.f());
System.out.println("B: " + b.f());
}
}