class Base {
Base() {
System.out.println("Base Constructor");
}
}
class Derived1 extends Base {
private static String pattern = "a+b+";
Derived1() {
super();
System.out.println("Derived 1 Constructor");
}
public static boolean doesMatch(String v) {
return v.matches(pattern);
}
}
class Derived2 extends Base {
private static String pattern = "c+";
Derived2() {
super();
System.out.println("Derived 2 Constructor");
}
public static boolean doesMatch(String v) {
return v.matches(pattern);
}
}
class Builder {
public static Base baseFromString(String v) throws Exception {
if (Derived1.doesMatch(v)) return new Derived1();
if (Derived2.doesMatch(v)) return new Derived2();
throw new Exception("Could not match " + v + " to any derived type.");
}
}
class Test {
public static void main(String[] args) throws Exception {
Base b = Builder.baseFromString("aaab");
}
}
The code above has a primary problem I want to solve:
The doesMatch method is repeated code for the two derived classes. I'd like to move it to the base class, but then it won't be able to access the pattern member. How do I structure my code better so that each derived class can have its own static pattern, while they all share the same base doesMatch method?
I've tried messing around with abstract classes and interfaces, but I couldn't get anything to work. I am fine with those types of solutions as long as there is a hierarchy where the derived classes either extend or implement the base class.
Secondary question (from original post)
I might want to add several more derived classes. I'd like to not have to update the baseFromString method with another if every time I extend the base class. Is this something that can be solved with polymorphism?
A functional technique (Java 9+), but there is some performance overhead:
class Base {
Base() {
System.out.println("Base Constructor");
}
}
class Derived1 extends Base {
Derived1() {
super();
System.out.println("Derived 1 Constructor");
}
}
class Derived2 extends Base {
Derived2() {
super();
System.out.println("Derived 2 Constructor");
}
}
interface NewBase {
Base create();
}
final class Pattern {
final private String pattern;
final private NewBase newBase;
public Pattern(String pattern, NewBase newBase) {
this.pattern = pattern;
this.newBase = newBase;
}
public String getPattern() {
return pattern;
}
public NewBase getNewBase() {
return newBase;
}
}
class Builder {
final private static List<Pattern> newObjects = new ArrayList<>();
private static void addPattern(String pattern, NewBase newObject) {
newObjects.add(new Pattern(pattern, newObject));
}
static {
addPattern("a+b+", Derived1::new);
addPattern("c+", Derived2::new);
}
public static Base baseFromString(String v) throws Exception {
for (Pattern p : newObjects) {
if (v.matches(p.getPattern()))
return p.getNewBase().create();
}
throw new Exception("Could not match " + v + " to any derived type.");
}
}
Just update the static Builder initializer to call the addPattern for new patterns and derived classes.
You can't do that, at least not with static members. The problem is that static members cannot be overridden.
public class Driver {
public static void main(String args[]) {
Derived aDerived = new Derived();
aDerived.print(); // prints "Value is 5", not "Value is 10"
}
}
public class Base {
protected static final int VALUE = 5;
public Base() {}
protected void print() {
System.out.println("Value is " + VALUE);
}
}
public class Derived extends Base {
protected static final int VALUE = 10; // does not override base Value
public Derived() {}
}
Each subclass can have its own value, and they would all inherit print(). But it doesn't do what you want because print() will always reference Base.VALUE even in the inherited Derived.print().
So, static doesn't work. I assume that you feel the pattern member needs to be static because there only needs to be one copy of the pattern value for the entire class. That one copy part should have tipped you off to a handy little design pattern: the singleton pattern!
public class Driver {
public static void main(String args[]) {
Derived aDerived = new Derived();
aDerived.print(); // prints "Value is 10"
}
}
public class Base {
protected Value val = Value.getInstance();
public Base() {}
protected void print() {
System.out.println("Value is " + val.value());
}
}
public class Derived extends Base {
public Derived() { val = DerivedValue.getInstance(); }
}
public class Value {
private int value = 5;
public int value() { return value; }
private static Value instance = new Value();
protected Value() {}
public static Value getInstance() { return instance; }
}
public class DerivedValue extends Value {
private int value = 10;
public int value() { return value; }
private static DerivedValue instance = new DerivedValue();
private DerivedValue() {}
public static DerivedValue getInstance() { return instance; }
}
There's a lot more code in this version, but now there is only one copy of the two different values used.
Note: Below is how you can make the value members final. You'll have to set up your packages appropriately so the protected Base(Value v) constructor is only visible to the Derived class.
public class Base {
protected final Value val;
public Base() { val = Value.getInstance(); }
protected Base(Value v) { val = v; }
protected void print() {
System.out.println("Value is " + val.value());
}
}
public class Derived extends Base {
public Derived() { super(DerivedValue.getInstance()); }
}
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;
}
While trying to get child class field getting parent field. In this example, when new DarkRoom is being created the int zero is passed to DarkRoom constructor. It supposed to assign zero int to thisRoomNumber field. The next step- creating Tenant and passing one object DarkRoom which we initialized. For some reason in Semaphore class the nextRoom has field thisRoomNumber which inherited from Room and has value 96 despite the fact we assigned zero. What is missing? Why I cannot get 0 from nextRoom.thisRoomNumber The design is quite complex, maybe there is a bit simpler solution?
public abstract class Room {
public int thisRoomNumber = 96;
}
Main method:
public class TestCase {
public static void main(String[] args) {
Room s[] = new Room[1];
{
s[0] = new DarkRoom(0, s);
}
new Tenant("Joe", s[0], s);
}
}
DarkRoom class:
public class DarkRoom extends Room {
private Room thisRoomType = this;
public int thisRoomNumber;
private Room[] rooms;
private Semaphore semaphore= new Semaphore();
#Override
void leave(Tenant t) {
semaphore.release(thisRoomType, rooms, t);
}
public int thisRoomNumber;
public DarkRoom(int i, Room[] s) {
this.thisRoomNumber = i;
this.rooms = s;
}
}
Semaphore class:
public class Semaphore {
public synchronized void release( Room thisRoom, Room[] allRooms, Tenant t) {
Room nextRoom;
while(j < allRooms.length &&
allRooms[j].negativeCounter == 0 &&
thisRoom.getClass().equals(allRooms[j].getClass())){
j++;
}
nextRoom = allRooms[j];
System.out.println(nextRoom.getClass().getName() +"' "+nextRoom.thisRoomNumber);
You are hiding the field thisRoomNumber by declaring it in the subclass as well. Remove the declaration of it in the sub class (DarkRoom).
Fields are not polymorphic in Java. Only methods are. Do what every decent book suggests, and never use public fields. Use a getter instead, and override the getter in the subclass. Fields can't be overridden.
I'm developing an application for Android, and at one point the user chooses a region, after which the contents of all parts of the app is changed. Therefore I need to access an integer throughout the program. I have considered a singleton class, but I can't figure out how to add just an int, a get() and a set() to it (I want to be able to read everywhere and write in two classes(everywhere is fine)).
Should I simply declare it global?
This is what I've got going now, is it ok?
public enum Region {
INSTANCE;
private int rID =0;
public void setRID(int rID) {
this.rID=rID;
}
public int getRID()
{
return rID;
}
}
To be accessed with
Region.INSTANCE.setRID(5);
Try something like
public class State
{
static State instance = new State ();
public static State getInstance() { return instance; }
private int value;
public void setValue (int value) { this.value = value; }
public int getValue () { return value; }
}
Don't forget to store your value when your application exits, and restore it when it reloads.
This is not really a Singleton, but a static class. A class that contains a static field (a field not bounded to an instance):
public class StaticClass {
private static int value;
private Singleton () {}
public static int getValue () {
return value;
}
public static void setValue (int val) {
value = val;
}
}
you can access the value by StaticClass.getValue() and set the value by StaticClass.getValue(4). ('StaticClass must not be replaced by an object).
The private constructor prevents one to create an instance by accident.
Create a public static class and declare the setter and getter as public.
I went with an enum:
public enum Region {
INSTANCE;
private int rID =0;
public void setRID(int rID)
{
this.rID=rID;
}
public int getRID()
{
return rID;
}
}
Which I access with
Region.INSTANCE.getRID();
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";
}
}