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Java invoke child method from parent object

I've got next situation:
There is an abstract class
public abstract class SuperClass {
public abstract void getString();
public abstract void method2();
}
public class InheritClass1 extends SuperClass {
#Override
public void getString(){...};
#Override
public void method2(){...};
}
public class InheritClass2 extends SuperClass {
#Override
public void getString{...};
#Override
public void method2(){...};
public void customMethod(){...};
}
There is another class that has a method that accepts SuperClass object as an argument. Depending on what kind of String is returned from getString I perform different actions. My case is that I am trying to call a child method while the object is of parent class:
public class Processor {
public String method(SuperClass type) {
switch (type.getString()) {
case "1":
return "OK"
case "2":
return ((InheritClass2) type).customMethod()
}
}
I do understand that this is BAD DESIGN, could you please help me with finding the best solution for this problem. Maybe generics are suitable in this case somehow. Also the thing is that customMethod() should be a part of not all classes.

Since only some (sub)classes implements customMethod, I would suggest to create an interface that contains this method:
public interface CustomInterface {
public String customMethod();
}
Your SuperClass can then remain just as it is. Only the subclasses/child classes that have customMethod, would then extend your SuperClass as well as implement this CustomInterface. This way, the child classes that do not implement CustomMethod (does not have the method in their class, such as InheritClass1 in your example), also remain just as they are.
Only child classes that have CustomMethod, such as InheritClass2 would then need to change slightly by saying it implements this new interface:
public class InheritClass2 extends SuperClass implements CustomInteface {
// the rest stays the same
}
Then in the section where you want to do the casting, you rather do the following:
public class Processor {
public String method(SuperClass type) {
switch (type.getString()) {
case "1":
return "OK"
case "2":
String s = "";
if (type instance of CustomInterface) {
s = (CustomInterface type).customMethod();
}
return s;
}
}
}
Using the interface in this way will help that you can implement all child classes and not just one as implementing the CustomInterface, and thus, all child classes will work with using instanceof and casting to the interface to call customMethod() - you won't have to handle each child that needs this method separately.
NOTE: Your code is clearly simplified example, it is unclear if the getString() method is just returning an identifier of the child classes in order for you to know which ones you can cast and then call custom Method on... If this is the purpose of your switch and getString methods - to identify which types implement the customMethod() and to call that method, and for any child class that does not have that method to return just "OK" - then you could instead do the following:
public class SubClass1 extends SuperClass implements CustomInterface {
// other mehtods...
public String CustomMethod() { return "SomeString1"; }
}
public class SubClass2 extends SuperClass {
// other methods...
// this subclass does not have the CustomMethod()
}
public class SubClass3 extends SuperClass implements CustomInterface {
// other methods...
public String CustomMethod() { return "SomeString3"; }
}
Then your Processor could look like this:
public class Processor {
public String method(SuperClass type) {
return (type instanceof CustomInterface) ? ((CustomInterface) type).CustomMethod() : "OK";
}
public static void main(String[] args) {
Processor p = new Processor();
SuperClass obj1 = new SubClass1();
SuperClass obj2 = new SubClass2();
SuperClass obj3 = new SubClass3();
System.out.println(p.method(obj1)); // prints: "SomeString1"
System.out.println(p.method(obj2)); // prints: "OK"
System.out.println(p.method(obj3)); // prints: "SomeString3"
}
}
If you don't understand the ternary operator then you can read about it here That's the condition ? exprTrue : exprFalse syntax. It's a short if else statement basically.

You can create an interface, with default custom method implementation, like:
interface A {
default String customMethod() {
return "";
}
}
And abstract class will implement this interface:
public abstract class SupperClass implements A {
public abstract String getString();
public abstract void method2();
}

Bad design will cause you to get bad answers. If you don't want to cast your object to a child object. You could use reflection.
import java.lang.reflect.Method;
public class Processor {
public String method(SuperClass type) {
Method[] methods = type.getClass().getMethods();
for (Method m : methods) {
if (m.getName().equals("customMethod")) {
try {
return m.invoke(type);
} catch (Exception ex) {
// throw
}
}
}
return "OK";
}
}

Depending on your design you could apply:
if (type instanceof InheritClass2.class) return type.customMethod();
or
if (type.getClass() == InheritClass2.class) return type.customMethod();

Can a super class method implementation depend on child class field

I am in a situation as follows.
I have an interface A which is inherited by class B,C,D (B,C,D implements A).
public interface A{
public String someMethod();
}
class B implements A{
ObjectType1 model;
#Override
public String someMethod(){
if(model instanceof X){
System.out.print(true);
}
}
}
class C implements A{
ObjectType2 model;
#Override
public String someMethod(){
if(model instanceof X){
System.out.print(true);
}
}
class D implements A{
ObjectType3 model;
#Override
public String someMethod(){
if(model instanceof X){
System.out.print(true);
}
}
As you can see all method implementations are the same. So I am duplicating code. My plan was to move the method to A and make A an abstract class. But the problem is my method depends on the model field. So what would be my options to make this code better?
bdw class A,B,C extends and implements other classes too.
EDIT
modification in code. check field

I don't see any problem related to the model field transforming the interface A into an abstract class.
There is no need to reimplement the method in the subclasses if it is the same, unless you want to change its behavior (override it).
public abstract class A {
// Make it protected so it can accessible by subclasses
protected Object model;
// Common behavior that will be inherited by subclasses
public String someMethod() {
if (model instanceof X) {
return "x";
} else {
return "not x";
}
}
}
public class B extends A {
// Subclasses may access superclasses fields if protected or public.
public void someOtherMethod() {
System.out.println(super.model.toString());
}
}
public class C extends A {
// You may wish to override a parent's method behavior
#Override
public String someMethod() {
return "subclass implements it different";
}
}
For your new code example, if you really want to do that in a procedural way you can create an abstract superclass ObjectType and then it will be accessible for the parent as well.
However I wouldn't do that. It seems to me that in doing so is the very opposite of what object orientation tries to solve.
By using a subclass to define the behavior, you wouldn't need to do it in a procedural logic. That's precisely then point of using objects, inheritance and overriding/implementing behavior as needed.

Create a parent class A with said field, and said function. Have the other classes extend A. No need to override them if they function the same.

To deduplicate, you can either make A an abstract class and move the implementation of the method and the field there, or create an abstract class, say E, that implements the interface with that method and field and then have B, C and D extend that class E.
For the more general question of depending on a subclass's field, you can create an abstract method getModel which the subclasses decide how to implement -- by returning a model field or doing something else.

If you are using java 8 you could use default method in interface A, with a getter method for model.
public interface A{
default public String someMethod() {
if(getModel() instanceof X){
System.out.print(true);
}
}
public Object model getModel();
}
Then implement getModel method in all child interfaces.

If you're going to do this you must have model to be of the same (basic) type in all derived objects. If it were of the same type there's a case for putting the model to a base class. Anyway if they are of different derived types you would need to have an accessor to get it.
interface B {
BaseModel getModel();
default public strict doSomething() {
BaseModel m = getModel();
// do something with m
}
}
class D implements B {
DerivedModel model;
public getModel() {
return model;
}
}

If I was given a chance to refactor it, I will follow below approach, leveraging Java 8 Default Methods:
interface A {
default String someMethod(X objectType) {
if (objectType instanceof X) {
System.out.println(true);
}
// return something, for now returning class
return objectType.getClass().toString();
}
}
class B implements A {
#Override
public String someMethod(X objectType) {
if (objectType instanceof X) {
System.out.println(true);
}
// return "Hello"
return "Hello";
}
}
class C implements A {}
class D implements A {}
Usage:
public class Main implements A {
public static void main(String[] args) {
B b = new B();
C c = new C();
D d = new D();
Main main = new Main();
main.call(b);
main.call(c);
main.call(d);
}
public void call(A clazz) {
ObjectType1 objectType1 = new ObjectType1();
String type = clazz.someMethod(objectType1);
System.out.println(type);
}
}
interface X {
}
class ObjectType1 implements X {
}

Calling method from abstract class

I have the following abstract class
public abstract class ReturnAgentFromTab extends BasePage{
#Persist("session")
public abstract Agent getAgent();
public abstract void setAgent(Agent agent);
#InjectObject("spring:agentApplicationModeResolver")
public abstract AgentApplicationModeResolver getAgentApplicationModeResolver();
.... more #InjectObject()
public void nextPage(IRequestCycle cycle) {
setApplicationModeUsingAgentStatus(getAgent());
AgentSearchNavigationManager navManager = getAgentSearchNavigationManagerFactory().getAgentSearchNavigationManager();
FlowStage stage = getFlowStage();
if (stage == null) {
setApplicationModeUsingAgentStatus(getAgent());
stage = getUserDefaultFlowStageService().getDefaultFlowStage(UserHolder.getUser(), getVisitClass().getApplicationMode());
}
Class nextPageClass = navManager.getNextPage(getUserDefaultFlowStageService());
String nextPageQualifier = getUserDefaultFlowStageService().getPageQualifier(getAgent(), nextPageClass, getVisitClass().getApplicationMode());
IPage nextPage = getPageUtils().getPage(nextPageClass, nextPageQualifier);
if ((getFlowStage() instanceof PSDFlowStage)) {
nextPageQualifier = getFlowStage().getValue();
}
nextPage = getPageUtils().getPage(nextPageClass, nextPageQualifier);
if (navManager instanceof NameBasedAgentSearchNavigationManager && nextPageClass != SignOffStatusPage.class) {
NameBasedAgentSearchNavigationManager nameBasedNavManager = (NameBasedAgentSearchNavigationManager) navManager;
String nextPageName = nameBasedNavManager.getNextPageName(stage);
if (!nextPageName.equals(nextPageClass.getSimpleName())) {
nextPage = getPageUtils().getPage(nextPageName, nextPageQualifier);
}
}
if (isNextPageActivateAgentGeneral(nextPage)) {
initialisePageLink(nextPageClass, nextPage);
}
((WuamsBasePage) nextPage).init(getAgent().getAgentId());
getPageUtils().navigateTo(nextPage);
}
private void setApplicationModeUsingAgentStatus(Agent agent) {
getVisitClass().setApplicationMode(getHomeLinksFactory().getRegionHomeLinksService().getApplicationMode(agent));
}
private boolean isNextPageActivateAgentGeneral(IPage nextPage) {
return nextPage instanceof ActiveAgentGeneralPage;
}
private void initialisePageLink(Class nextPageClass, IPage nextPage) {
if (getVisitClass().getPageLink() == null) {
getVisitClass().setPageLink(PageLinkUtil.getPageLinkMessageKeyFromPageClass(nextPageClass,
getUserDefaultFlowStageService().getDefaultFlowStage(UserHolder.getUser(), getVisitClass().getApplicationMode()).getValue()));
}
}
}
What I want to do is call my nextPage(cycle) from another class that is abstract and extends ReturnAgentFromTab, but when I try
public abstract class DoSomethingWithAgent extends ReturnAgentFromTab {
#Persist("session")
public abstract ReturnAgentFromTab getReturnAgentFromTab();
public abstract void setReturnAgentFromTab(ReturnAgentFromTab returnAgentFromTab);
....
getReturnAgentFromTab().nextPage(cycle);
I get a null pointer exception, I know this is because I am not actually setting ReturnAgentFromTab anywhere but I do not understand how to set it using abstract classes. Can anybody help?
If ye need more code just ask

The point of abstract classes is to simply not implement certain things, such as providing certain objects. The method getReturnAgentFromTab() is a perfect example: the class itself does not care where that object comes from because that is the sole responsibility of the subclass. So extend that class, write that method, and all of a sudden the base class does its thing.

well, you cant intialize abstract class, the only way is to make some other concrete class extend your abstract class, and call the non abstract method with the concrate classes instance.
abstarct class ABS1 {
//abstract methods
//concreate method
public void concMethod() {
}
}
public class ABS1Impl extends ABS1 {
//implement all the abstract methods
}
public abstract class ABS2 {
ABS1 abs = new ABSImpl();
abs.concMethod //
}

Super class which uses the values from children

I wanted to implement a method in a abstract class that is called by the inherited classes and uses their values.
For instance:
abstract class MyClass{
String value = "myClass";
void foo(){System.out.println(this.value);}
}
public class childClass{
String value="childClass";
void foo(){super.foo();}
}
public static void main(String[] args){
new childClass.foo();
}
This will output "myClass" but what I really want is to output "childClass". This is so I can implement a "general" method in a class that when extended by other classes it will use the values from those classes.
I could pass the values as function arguments but I wanted to know if it would be possible to implement the "architecture" I've described.
A super method called by the inherited class which uses the values from the caller not itself, this without passing the values by arguments.

You could do something like this:
abstract class MyClass {
protected String myValue() {
return "MyClass";
}
final void foo() {
System.out.println(myValue());
}
}
public class ChildClass extends MyClass {
#Override
protected String myValue() {
return "ChildClass";
}
}
and so on

This is a place where composition is better than inheritance
public class Doer{
private Doee doee;
public Doer(Doee doee){
this.doee = doee;
}
public void foo(){
System.out.println(doee.value);
}
}
public abstract class Doee{
public String value="myClass"
}
public ChildDoee extends Doee{
public String= "childClass"
}
...
//Excerpt from factory
new Doer(new ChildDoee);

I believe you are asking whether this is possible:
public class MyClass {
void foo() {
if (this instanceof childClass) // do stuff for childClass
else if (this intanceof anotherChildClass) // do stuff for that one
}
}
So the answer is "yes, it's doable", but very much advised against as it a) tries to reimplement polymorphism instead of using it and b) violates the separation between abstract and concrete classes.

You simply want value in MyClass to be different for an instance of childClass.
To do this, change the value in the childClass constructor:
public class childClass {
public childClass() {
value = "childClass";
}
}
Edited:
If you can't override/replace the constructor(s), add an instance block (which gets executed after the constructor, even an undeclared "default" constructor):
public class childClass {
{
value = "childClass";
}
}

Can I have an abstract builder class in java with method chaining without doing unsafe operations?

I'm trying to have an abstract base class for some builder classes so I can easily reuse code between the Builder implementations. I want my builders to support method chaining therefore a method has to return "this" instance of the most specific type. I figured I could probably do this with generics. Unfortunatly I did not manage to do it without using unsafe operations. Is it possible?
Sample code of how I'm trying it (and how it works) below. I'd like to avoid casting to T in "foo()" (which causes an unchecked warning), can this be done?
public class Builders
{
public static void main( final String[] args )
{
new TheBuilder().foo().bar().build();
}
}
abstract class AbstractBuilder<T extends AbstractBuilder<?>>
{
public T foo()
{
// set some property
return (T) this;
}
}
class TheBuilder extends AbstractBuilder<TheBuilder>
{
public TheBuilder bar()
{
// set some other property
return this;
}
public Object build()
{
return new Object();
}
}

You want to declare T as extends AbstractBuilder<T> in AbstractBuilder.
Use an abstract protected method to get this of type T.
abstract class AbstractBuilder<T extends AbstractBuilder<T>> {
protected abstract T getThis();
public T foo() {
// set some property
return getThis();
}
}
class TheBuilder extends AbstractBuilder<TheBuilder> {
#Override protected TheBuilder getThis() {
return this;
}
...
}
Alternatively, drop the generic type parameter, rely on covariant return types and make the code cleaner for clients (although usually they would be using TheBuilder rather than the largely implementation detail of the base class), if making the implementation more verbose.

One alternative is not to use generics, but use overrides:
abstract class AbstractBuilder
{
public AbstractBuilder foo()
{
// set some property
return this;
}
}
class TheBuilder extends AbstractBuilder
{
#Override public TheBuilder foo()
{
super.foo(); return this;
}
public TheBuilder bar()
{
// set some other property
return this;
}
public Object build()
{
return new Object();
}
}

This should help:
abstract class AbstractBuilder<T extends AbstractBuilder<?>>
{
public AbstractBuilder<T> foo()
{
// set some property
return (AbstractBuilder<T>) this;
}
abstract AbstractBuilder<T> bar();
abstract Object build();
}