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Looking for an implementation of an abstract method

I need to make a programm which is like a rally, theres 2 types of vehicles, motorcycle and cars, two types of motorcycle, with and without sidecar, the thing is that I need to verify if there is just a motorcycle in an array list, I mean, two wheels vehicle. That verification should be done in a method called esDe2Ruedas(), which is called by an abstract overrided method called check() that should be the one that verifies if a group of vehicles from an array are able to run in the rally, if its true all the elements of the array must be from the same type.
Here is the code
this is how the program arrays the vehicles
GrandPrix gp1 = new GrandPrix();
gp1.agregar(v1);
//gp1.mostrar(v1);
gp1.agregar(v2);
System.out.println(gp1.check());
GrandPrix gp2 = new GrandPrix();
gp2.agregar(vt1);
gp2.agregar(vt2);
gp2.agregar(m2);
System.out.println(gp2.check());
GrandPrix gp3 = new GrandPrix();
gp3.agregar(vt1);
gp3.agregar(vt2);
gp3.agregar(m1);
System.out.println(gp3.check());
GrandPrix gp4 = new GrandPrix();
gp4.agregar(m1);
gp4.agregar(m2);
System.out.println(gp4.check());
This is the class that is using
import java.util.ArrayList;
public class GrandPrix extends Rally{
ArrayList<Vehiculo> ve = new ArrayList<Vehiculo>();
public void agregar(Vehiculo v) {
ve.add(v);
}
public void agregar(Carro c) {
ve.add(c);
}
public void agregar(Moto m) {
ve.add(m);
}
#Override
boolean check() {// HERE I VERIFY IF THE VEHICLES ARE COMPATIBLE
return false;
}
}
This is the class where everything goes on
public class Vehiculo {
private String Nombre;
private double velocidad_max;
private int peso;
private int comb;
public Vehiculo() {
setNombre("Anónimo");
setVel(130);
setPeso(1000);
setComb(0);
}
public Vehiculo(String string, double d, int i, int j) {
setNombre(string);
setVel(d);
setPeso(i);
setComb(j);
}
double rendimiento() {
return velocidad_max/peso;
}
public boolean mejor(Vehiculo otroVehiculo) {
return rendimiento()>otroVehiculo.rendimiento();
}
public String toString() {
return getNombre()+"-> Velocidad máxima = "+getVel()+" km/h, Peso = "+getPeso()+" kg";
}
/**************************************
---------SET And GET Nombre------------
***************************************/
public String getNombre() {
return Nombre;
}
public void setNombre(String nuevoNombre) {
this.Nombre=nuevoNombre;
}
/**************************************
---------SET And GET velocidad_max------------
***************************************/
public double getVel() {
return velocidad_max;
}
public void setVel(double nuevaVel) {
this.velocidad_max=nuevaVel;
}
/**************************************
---------SET And GET peso------------
***************************************/
public double getPeso() {
return peso;
}
public void setPeso(int nuevoPeso) {
this.peso=nuevoPeso;
}
/**************************************
---------SET And GET comb------------
***************************************/
public int getComb() {
return comb;
}
public void setComb(int comb) {
this.comb = comb;
}
boolean esDe2Ruedas() {
return false;
}
}
This is the class of motorcycles, which is in theory the same as the car's class, without sidecar thing
public class Moto extends Vehiculo{
private boolean sidecar;
public Moto(String string, double d, int i, int j) {
setNombre(string);
setVel(d);
setPeso(i);
setComb(j);
setSidecar(false);
}
public Moto(String string, double d, int i, int j, boolean b) {
setNombre(string);
setVel(d);
setPeso(i);
setComb(j);
setSidecar(b);
esDe2Ruedas(false);
}
public String toString() {
String str = null;
if(isSidecar())
str =super.toString()+", Moto, con sidecar";
else
str =super.toString()+", Moto";
return str;
}
public boolean isSidecar() {
return sidecar;
}
public void setSidecar(boolean sidecar) {
this.sidecar = sidecar;
}

I guess what you presented is what is given. If you came up with the design it is ok, but I believe it could be improved. Anyway, I try to respond to what I believe was your question straight away.
Vehiculo is the super type of Moto (which can have a side car and becomes 3 wheeler).
Vehiculo has a method esDe2Ruedas, which returns false.
Moto inherits that method <-- this is wrong, it should override it and, depending on side car, return the expected boolean value.
In the check method you can now distinguish between Moto and "Moto with sidecar" by using that method.

toString method in self-defined class using ArrayLists

I am using three classes in my program:
Term class with variables coefficient and exponent, toString() method etc.
Polynome class, using an ArrayList to store the different Term objects.
Main class that runs the program.
Can I use the toString method of ArrayList in my Polynome class? I'm trying to, but I can't.
I need my polynome to output like this: [3x^2, 3x^1, 1x^0]
I am really confused, I'm calling the toString method of Term, using a for-loop to access each term separately.
My code:
public class Term {
private int coëfficiënt;
private int exponent;
public Term(int coëfficiënt, int exponent) {
this.coëfficiënt = coëfficiënt;
this.exponent = exponent;
}
public int getCoef() {
return coëfficiënt;
}
public int getExp() {
return exponent;
}
public String toString() {
return coëfficiënt + "x^" + exponent;
}
}
Polynome class:
public class Polynoom {
private ArrayList<Term> polynoom;
public Polynoom() {
polynoom = new ArrayList<Term>();
}
public void add(Term term) {
polynoom.add(term);
}
public Term get(int i) {
return polynoom.get(i);
}
public int size() {
return polynoom.size();
}
public String toString() {
// what should I write here?
}
}
Main class:
public class opgave3 {
public static void main(String[] args) {
Polynoom polynoom1, polynoom2, sompolynoom;
polynoom1 = new Polynoom();
polynoom1.add(new Term(1, 2));
polynoom1.add(new Term(3, 1));
polynoom1.add(new Term(1, 0));
polynoom2 = new Polynoom();
polynoom2.add(new Term(-1, 3));
polynoom2.add(new Term(2, 2));
polynoom2.add(new Term(-5, 0));
System.out.println("Tests: ");
System.out.println(polynoom1.toString());
for (int i = 0; i < polynoom1.size(); i++) {
System.out.println(polynoom1.get(i).toString());
}
System.out.println(polynoom1.get(0).toString());
}
}

You just need to use your ArrayList's toString() method as the results of Polynome's toString() method.
public class Polynome {
public ArrayList<Term> terms;
#Override
public String toString() {
if (terms != null) {
return terms.toString();
} else {
return "";
}
}
}

EDIT: The quick answer, since you put your code up is to put
return polynoom.toString();
where you have indicated. Then in your Main class you can simply write
System.out.println(polynoom1);
to show the contents in the desired format.
As Tenner said, use the toString() method of your ArrayList to get the desired output. But also make sure your Term class has a useful toString method of its own:
public class Term {
private int co, ex;
public Term(int coeff, int exp) {
co = coeff;
ex = exp;
}
#Override
public String toString() {
return co + "x^" + ex;
}
}

Add #Override toString() to your Term & Polynome class. The Term class toString() should return a string in the format of coefficientx^exponent.
Then have the Polynome class toString() return yourArrayList.toString()
public static void main(String[] args) throws Exception {
Polynome polynome = new Polynome();
polynome.addTerm(3, 2);
polynome.addTerm(3, 1);
polynome.addTerm(1, 0);
System.out.println(polynome);
}
public static class Term {
private int coefficient;
private int exponent;
public Term(int c, int e) {
coefficient = c;
exponent = e;
}
#Override
public String toString() {
return coefficient + "x^" + exponent;
}
}
public static class Polynome {
private List<Term> terms = new ArrayList<>();
public void addTerm(int coefficient, int exponent) {
terms.add(new Term(coefficient, exponent));
}
#Override
public String toString() {
return terms.toString();
}
}
Results:

Long story short, you can ALWAYS use toString() on anything, even if it's a user defined class. When you call the method, it calls the closest parent class's toString() method, which is guaranteed to be there as Object has one. If you want to control the output of toString() called on your object, you must override it. As it is, if you have an object with a member of type ArrayList, calling your object's toString() will include a ton of extra information that you probably don't want. In order to get the output you want, you need to have the code given by #Tenner's answer, which is
public class Polynome {
public ArrayList<Term> terms;
#Override
public String toString() {
if (terms != null) {
return terms.toString();
} else {
return "";
}
}
}
But you also need to override toString() in the Term class, so that each term outputs in the form desired. The reason this is required is that when you call toString() on an ArrayList, or any other container for that matter, it iterates through the container, calling each object's toString() in turn, adding whatever formatting the container class defines. Ultimately, Term's toString() will be called, and you can control that output by overriding it in the Term class.
As for the last part of the question, you need not call Term's toString() directly, as calling the toString() method of the ArrayList will do this on its own.

Java enum instatiate using constructor parameter

I have this enum:
public enum Operation {
ADD {
public double apply(double a, double b) {
return a + b;
}
},
SUBTRACT {
public double apply(double a, double b) {
return a - b;
}
}
} ;
public abstract double apply(double a, double b);
`}`
I want to instantiate this like:
Operation op=new Operation("+");
op.aply(2,3);//now use ADD
Is it possible to write a constructor with string parameter that tells to enum which operation to aply?

You can define a public static method to get the correct operation based on a String:
public static Operation get(String input) {
if(input.equals("+")) {
return ADD;
}
if(input.equals("-")) {
return SUBTRACT;
}
throw new IllegalArgumentException();
}
Then you would call it like:
Operation op = Operation.get("+");
op.apply(2, 3);

Unfortunately, no. You can create a method with a String parameter that returns the specific Operation depending on what is the parameter. Example :
public enum Operation {
ADD("+") {
public double apply(double a, double b) {
return a + b;
}
},
SUBTRACT("-") {
public double apply(double a, double b) {
return a - b;
}
};
private String operationChar;
private Operation(String s) {
this.operationChar = s;
}
public static Operation getOperation(String s) {
for (Operation o : Operation.values()) {
if (o.operationChar.equals(s))
return o;
}
return null;
}
public abstract double apply(double a, double b);
}
You can get the instance like this :
Operation op = Operation.getOperation("+");
op.apply(2,3); // ADD will be used here
(You can use a char instead of a String, since most operations only have one char (1+1, 1-1, 1*1, 1/1))

You can try the following example.
It's a bit over-engineered (you could ditch the whole "+" or "-" and call elements by name) but it works.
public enum Operation {
ADD("+") {
public double apply(double a, double b) {
return a + b;
}
},
SUBTRACT("-") {
public double apply(double a, double b) {
return a - b;
}
};
public abstract double apply(double a, double b);
String type;
Operation(String type) {
this.type = type;
}
public String getType() {
return type;
}
static Operation get(String type) {
for (Operation o: Operation.values()) {
if (o.getType().equals(type)) {
return o;
}
}
throw new NoSuchElementException();
}
public static void main(String[] args) throws Exception {
System.out.println(Operation.get("+").apply(1, 2));
}
}
Output
3.0

You can define a constructor in an enum, but you cannot use it outside of the enum definition. The available enum values are fixed at compile time and you can't create more at runtime by using new. If you want to look up the enum by symbol, you need to define a lookup in the code.
For example by switching on the symbol:
public enum Operation {
ADD {/*...*/};
// ...
public static Operation forSymbol(String symbol) {
switch(symbol) {
case "+":
return ADD;
//...
default:
throw new IllegalArgumentException("Unsupported symbol: " + symbol);
}
}
}
Or by defining the symbol together with the enum, and using that in the look up:
public enum Operation {
ADD("+") {/*...*/};
private final String symbol;
private Operation(String symbol) {
this.symbol = symbol;
}
// ...
public static Operation forSymbol(String symbol) {
for (Operation operation : Operation.values()) {
if (operation.symbol.equals(symbol)) return operation
}
throw new IllegalArgumentException("Unsupported symbol: " + symbol);
}
}

It is not possible, due to the fact that enums cannot have public constructors (more information in this question). To use an Operation you will have to manually tell the compiler which Operation to use. This can be done by:
Operation op = ADD;
double x = op.apply(2,3);
which uses ADD as the Operation.

How to write a method that returns an instance of an abstract class?

I am a beginner in Java and i trying to understand the abstract classes.
Below is the code that I've written; the question is: how do i write a method that will return an instance of that class.
public abstract class VehicleEngine
{
protected String name;
protected double fabricationCons;
protected double consum;
protected int mileage;
public VehicleEngine(String n, double fC)
{
name = n;
fabricationCons = fC;
mileage = 0;
consum = 0;
}
private void setFabricationCons(double fC)
{
fabricationCons = fC;
}
public abstract double currentConsum();
public String toString()
{
return name + " : " + fabricationCons + " : " + currentConsum();
}
public void addMileage(int km)
{
mileage += km;
}
public double getFabricationConsum()
{
return fabricationCons;
}
public String getName()
{
return name;
}
public int getMileage()
{
return mileage;
}
//public VehicleEngine get(String name){
//if(getName().equals(name)){
//return VehicleEngine;
//}
//return null;
//}
}
public class BenzinVehicle extends VehicleEngine
{
public BenzinVehicle(String n, double fC)
{
super(n, fC);
}
#Override
public double currentConsum()
{
if (getMileage() >= 75000) {
consum = getFabricationConsum() + 0.4;
} else {
consum = getFabricationConsum();
}
return consum;
}
}
public class DieselVehicle extends VehicleEngine
{
public DieselVehicle(String n, double fC)
{
super(n, fC);
}
#Override
public double currentConsum()
{
int cons = 0;
if (getMileage() < 5000) {
consum = getFabricationConsum();
} else {
consum = getFabricationConsum() + (getFabricationConsum() * (0.01 * (getMileage() / 5000)));
}
return consum;
}
}
This is the main.
public class Subject2
{
public static void main(String[] args)
{
VehicleEngine c1 = new BenzinVehicle("Ford Focus 1.9", 5.0);
DieselVehicle c2 = new DieselVehicle("Toyota Yaris 1.4D", 4.0);
BenzinVehicle c3 = new BenzinVehicle("Citroen C3 1.6",5.2);
c1.addMileage(30000);
c1.addMileage(55700);
c2.addMileage(49500);
c3.addMileage(35400);
System.out.println(c1);
System.out.println(c2);
System.out.println(VehicleEngine.get("Citroen C3 1.6")); //this is the line with problems
System.out.println(VehicleEngine.get("Ford Focus "));
}
}
And the output should be:
Ford Focus 1.9 : 5.0 : 5.4
Toyota Yaris 1.4D : 4.0 : 4.36
Citroen C3 1.6 : 5.2 : 5.2
null

You can not return an instance of an abstract class, by definition. What you can do, is return an instance of one of the concrete (non-abstract) subclasses that extend it. For example, inside the VehicleEngine you can create a factory that returns instances given the type of the instance and the expected parameters, but those instances will necessarily have to be concrete subclasses of VehicleEngine

Have a look at the Factory Method pattern. Your concrete classes will implement an abstract method that returns a class instance.

Abstract classes do not keep a list of their instances. Actually no Java class does that. If you really want to do that, you could add a static map to VehicleEngine like this:
private static Map<String, VehicleEngine> instanceMap = new HashMap<String, VehicleEngine>();
and change your get method to a static one like this:
public static VehicleEngine get(String name) {
return instanceMap.get(name);
}
and add this line to the end of the constructor of VehicleEngine:
VehicleEngine.instanceMap.put(n, this);
this way every new instance created puts itself into the static map. However this actually is not a good way to implement such a functionality. You could try to use a factory to create instances, or you could consider converting this class into an enum if you will have a limited predefined number of instances.

How to return 2 values from a Java method?

I am trying to return 2 values from a Java method but I get these errors. Here is my code:
// Method code
public static int something(){
int number1 = 1;
int number2 = 2;
return number1, number2;
}
// Main method code
public static void main(String[] args) {
something();
System.out.println(number1 + number2);
}
Error:
Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - missing return statement
at assignment.Main.something(Main.java:86)
at assignment.Main.main(Main.java:53)
Java Result: 1

Instead of returning an array that contains the two values or using a generic Pair class, consider creating a class that represents the result that you want to return, and return an instance of that class. Give the class a meaningful name. The benefits of this approach over using an array are type safety and it will make your program much easier to understand.
Note: A generic Pair class, as proposed in some of the other answers here, also gives you type safety, but doesn't convey what the result represents.
Example (which doesn't use really meaningful names):
final class MyResult {
private final int first;
private final int second;
public MyResult(int first, int second) {
this.first = first;
this.second = second;
}
public int getFirst() {
return first;
}
public int getSecond() {
return second;
}
}
// ...
public static MyResult something() {
int number1 = 1;
int number2 = 2;
return new MyResult(number1, number2);
}
public static void main(String[] args) {
MyResult result = something();
System.out.println(result.getFirst() + result.getSecond());
}

Java does not support multi-value returns. Return an array of values.
// Function code
public static int[] something(){
int number1 = 1;
int number2 = 2;
return new int[] {number1, number2};
}
// Main class code
public static void main(String[] args) {
int result[] = something();
System.out.println(result[0] + result[1]);
}

You could implement a generic Pair if you are sure that you just need to return two values:
public class Pair<U, V> {
/**
* The first element of this <code>Pair</code>
*/
private U first;
/**
* The second element of this <code>Pair</code>
*/
private V second;
/**
* Constructs a new <code>Pair</code> with the given values.
*
* #param first the first element
* #param second the second element
*/
public Pair(U first, V second) {
this.first = first;
this.second = second;
}
//getter for first and second
and then have the method return that Pair:
public Pair<Object, Object> getSomePair();

You can only return one value in Java, so the neatest way is like this:
return new Pair<Integer>(number1, number2);
Here's an updated version of your code:
public class Scratch
{
// Function code
public static Pair<Integer> something() {
int number1 = 1;
int number2 = 2;
return new Pair<Integer>(number1, number2);
}
// Main class code
public static void main(String[] args) {
Pair<Integer> pair = something();
System.out.println(pair.first() + pair.second());
}
}
class Pair<T> {
private final T m_first;
private final T m_second;
public Pair(T first, T second) {
m_first = first;
m_second = second;
}
public T first() {
return m_first;
}
public T second() {
return m_second;
}
}

Here is the really simple and short solution with SimpleEntry:
AbstractMap.Entry<String, Float> myTwoCents=new AbstractMap.SimpleEntry<>("maximum possible performance reached" , 99.9f);
String question=myTwoCents.getKey();
Float answer=myTwoCents.getValue();
Only uses Java built in functions and it comes with the type safty benefit.

Use a Pair/Tuple type object , you don't even need to create one if u depend on Apache commons-lang. Just use the Pair class.

you have to use collections to return more then one return values
in your case you write your code as
public static List something(){
List<Integer> list = new ArrayList<Integer>();
int number1 = 1;
int number2 = 2;
list.add(number1);
list.add(number2);
return list;
}
// Main class code
public static void main(String[] args) {
something();
List<Integer> numList = something();
}

public class Mulretun
{
public String name;;
public String location;
public String[] getExample()
{
String ar[] = new String[2];
ar[0]="siva";
ar[1]="dallas";
return ar; //returning two values at once
}
public static void main(String[] args)
{
Mulretun m=new Mulretun();
String ar[] =m.getExample();
int i;
for(i=0;i<ar.length;i++)
System.out.println("return values are: " + ar[i]);
}
}
o/p:
return values are: siva
return values are: dallas

I'm curious as to why nobody has come up with the more elegant callback solution. So instead of using a return type you use a handler passed into the method as an argument. The example below has the two contrasting approaches. I know which of the two is more elegant to me. :-)
public class DiceExample {
public interface Pair<T1, T2> {
T1 getLeft();
T2 getRight();
}
private Pair<Integer, Integer> rollDiceWithReturnType() {
double dice1 = (Math.random() * 6);
double dice2 = (Math.random() * 6);
return new Pair<Integer, Integer>() {
#Override
public Integer getLeft() {
return (int) Math.ceil(dice1);
}
#Override
public Integer getRight() {
return (int) Math.ceil(dice2);
}
};
}
#FunctionalInterface
public interface ResultHandler {
void handleDice(int ceil, int ceil2);
}
private void rollDiceWithResultHandler(ResultHandler resultHandler) {
double dice1 = (Math.random() * 6);
double dice2 = (Math.random() * 6);
resultHandler.handleDice((int) Math.ceil(dice1), (int) Math.ceil(dice2));
}
public static void main(String[] args) {
DiceExample object = new DiceExample();
Pair<Integer, Integer> result = object.rollDiceWithReturnType();
System.out.println("Dice 1: " + result.getLeft());
System.out.println("Dice 2: " + result.getRight());
object.rollDiceWithResultHandler((dice1, dice2) -> {
System.out.println("Dice 1: " + dice1);
System.out.println("Dice 2: " + dice2);
});
}
}

You don't need to create your own class to return two different values. Just use a HashMap like this:
private HashMap<Toy, GameLevel> getToyAndLevelOfSpatial(Spatial spatial)
{
Toy toyWithSpatial = firstValue;
GameLevel levelToyFound = secondValue;
HashMap<Toy,GameLevel> hm=new HashMap<>();
hm.put(toyWithSpatial, levelToyFound);
return hm;
}
private void findStuff()
{
HashMap<Toy, GameLevel> hm = getToyAndLevelOfSpatial(spatial);
Toy firstValue = hm.keySet().iterator().next();
GameLevel secondValue = hm.get(firstValue);
}
You even have the benefit of type safety.

Return an Array Of Objects
private static Object[] f ()
{
double x =1.0;
int y= 2 ;
return new Object[]{Double.valueOf(x),Integer.valueOf(y)};
}

In my opinion the best is to create a new class which constructor is the function you need, e.g.:
public class pairReturn{
//name your parameters:
public int sth1;
public double sth2;
public pairReturn(int param){
//place the code of your function, e.g.:
sth1=param*5;
sth2=param*10;
}
}
Then simply use the constructor as you would use the function:
pairReturn pR = new pairReturn(15);
and you can use pR.sth1, pR.sth2 as "2 results of the function"

You also can send in mutable objects as parameters, if you use methods to modify them then they will be modified when you return from the function. It won't work on stuff like Float, since it is immutable.
public class HelloWorld{
public static void main(String []args){
HelloWorld world = new HelloWorld();
world.run();
}
private class Dog
{
private String name;
public void setName(String s)
{
name = s;
}
public String getName() { return name;}
public Dog(String name)
{
setName(name);
}
}
public void run()
{
Dog newDog = new Dog("John");
nameThatDog(newDog);
System.out.println(newDog.getName());
}
public void nameThatDog(Dog dog)
{
dog.setName("Rutger");
}
}
The result is:
Rutger

You can create a record (available since Java 14) to return the values with type safety, naming and brevity.
public record MyResult(int number1, int number2) {
}
public static MyResult something() {
int number1 = 1;
int number2 = 2;
return new MyResult(number1, number2);
}
public static void main(String[] args) {
MyResult result = something();
System.out.println(result.number1() + result.number2());
}

First, it would be better if Java had tuples for returning multiple values.
Second, code the simplest possible Pair class, or use an array.
But, if you do need to return a pair, consider what concept it represents (starting with its field names, then class name) - and whether it plays a larger role than you thought, and if it would help your overall design to have an explicit abstraction for it. Maybe it's a code hint...
Please Note: I'm not dogmatically saying it will help, but just to look, to see if it does... or if it does not.