Instanceof

[KevinNguyen]

Hi guys, for example, I have class A, B extends A. I create an ArrayList<A> to contain both A and B

ArrayList<A> lst = new ArrayList<A>();
lst.add(new B(args));

So when I check if an object is instance of B

if (lst.get(0) instanceof B) {
//Do something
}
else if (lst.get(0) instanceof A) {
//Do something 
}

two conditions will be true. I wanna ask if there is any way to recognise that lst.get(0) is only an instance of B so the second condition should not be done.
Thanks.

[KevinWorkman]

You're using an else if. If the first condition is true, the second condition will never even be evaluated.

[KevinNguyen]

Oh, sorry. In my case

for (int j = 0; j < lst.size(); j++) 
                    	if (lst.get(j) instanceof B) {
                    		System.out.println(lst.get(j));
                    	}

for (int j = 0; j < lst.size(); j++) 
                    	if (lst.get(j) instanceof A) {
                    		System.out.println(lst.get(j));
                    	}

the second code block will also print the first result.

[KevinWorkman]

So why don't you just do the checks together, like in your first example? Or use a compound boolean expression (using && or ||).

[KevinNguyen]

Yo, I use switch() to select what to do so I have to split them. I will also post source here:

package QuestionDatabaseManagement;

import java.io.*;
import java.util.*;
 
public class Question {
	protected String ques, ans;	
	protected int type; //0:NOTSET(only use to initial beginning value)
    //1:MATH | 2.PHYSICS | 3.ENG | 4.LITERATURES | 5.CHE | 6. ASTRO
    //7. GEO | 8. BIO | 9: HIS | 10: LAW | 11: ARTS&CULTURE
    //12.UNDEFINED
	protected String[] Category = {"NOTSET","MATH","PHY","ENG",
               "LIT","CHE","ASTRO","GEO","BIO","HIS","LAW","ARTS & CULTURE","UNDEFINED"};
	
	protected int dif; 	//0: NOTSET
						//1: EASY | 2: MEDIUM | 3: HARD
						//4: UNDIFINED
	protected String[] Dif = {"NOTSET","EASY","MEDIUM","HARD","UNDEFINED"};
	
	public String getQues() {
		return ques;
	}

	public void setQues(String ques) {
		this.ques = ques;
	}

	public String getAns() {
		return ans;
	}

	public void setAns(String ans) {
		this.ans = ans;
	}

	public int getType() {
		return type;
	}

	public void setType(int type) {
		this.type = type;
	}

	public int getDif() {
		return dif;
	}

	public void setDif(int dif) {
		this.dif = dif;
	}
	
	public Question(String ques, String ans, int type, int dif) {
		this.ques = ques;
		this.ans = ans;
		this.type = type;
		this.dif = dif;
    }
 
    public Question() {
        this("","",0,0);
    }
    
    public String toString() {
        return 	"Type: " + type +
        		"\nDificulty: " + dif +
        		"\nQuestion: " + ques +
        		"\nAnswer: " + ans +
        		"\n***************************************************************************************";
    }
    
    public void input() {
    	System.out.print("Enter type: ");
 	    type = new Scanner(System.in).nextInt();
 	    System.out.print("Enter difficulty: ");
 	    dif = new Scanner(System.in).nextInt();
 	    System.out.print("Enter question: ");
 	    ques = new Scanner(System.in).nextLine();  
 	    System.out.print("Enter answer: ");
	    ans = new Scanner(System.in).nextLine();
    }
    
}

package QuestionDatabaseManagement;

import java.io.*;
import java.util.*;
 
public class MultiChoiceQuestion extends Question implements Serializable {

	private static final long serialVersionUID = 7189152575673931024L;
	
	private String optA, optB, optC, optD;
	private static int count = 0;
	
	public int getCount() {
		return count;
	}
	
    public String getOptA() {
        return optA;
    }
 
    public void setOptA(String optA) {
        if (optA.length()>0) 
            this.optA = optA;
    }
 
    public String getOptB() {
        return optB;
    }
 
    public void setOptB(String optB) {
        if (optB.length()>0)
            this.optB = optB;
    }
 
    public String getOptC() {
        return optC;
    }
 
    public void setOptC(String optC) {
        if (optC.length()>0)
            this.optC = optC;
    }
 
    public String getOptD() {
        return optD;
    }
 
    public void setOptD(String optD) {
        if (optD.length()>0)
            this.optD = optD;
    }

    public MultiChoiceQuestion(String ques,
			String optA, String optB, String optC, String optD, String ans, int type, int dif) {
    	super(ques,ans,type,dif);
		this.optA = optA;
		this.optB = optB;
		this.optC = optC;
		this.optD = optD;
		count++;
	}

	public MultiChoiceQuestion() {
        this("","","","","","",0,0);
    }

	 public void input() {
	    System.out.print("Enter type: ");
	    type = new Scanner(System.in).nextInt();
	    System.out.print("Enter difficulty: ");
	    dif = new Scanner(System.in).nextInt();
	    System.out.print("Enter question: ");
	    ques = new Scanner(System.in).nextLine();
	    System.out.print("Enter option A: ");
	    optA = new Scanner(System.in).nextLine();
	    System.out.print("Enter option B: ");
	    optB = new Scanner(System.in).nextLine();
	    System.out.print("Enter option C: ");
	    optC = new Scanner(System.in).nextLine();
	    System.out.print("Enter option D: ");
	    optD = new Scanner(System.in).nextLine();
	    System.out.print("Enter answer: ");
	    ans = new Scanner(System.in).nextLine();
	}

    public String toString() {
        return "Type: " + Category[type] +
        "\nDifficulty: " + Dif[dif] +
        "\nQuestion: " + ques +
        "\n\tA. " + optA +
        "\n\tB. " + optB +
        "\n\tC. " + optC +
        "\n\tD. " + optD +
        "\nAnswer: " + ans +
    	"\n***************************************************************************************";
    }
    
}

package QuestionDatabaseManagement;
 
import java.util.*;
import java.io.*;
  
public class QuestionManagement {
	public static void main(String[] args) throws NoSuchElementException, IOException, ClassNotFoundException {
    	ArrayList<Question> Data = new ArrayList<Question>();
    	
    	FileOutputStream file_out = new FileOutputStream("data.dat");
    	Data.add(new MultiChoiceQuestion("Bộ phim hoạt hình nổi tiếng Shrek của hãng nào sản xuất?","Walt Disney","Pixar","DreamWorks Animation","Madhouse","C",11,2));
    	Data.add(new MultiChoiceQuestion("Số tiếp tục trong dãy số dưới đây là số nào?: 6 20 62 188","564","566","568","600","B",1,1));
    	Data.add(new MultiChoiceQuestion("Nước ở đâu rộng nhất Thế Giới ?","Mỹ","Nga","Trung Quốc","Khác","D",7,1));
    	Data.add(new MultiChoiceQuestion("Bạn có biết có khoảng bao nhiêu ngôn ngữ có trên trái đất?","3500","4700","6500","7300","C",11,2));
    	Data.add(new MultiChoiceQuestion("Trong 4 con vật sau đây, thì con nào có đặc điểm ít giống nhất với 3 con còn lại?","Chó","Voi","Mèo","Rùa","D",8,1));
    	Data.add(new Question("Sông nào có một bởi ở châu Âu, một bởi ở châu Á??","Sông U-ran",7,2));
    	Data.add(new Question("Hành tinh nào lớn nhất trong hệ Mặt Trời?","Mộc tinh",6,1));
    	Data.add(new Question("Trong bức tranh đám cưới chuột có tất cả bao nhiêu con chuột?","12",3,3));
    	Data.add(new Question("Đồng bằng nào lớn nhất thế giới?","Amazon",4,4));
    	Data.add(new Question("Chữ cái cuối cùng trong bảng chữ cái Hy Lạp?","Omega",5,0));
    	ObjectOutputStream StreamOut_1 = new ObjectOutputStream(file_out);
    	StreamOut_1.writeObject(Data);
 		StreamOut_1.close();
 		
    	FileInputStream file_in = new FileInputStream("data.dat");
    	ObjectInputStream StreamIn = new ObjectInputStream(file_in);
    	Data = (ArrayList)StreamIn.readObject();
    	StreamIn.close();
    	
        Scanner Sc = new Scanner(System.in);
        boolean isReturnable = true;
        int index;
	        
        while (isReturnable == true) {
        	int s;
            //System.out.println("***************************************************************************************");
        	System.out.println("Please select one: ");
            System.out.println("\t1. Add a multiple choice question");
            System.out.println("\t2. Add a question");
            System.out.println("\t3. Delete a multiple choice question");
            System.out.println("\t4. Delete a question");
            System.out.println("\t5. Edit a multiple choice question");
            System.out.println("\t6. Edit a question");
            System.out.println("\t7. Report: Multiple choice question database");
            System.out.println("\t8. Report: Question database");
            System.out.println("\t9. Report: All database");
            System.out.println("\t10. Save");
            System.out.println("\t11. Exit");
		    System.out.printf("Enter your choice: ");
		    s = Sc.nextInt();
            System.out.println("***************************************************************************************");
            
            switch (s) {
                case 1:
                    MultiChoiceQuestion MQue = new MultiChoiceQuestion();
                    MQue.input();
                    Data.add(MQue);
                    Sc.nextLine();
                    isReturnable = true;
                    break;
                    
                case 2:
                	Question Que = new Question();
                	Que.input();
                	Data.add(Que);
                	Sc.nextLine();
                    isReturnable = true;
                    break;
                    
                case 3:
                	System.out.print("Please enter index number of the question, 0 to go back to menu " +
                			"you want to delete (if you don't know please consider use option 9 to see): " );
                	index = Sc.nextInt();
                	if (index == 0) break;
                	else
                		if ((index-1)<Data.size() && Data.get(index-1) instanceof MultiChoiceQuestion) {
                			Data.remove(index-1);
                			System.out.println("Entry deleted.");
                		}
                		else System.out.println("Cannot delete entry due to wrong type of question. Operation aborted.");
                	isReturnable = true;
                    break;
                    
                case 4:
                	System.out.print("Please enter index number of the question, 0 to go back to menu " +
                			"you want to delete (if you don't know please consider use option 9 to see): " );
                	index = Sc.nextInt();
                	if (index == 0) break;
                	else
                		if ((index-1)<Data.size() && Data.get(index-1) instanceof Question) {
                			Data.remove(index-1);
                			System.out.println("Entry deleted.");
                		}
                		else System.out.println("Cannot delete entry due to wrong type of question. Operation aborted.");
                	isReturnable = true;
                    break;
                    
                case 5:
                	System.out.print("Please enter index number of the question, 0 to go back to menu " +
                			"you want to edit (if you don't know please consider use option 7 to 9 to see): " );
                	index = Sc.nextInt();
                	if (index == 0) break;
                	else
                		if ((index-1)<Data.size() && Data.get(index - 1) instanceof MultiChoiceQuestion) {
                			MultiChoiceQuestion temp = new MultiChoiceQuestion();
                			temp.input();
                			Data.set(index - 1, temp);
                			System.out.println("Entry edited.");
                		}
                		else System.out.println("Cannot edit entry due to wrong type of question. Operation aborted.");
                	isReturnable = true;
                    break;

                case 6:
                	System.out.print("Please enter index number of the question, 0 to go back to menu " +
                			"you want to edit (if you don't know please consider use option 7 to 9 to see): " );
                	index = Sc.nextInt();
                	if (index == 0) break;
                	else
                		if ((index-1)<Data.size() && Data.get(index - 1) instanceof Question) {
                			Question temp = new Question();
                			temp.input();
                			Data.set(index - 1, temp);
                			System.out.println("Entry edited.");
                		}
                		else System.out.println("Cannot edit entry due to wrong type of question. Operation aborted.");
                	isReturnable = true;
                    break;
                    
                case 7:
                    for (int j = 0; j < Data.size(); j++) 
                    	if (Data.get(j) instanceof MultiChoiceQuestion) {
                    		System.out.println("Question index: " +(j+1));
                    		System.out.println(Data.get(j));
                    	}
                    isReturnable = true;
                    break;
                    
                case 8:
                    for (int j = 0; j < Data.size(); j++) 
                    	if (Data.get(j) instanceof Question) {
                    		System.out.println("Question index: " +(j+1));
                    		System.out.println(Data.get(j));
                    	}
                    isReturnable = true;
                    break;
                    
                case 9: 
                	for (int j = 0; j < Data.size(); j++) {
                		System.out.println("Question index: " +(j+1));
                		System.out.println(Data.get(j));
                	}
                	isReturnable = true;
                    break;
                
                case 10: 
                	ObjectOutputStream StreamOut_2 = new ObjectOutputStream(new FileOutputStream("data.dat"));
                	StreamOut_2.writeObject(Data);
             		StreamOut_2.close();
                default: break;
            }  
        }
    } 
}

After add new MultiChoiceQuestion I select option 8 to display Question only but it will also display the MultiChoiceQuestion that I just added. So that I mean if there is some way to let it only display Question instead of display the MultiChoiceQuestion also cause I know that ì B extends A so instanceof B will also an instanceof A.

[KevinWorkman]

Yo, I use switch() to select what to do so I have to split them. I will also post source here:

I'm not sure how that changes my advice?

Although one might wonder why you have to call instanceof so many times. Usually that's a sign that some refactoring is in order.

[KevinNguyen]

My problem is if I want to display Question's object it will also display MultiChoiceQuestion. I want to make it display only Question's objects :)

[Tolls]

But that implies your model is wrong, since you have said (by your code) that a MultiChoiceQuestion is-a Question.
If they are not, then they should not be extending Question.

[KevinNguyen]

Ya sure that, but MultiChoiceQuestion is-a Question.
My java assignment topic is to create a program to manage question for a gameshow. So there will be MultiChoiceQuestion and Question(don't know if I use wrong word, you know Question - Answer not Question - A, B, C, D) so if I create a parent class, it will also the SAME as Question class ( String ques, String ans, int type and int difficulty). That's why I create MultiChoiceQuestion extends Question. Sure that my solution is not good and maybe wrong. That's why I need everybody helps :D

[Tolls]

But in your app it clearly isn't the case, because otherwise you wouldn't have to somehow split them up.
Your own logic is showing that this is not the correct relationship.

[KevinNguyen]

Sure, so the problem is I can't find the way to design the parent class for MultiChoiceQuestion and Question cause if I create it, it will be the same as Question. You have any idea?

[KevinWorkman]

Perhaps instead of having all these different kinds of questions, have a single Question Object with several boolean attributes: isMultipleChoice, isTrueFalse, etc. Then just check them instead of using the instanceof keyword.

[Tolls]

I'd have them as two separate classes, in two arrays.
One Multi, one "normal".

They would each implement a AskQuestion interface (or some better name).
That would have an askQuestion method and a checkAnswer (or something) method.

I say this, since you seem to be treating a MultiQuestion and a Question as two very different things (looking at the options).
It's only when you actually ask the question that they do similar things (and there's no code for that at the moment).