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Thread: Removing an object from an ArrayList

  1. #1
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    Default Removing an object from an ArrayList

    How can I remove an object from an ArrayList? If you run my code, my delete method in the Team class does not seem to be able to remove the object that I typed from the keyboard.

    the remove() method seems so have no effect even though that is how you remove an object from arraylist object that I removed is still there

    delete method I created to remove object from ArrayList:

    HTML Code:
    	public void delete() throws IOException {
    		System.out.println("Enter the name of the file that you wish to delete information from: ");
    		String name = scan.next();
    		File w = new File(name);
    		if(w.exists())
    		{
    		    ArrayList<Player> x = new ArrayList<Player>();
    		    try {
    		       Stream<String> lines = Files.lines(Paths.get(name));
    		       lines.forEach((String t) -> {
    		          String[] parse = t.split(" ");
    		          if(parse.length<5) return;
    		          x.add(new Player(parse[0], parse[1], parse[2], parse[3], parse[4]));
    		       });
    		    } catch (IOException ex) {
    		       System.out.println("Unable to open student file." + ex.toString());
    		    }
    			System.out.println("Size of Player ArrayList: " + x.size());
    			System.out.println();
    
    			for(int i=0;i<x.size();i++){  //Prints objects from chosen file to console
    		    	System.out.println(x.get(i));
    		    }
    
    		    System.out.println("How many people would you like to remove? ");
    			int num_people = scan.nextInt();
    			System.out.println("Enter first, last, goals, caps, and assists:");
    			for(int i=0; i<num_people; i++){
    				String fstname2 = scan.next();
    				String lstname2 = scan.next();
    				String goals = scan.next();
    				String caps = scan.next();
    				String assists = scan.next();
    				x.remove(new Player(fstname2,lstname2,goals,caps,assists));
    			}
    			System.out.println("Done.");
    			System.out.println();
    			for(int i=0;i<x.size();i++){ 
    		    	System.out.println(x.get(i));
    		    }
    			BufferedWriter q = new BufferedWriter(new FileWriter(w));
    		    for(Player p : x){
    		    	q.write(p + "\n");
    		    	q.newLine();
    		    }
    		    q.close();
    		}
    		else{
    			System.out.println("File does not exist.");
            }
    	}

  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Removing an object from an ArrayList

    How can I remove an object from an ArrayList?
    Look at the API doc for the ArrayList class. It has a methods to do that.
    http://docs.oracle.com/javase/8/docs/api/index.html

    The code should test the boolean value returned by the method to see if it was successful.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: Removing an object from an ArrayList

    I'm already using the correct method to delete the certain object though. I don't need a boolean because I already know that it doesn't work

  4. #4
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    Default Re: Removing an object from an ArrayList

    I already know that it doesn't work
    If you know it doesn't work, why are you using it?
    If you don't understand my answer, don't ignore it, ask a question.

  5. #5
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    Default Re: Removing an object from an ArrayList

    Because there is no other way that I know of to delete an object. Is there an alternative to remove()?

  6. #6
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    Default Re: Removing an object from an ArrayList

    Yes, the remove method is used for removing objects that are in the ArrayList from the ArrayList.
    The code needs to use a reference to an object that is in the ArrayList, not create a new object which will not be in the list.
    Unless the equals() method says the objects are the same.
    If you don't understand my answer, don't ignore it, ask a question.

  7. #7
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    Default Re: Removing an object from an ArrayList

    And how can I reference an object that is already in the ArrayList?

  8. #8
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    Default Re: Removing an object from an ArrayList

    And how can I reference an object that is already in the ArrayList?
    The get method returns a reference to an object in the list.

    Does the Player class have an equals() method?
    If you don't understand my answer, don't ignore it, ask a question.

  9. #9
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    Default Re: Removing an object from an ArrayList

    How does the get method return a reference to an object in the list?

    I created an equals method in the Player class but have not used it.

    --- Update ---

    This is my method when I added the equals function and used a getter to get all of the objects in the existing arraylist. I made a loop to test if the object I entered in from the keyboard is equal to any of the ones in the arraylist, but it still doesn't work.


    HTML Code:
    	public void delete() throws IOException {
    		System.out.println("Enter the name of the file that you wish to delete information from: ");
    		String name = scan.next();
    		File w = new File(name);
    		if(w.exists())
    		{
    		    ArrayList<Player> x = new ArrayList<Player>();
    		    try {
    		       Stream<String> lines = Files.lines(Paths.get(name));
    		       lines.forEach((String t) -> {
    		          String[] parse = t.split(" ");
    		          if(parse.length<5) return;
    		          x.add(new Player(parse[0], parse[1], parse[2], parse[3], parse[4]));
    		       });
    		    } catch (IOException ex) {
    		       System.out.println("Unable to open student file." + ex.toString());
    		    }
    			System.out.println("Size of Player ArrayList: " + x.size());
    			System.out.println();
    
    			for(int i=0;i<x.size();i++){
    		    	System.out.println(x.get(i));
    		    }
    
    		    System.out.println("How many people would you like to remove? ");
    			int num_people = scan.nextInt();
    			System.out.println("Enter first, last, goals, caps, and assists:");
    			for(int i=0; i<num_people; i++){
    				String fstname2 = scan.next();
    				String lstname2 = scan.next();
    				String goals = scan.next();
    				String caps = scan.next();
    				String assists = scan.next();
    				Player y = new Player(fstname2,lstname2,goals,caps,assists);
    				for(int j=0;j<x.size();j++){
    					if(y.equals(x.get(j))){
    						x.remove(j);
    					}
    			    }
    			}
    			
    			System.out.println("Done.");
    			System.out.println();
    			for(int i=0;i<x.size();i++){
    		    	System.out.println(x.get(i));
    		    }
    			BufferedWriter q = new BufferedWriter(new FileWriter(w));
    		    for(Player p : x){
    		    	q.write(p + "\n");
    		    	q.newLine();
    		    }
    		    q.close();
    		}
    		else{
    			System.out.println("File does not exist.");
            }
    	}

  10. #10
    Super Moderator Norm's Avatar
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    Default Re: Removing an object from an ArrayList

    I created an equals method in the Player class but have not used it.
    What about this statement:
       if(y.equals(x.get(j))){


    it still doesn't work.
    What results does the equals method return? Try debugging the equals method. One way is to add some print statements that show what is a happening in the method and what value it is returning.
    If you don't understand my answer, don't ignore it, ask a question.

  11. The Following User Says Thank You to Norm For This Useful Post:

    Markusovich_ (July 26th, 2019)

  12. #11
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    Default Re: Removing an object from an ArrayList

    I think I got it now, thanks!

    I created my equals method like this:

    HTML Code:
    	public boolean equals(Player other){
    		return(true);
    	}
    Now it works perfectly fine.

  13. #12
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    Default Re: Removing an object from an ArrayList

    Now it works perfectly fine.
    Looks like that equals will match ALL the objects in the list or anywhere else.
    Have you tested it with different values?
    If you don't understand my answer, don't ignore it, ask a question.

  14. #13
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    Default Re: Removing an object from an ArrayList

    I replaced

    HTML Code:
    public boolean equals(Player other){
    	return(true);
    }
    with

    HTML Code:
    public boolean equals(Object other) {
        if (this == other)
            return true;
        if (other == null || getClass() != other.getClass())
            return false;
        Player player = (Player) other;
        return (fstname.equals(player.fstname) && lstname.equals(player.lstname));
        }
    and now it works.

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