Java remove an item within a chained list Simple

Question

I am having difficulties in generating a code to remove an item from a certain position in a chained list, this is not an Arraylist, but a chained list, for this I am creating a method as below:

public Object remove(int posicao){

I know that the creation is similar to the method in the position, but I’m not able to develop the method in the form of removing items in the middle of the list.

package slist;

import java.util.Scanner;

public class SList {

private SListNode head;
  private int tamanho;

  public SList() {
    tamanho = 0;
    head = null;
  }

  public boolean eVazio() {
    return tamanho == 0;
  }

  public int length() {
    return tamanho;
  }

  public void insereAntes(Object obj) {
    head = new SListNode(obj, head);
    tamanho++;
  }

  public void insereApos(Object obj) {
    if (head == null) {
      head = new SListNode(obj);
    } else {
      SListNode no = head;
      while (no.next != null) {
        no = no.next;
      }
      no.next = new SListNode(obj);
    }
    tamanho++;
  }

  public Object naPosicao(int posicao) {
    SListNode noAtual;

    if ((posicao < 1) || (head == null)) {
      return null;
    } else {
      noAtual = head;
      while (posicao > 1) {
        noAtual = noAtual.next;
        if (noAtual == null) {
          return null;
        }
        posicao--;
      }
      return noAtual.item;
    }
  } 

  public Object remove(int posicao){

}  

  public String toString() {
    int i;
    Object obj;
    String res = "[  ";

    SListNode atual = head;

    while (atual != null) {
      obj = atual.item;
      res = res + obj.toString() + "  ";
      atual = atual.next;
    }
    res = res + "]";
    return res;
  }

    public static void main (String[] args) {
    SList lst1 = new SList();
    SList lst2 = new SList();
    int escolha;
    Scanner opcao = new Scanner(System.in);

    do {
        System.out.println("Menu de opcoes \n"
        + "1 - Coloca na lista \n"
        + "2 - Mostra na Lista \n"
        + "3 - Listar conteudo de determinada posicao\n"
        + "4 - Remover conteudo de determinada posicao \n");

        escolha = opcao.nextInt();

            switch (escolha) {
                case 1: System.out.println(" Diga o nome do aluno");
                opcao.nextLine();
                String item = opcao.nextLine();
                    lst1.insereApos(item);
                    break;
                case 2: System.out.println("Listando os nomes da lista\n"
                                            + lst1.toString());
                    break;
                case 3: System.out.println("informe a posicao desejada");
                    int esc = opcao.nextInt();
                    System.out.println("Objeto na posicao " + esc + " " + lst1.naPosicao(esc));
                case 4:     


            }
    } while (escolha <=4);
  }
}

And here the Class Slistnode

package slist;

public class SListNode {
  Object item;
  SListNode next;

  SListNode(Object obj) {
    item = obj;
    next = null;
  }

  SListNode(Object obj, SListNode next) {
    item = obj;
    this.next = next;
  }

}

Answer 1

You just have to swap the link by removing the current one and pass the next one from the current one to the predecessor.

The logic is quite simple:

List

{Node-Previous} -> {Node to Be Deleted} -> {Node-Posterior to Excluded}

Operation to be done:

{Previous Node}. next = {Posterior to Deleted Node}

For that I modified your method Object naPosicao() to return a node from the list:

public SListNode getNo(int posicao) { //retorna direto um nó e não um objeto
    SListNode noAtual;

    if ((posicao < 1) || (head == null)) {
        return null;
    } else {
        noAtual = head;
        while (posicao > 1) {
            noAtual = noAtual.next;
            if (noAtual == null) {
                return null;
            }
            posicao--;
        }
        return noAtual; // única mudança real no método.
    }
}

Now that you can get the knots with this method just use it to make the logic I described before:

public void remove(int posicao) {
    try{
        SListNode predecessor = getNo(posicao - 1);
        SListNode excluido = predecessor.next;
        predecessor.next = excluido.next;
        excluido = null;  // Para tentar facilitar a exclusão para o garbage collector
    }catch(Exception e){
        e.printStackTrace();
    }
}

Note that I did not delete the object as this is not possible to do directly in Java, but when defining excluido = null; I delete the references so that it is caught (in a way more explicit) by the Garbage Ector. This is not a necessary step, as the reference will disappear from the program after this method is called, but it is clear that it is not intended to use the deleted node again.