如何在ArrayList中搜索StringBuffer对象？

Question

Following is the code snippet I am working with.

Scanner sc = new Scanner(System.in);
int N = sc.nextInt();
ArrayList<StringBuffer> al = new ArrayList<StringBuffer>();
while (N-- > 0) {
   str = new StringBuffer(sc.next());
   if (al.contains(str)) {
       System.out.println("Duplicate value " + str);
   } else {
       al.add(str);
   }    
}

If the input is: 4

abc

fgh

dfg

abc

It is showing blank output when the expected output is:

Duplicate value abc

Where am I going wrong here?

Answer 1

StringBuffer doesn't override Object 's equals , so when you search if your List contains a certain StringBuffer instance, you are checking if the exact reference appears in the List .

You could use a HashSet<String> to avoid duplicates, since String overrides equals , and then (if you must) create a List<StringBuffer> from the elements of the HashSet .

BTW, StringBuilder is more efficient than StringBuffer (which should only be used if you plan to access it from multiple threads).

Scanner sc = new Scanner(System.in);
int N = sc.nextInt();
ArrayList<StringBuffer> al = new ArrayList<StringBuffer>();
Set<String> uniques = new HashSet<>();
while (N-- > 0) {
   uniques.add(sc.next());
}
for (String s : uniques)
    al.add (new StringBuffer(s));

If you have to report the duplicates, a small change will be required :

Scanner sc = new Scanner(System.in);
int N = sc.nextInt();
ArrayList<StringBuffer> al = new ArrayList<StringBuffer>();
Set<String> uniques = new HashSet<>();
while (N-- > 0) {
   String str = sc.next();
   if (!uniques.add(str))
       System.out.println("Duplicate value " + str);
}
for (String s : uniques)
    al.add (new StringBuffer(s));

Answer 2

You can edit your code as below:

Scanner sc = new Scanner(System.in);
int N = sc.nextInt();
ArrayList<StringBuffer> al = new ArrayList<>();
while (N-- > 0) {
    str = new StringBuffer(sc.next());
    //if (al.contains(str)) {
    if (al.stream().anyMatch(stringBuffer -> stringBuffer.toString().equals(str.toString()))) {
        System.out.println("Duplicate value " + str);
    } else {
        al.add(str);
    }

}

In code above

if (al.contains(str)) {

Changed to

if (al.stream().anyMatch(stringBuffer -> stringBuffer.toString().equals(str.toString()))) {

Answer 3

I am here posting the workaround to the problem that I was facing.

The actual problem statement is here .

The following is the code snippet of one of the ways which I implemented to solve the problem statement, using linkedlist:

import java.util.*;

public class Password {
    public static void main(String [] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        String str;
        LinkedList ll = new LinkedList();
        StringBuilder sb = new StringBuilder();
        for (int i=0; i<N; i++) {
            str = sc.next();
            ll.add(str);
        }
        for (int i=0; i<N; i++) {
            str = (sb.append(ll.get(i))).reverse().toString();
            if (ll.contains(str)) {
                System.out.print(str.length() + " " + str.charAt(str.length()/2)); 
                break;
            }
            sb.setLength(0);
        }
        sc.close(); 
    }
}