Java - print contents of a queue

I have a queue (where each element is an array of integer) defined as -

Queue<int[]> queue = new LinkedList<>();

When I try to print the queue, I get output like this as it prints the array object -

[[I@58d25a40]

How can I override the toString() method of Queue class to get the correct output ? I donot want to get the output by iterating over the queue. I hope to achieve it somehow by overriding the toString() method.

You can try something like this:

Queue<int[]> queue = new LinkedList<>() {
  @Override public String toString() {
    return "[" +
      stream()
      .map(Arrays::toString)
      .collect(Collectors.joining(", ")) +
      "]";
  }
};

queue.add(new int[] {1, 2});
queue.add(new int[] {3, 4});
System.out.println(queue);

printing:

[[1, 2], [3, 4]]

Which is presumably what you want.

This isn't necessarily a good idea. toString() should be used solely for debugging purposes: toString demands nothing of implementations, not even that they are consistent. Most things I can think of that would lead one to ask this question are problematic.

> How can I override the toString() method of Queue class to get the correct output ?

You can't.

For starters, Queue is not a class, it's an interface. The concrete class you're using is LinkedList.

More concretely what you could do is defining a custom class that implements Queue and extends, for example LinkedList, and that class can have a custom toString method (see implementation below).

> I donot want to get the output by iterating over the queue. I hope to achieve it somehow by overriding the toString() method.

You'll have to iterate eventually, one way or another. You can just hide that iteration in another method, but the iteration will still be there under the hood.

Here's a quick blueprint of what you can do

import java.util.Queue;
import java.util.LinkedList;
import java.util.Arrays;
import java.util.stream.Collectors;

public class ArrayQueue extends LinkedList<int[]> implements Queue<int[]> {

	@Override
	public String toString() {
		return this.stream()
			.map(Arrays::toString)
			.collect(Collectors.joining(", ", "[", "]"));
	}
}

And a test program

import java.util.Queue;

public class ArrayQueueTest {
	public static void main(String args[]) {
		Queue<int[]> q = new ArrayQueue();
		q.add(new int[]{1, 2, 3});
		q.add(new int[]{4, 5, 6});
		q.add(new int[]{7, 8, 9});
		System.out.println(q);
	}
}

Running this yields

[[1, 2, 3], [4, 5, 6], [7, 8, 9]]