Which Java data structure is used to implement the sorting algorithm without extra space?

The Java data structure that is used to implement the sorting algorithm without extra space is an Array. Here's a step-by-step guide on how you can do it:

1. Declare an Array: You can declare an array in Java using the following syntax: int[] arr = new int[n]; where n is the size of the array.

2. Input Array Elements: You can input elements into the array using a loop. For example, you can use a for loop to input elements into the array.

3. Implement Sorting Algorithm: You can implement a sorting algorithm directly on the array. For example, you can implement the Bubble Sort algorithm on the array without using extra space. Here's how you can do it:

for (int i = 0; i < n-1; i++) {     
    for (int j = 0; j < n-i-1; j++) { 
        if (arr[j] > arr[j+1]) { 
            // swap arr[j+1] and arr[j] 
            int temp = arr[j]; 
            arr[j] = arr[j+1]; 
            arr[j+1] = temp; 
        } 
    } 
}

4. Print Sorted Array: Finally, you can print the sorted array using a for loop.

Remember, the above example uses Bubble Sort which is not the most efficient sorting algorithm. For larger arrays, other sorting algorithms like Quick Sort, Merge Sort, etc., are more efficient but they require extra space.