Question

I have this Java program
I need Help plz

package ads.set2.select;

public class Selector {

public static void main(String[] args) {
  int index = 3;
  int[] numbers = { 23, 4, 33, 11, 99,24 };
  int results = select(numbers, index);
  System.out.println("Element at index: " + index + " in sorted array is " + results);

  // error case
  select(numbers, 9);
}

/**
* Returns the number that would be at index {@code index} in a sorted version
* of the array.
*
* @param numbers array with pairwise distinct numbers.
* @param index   the index of the number we're looking for in a sorted version
*                of {@code numbers}.
* @return the number at the given index.
* @throws IllegalArgumentException if {@code index} is not valid for the array.
*/
public static int select(int[] numbers, int index) {
  sort(numbers, numbers.length);
  if (index > numbers.length) {
   throw new IllegalArgumentException("Index is not valid for the array");
  }
  return numbers[index];
}

/**
* A function to implement bubble sort
*
* @param arr the arr
* @param n   the n
*/
public static void sort(int[] arr, int n) {
  // Base case
  if (n == 1)
   return;

  // One pass of bubble sort. After
  // this pass, the largest element
  // is moved (or bubbled) to end.
  for (int i = 0; i < n - 1; i++)
   if (arr[i] > arr[i + 1]) {
    // swap arr[i], arr[i+1]
    int temp = arr[i];
    arr[i] = arr[i + 1];
    arr[i + 1] = temp;
   }

  // Largest element is fixed,
  // recur for remaining array
  sort(arr, n - 1);
}

}

I have the Following Failures

There were 5 failures:
1) Test testEmptyArrayWithZeroIndex returned:
java.lang.Exception: Unexpected exception, expected but was
?at org.junit.internal.runners.statements.ExpectException.evaluate(ExpectException.java:28)
?at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:298)
?at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:292)
?at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
?at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.StackOverflowError
?at ads.set2.select.Selector.sort(Selector.java:41)
?at ads.set2.select.Selector.sort(Selector.java:57)

2) Test testEmptyArrayWithPositiveIndex returned:
java.lang.Exception: Unexpected exception, expected but was
?at org.junit.internal.runners.statements.ExpectException.evaluate(ExpectException.java:28)
?at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:298)
?at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:292)
?at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
?at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.StackOverflowError
?at ads.set2.select.Selector.sort(Selector.java:57)
?at ads.set2.select.Selector.sort(Selector.java:57)

3) Test testEmptyArrayWithNegativeIndex returned:
java.lang.Exception: Unexpected exception, expected but was
?at org.junit.internal.runners.statements.ExpectException.evaluate(ExpectException.java:28)
?at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:298)
?at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:292)
?at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
?at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.StackOverflowError
?at ads.set2.select.Selector.sort(Selector.java:57)
?at ads.set2.select.Selector.sort(Selector.java:57)

4) Test testArrayWithNegativeIndex returned:
java.lang.Exception: Unexpected exception, expected but was
?at org.junit.internal.runners.statements.ExpectException.evaluate(ExpectException.java:28)
?at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:298)
?at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:292)
?at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
?at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.ArrayIndexOutOfBoundsException: Index -1 out of bounds for length 10
?at ads.set2.select.Selector.select(Selector.java:30)
?at SelectorTest.testArrayWithNegativeIndex(SelectorTest.java:38)
?at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
?at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
?at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
?at java.base/java.lang.reflect.Method.invoke(Method.java:566)
?at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
?at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
?at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
?at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
?at org.junit.internal.runners.statements.ExpectException.evaluate(ExpectException.java:19)
?... 4 more

5) Test testArrayWithLargerIndex returned:
java.lang.Exception: Unexpected exception, expected but was
?at org.junit.internal.runners.statements.ExpectException.evaluate(ExpectException.java:28)
?at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:298)
?at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:292)
?at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
?at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.ArrayIndexOutOfBoundsException: Index 10 out of bounds for length 10
?at ads.set2.select.Selector.select(Selector.java:30)
?at SelectorTest.testArrayWithLargerIndex(SelectorTest.java:43)
?at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
?at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
?at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
?at java.base/java.lang.reflect.Method.invoke(Method.java:566)
?at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
?at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
?at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
?at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
?at org.junit.internal.runners.statements.ExpectException.evaluate(ExpectException.java:19)
?... 4 more

Answer 1

public class Selector {

    public static void main(String[] args) {
        int index = 3;
        int[] numbers = { 23, 4, 33, 11, 99,24 };
        int results = select(numbers, index);
        System.out.println("Element at index: " + index + " in sorted array is " + results);

        // error case
        select(numbers, 6);
    }

    /**
     * Returns the number that would be at index {@code index} in a sorted version
     * of the array.
     *
     * @param numbers array with pairwise distinct numbers.
     * @param index   the index of the number we're looking for in a sorted version
     *                of {@code numbers}.
     * @return the number at the given index.
     * @throws IllegalArgumentException if {@code index} is not valid for the array.
     */
    public static int select(int[] numbers, int index) {
        sort(numbers, numbers.length);
        if (index >= numbers.length || index < 0) {
            throw new IllegalArgumentException("Index is not valid for the array");
        }
        return numbers[index];
    }

    /**
     * A function to implement bubble sort
     *
     * @param arr the arr
     * @param n   the n
     */
    public static void sort(int[] arr, int n) {
        // Base case
        if (n <= 1)
            return;

        // One pass of bubble sort. After
        // this pass, the largest element
        // is moved (or bubbled) to end.
        for (int i = 0; i < n - 1; i++)
            if (arr[i] > arr[i + 1]) {
                // swap arr[i], arr[i+1]
                int temp = arr[i];
                arr[i] = arr[i + 1];
                arr[i + 1] = temp;
            }

        // Largest element is fixed,
        // recur for remaining array
        sort(arr, n - 1);
    }

}


Failures:
1. testEmptyArrayWithZeroIndex: When Input is empty array with Zero Index
In your sort method 
public static void sort(int[] arr, int n) {
  // Base case
  if (n == 1)
   return;

There is need to change base condition to handle empty array case, As sort method is recursive and array is having size 0, so base case never occurs, Causing stackOverflow

We can change the base condition to 
if (n <= 1)
   return;
It will be fixed
2. testEmptyArrayWithPositiveIndex: Same as first one
3. testEmptyArrayWithNegativeIndex: Same as first one
4. testArrayWithNegativeIndex: Array with negative index
Change is required here 
public static int select(int[] numbers, int index) {
  sort(numbers, numbers.length);
  if (index > numbers.length) {
   throw new IllegalArgumentException("Index is not valid for the array");
  }

In index < 0 then also we need to throw IllegalArgumentException as its not valid index

5. testArrayWithLargerIndex: In Java array index starts with 0. So, if there are 6 elements in array the index for 6th element will be 5. So we need to update the condition to this
if (index >= numbers.length || index < 0) {
   throw new IllegalArgumentException("Index is not valid for the array");
  }