单个链表Java

Question

System.out.println("insert after specified data node");
System.out.println("enter data");
int x = sc.nextInt();
Node s4 = head;
try
{
    while(s4.data != x)
    {
        s4 = s4.next;
    }
}
catch(NullPointerException e)
{
    e.printStackTrace();
}

System.out.println("hi");

Node specifiedNode = new Node(x);
//s4.next = specifiedNode;

try
{
    specifiedNode.next = s4.next;
    s4.next = specifiedNode;

}
catch(NullPointerException e)
{
    e.printStackTrace();
}

System.out.println("output after specified insertion");
Node s5 = head;
while(s5!=null)
{
    System.out.println(s5.data);
    s5 = s5.next;
}

}

This is the sample program to insert data after a specified node in single linked list. In the above program my question is why null pointer exception is happening at the following statements:

specifiedNode.next = s4.next;
s4.next = specifiedNode;

Answer 1

why do you keep accessing the .next of the s4 instance? the way to do that is to store the s4.next in an attribute for one time and keep using it in all your program since each time you access the .next() you are invoking the next record and probably one of them is null;

Answer 2

In order to avoid a NullPointerException, ensure you always check on s4 not being null.

Node s4 = head;      // Head initially is null (empty list).
while (s4 != null) { // End reached?
    if (s4.data == x) {
        // Wow, found x.
        break; // Jump out of the loop.
    }

    // Go to next node.
    s4 = s4.next; // s4 could become null.
}