Go Fish Program And Null Pointer Exception

I'm new to Java programming (taking a beginner's class) and our assignment is to program a game of Go Fish. I've got all my classes written (Card, CardPile, Player and GoFish). Now the problem is that I'm getting a NullPointerException when I run the GoFish class. The error seems to occur when I ask the program to check whether the player whose current turn it is, has a book (meaning 4 cards of the same value) in their hand. My idea was to run through the values 1 to 13, first checking to see if the value appeared in the player's hand:

if(players[currentTurn].hasCard(hasValue))

And if true, then remove all the cards matching the value from the player's hand, add to a new array of cards and then check to see if this array is of length 4 i.e. a book. If not, return the array of cards to the player's hand.

Here's the error message:

java.lang.NullPointerException
at CardPile.searchValue(CardPile.java:88)
at Player.hasCard(Player.java:15)
at GoFish.main(GoFish.java:98)

It references the following lines of code:

In GoFish.main:

// At the end of each turn, check to see if player currentTurn has made a book.
boolean hasBook = false;
int hasValue;
for (hasValue = 1; hasValue <= 13; hasValue++) {
// Runs through player's hand and removes all cards of the same value between 1 and 13 and places in a new array.

[Code] ....

I cannot figure out why I am getting the null pointer exception error!!!