General

Default Values

Java uses default values for all its fields. Meaning :

public  Node(){
    item = null;
    next = null;
}

Does nothing except takes up space for you. I would remove it, and the other lines / constructors like it). Some consider it bad practice to rely on the default values, you can then explicitly state the default value of a field on the same row as you are declaring it.

private Node head = null;

Reference

Inconsistency

You are inconsistent at spacing your code. I also recommend having fields at the right below the class declaration. I should not have to look for the fields, I want to find them instantly.

Bug in remove, insertBefore

If head is null, then current.next will throw a NullPointerException. I suggest guarding against null for head first in the method.

if (head == null) {
    System.out.println ("Item doesn't exist in the list.");
    return;
}

Nitpicks

public void add(T item)

You always execute the size++; why have it in two different places? And not directly under Node p = new Node (item);

public boolean isEmpty()

Looks perfect, why can't every method turn out to be like this? One option is to match against head == null as well, however in terms micro-optimization, it is slower.

public int getSize()

Nothing needs to be said, a simple getter.

public void remove(T item)

Instead of letting it handle the error printing, let remove return a boolean, true if removed, false if not, making it more flexible to use in future applications.

General

Default Values

Java uses default values for all its fields. Meaning :

public  Node(){
    item = null;
    next = null;
}

Does nothing except take up space for you. I would remove it, and the other lines / constructors like it. Some consider it bad practice to rely on the default values, you can then explicitly state the default value of a field on the same row as you are declaring it.

private Node head = null;

Reference

Inconsistency

You are inconsistent at spacing your code. I also recommend having fields at the right below the class declaration. I should not have to look for the fields, I want to find them instantly.

Bug in remove, insertBefore

If head is null, then current.next will throw a NullPointerException. I suggest guarding against null for head first in the method.

if (head == null) {
    System.out.println ("Item doesn't exist in the list.");
    return;
}

Nitpicks

public void add(T item)

You always execute the size++; why have it in two different places? And not directly under Node p = new Node (item);

public boolean isEmpty()

Looks perfect, why can't every method turn out to be like this? One option is to match against head == null as well, however in terms micro-optimization, it is slower.

public int getSize()

Nothing needs to be said, a simple getter.

public void remove(T item)

Instead of letting it handle the error printing, let remove return a boolean, true if removed, false if not, making it more flexible to use in future applications.