You will run into these runtime exceptions repeatedly as you write Java code:
NullPointerException happens when you call a method or access a field on a reference that is null. It is the single most common exception in Java.
ArrayIndexOutOfBoundsException happens when you use an index that is negative or greater than or equal to the array's length.
ArithmeticException happens when you divide an integer by zero. Note that dividing a double by zero produces Infinity, not an exception.
NumberFormatException happens when you pass a non-numeric string to Integer.parseInt() or similar parsing methods.
ClassCastException happens when you try to cast an object to a type it does not belong to.