program behaves not in expected way
2 + 2 =
52 + 2 = 4 … but takes 20 seconds to calculate
1 / 0 … 1.0 / 0.0
File.open(“???123321\5431`”);
a = null; a.equals(b);
Exception – problem, that can be detected as something “not expected to happen” (“exceptional”) and handled in your code. In Java language exceptions are implemented as specific kind of objects and operators to handle them.