The single ampersand & is the logical AND operator. The double ampersand && is again a logical AND operator that employs short-circuiting behaviour. Short-circuiting just means the second operand (right hand side) is evaluated only when the result is not fully determined by the first operand (left hand side)
A & B (A and B are evaluated)
A && B (B is only evaluated if A is true)
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
