Consider the expression
if (0 < amount < 1000) . . . // Error
This looks just like the mathematical notation for “amount is between 0 and 1000”. But in Java, it is a syntax error.
Let us dissect the condition. The first half, 0 < amount, is a test with outcome true or false. The outcome of that test (true or false) is then compared against 1000. This seems to make no sense. Is true larger than 1000 or not? Can one compare truth values and numbers? In Java, you cannot. The Java compiler rejects this statement.
Instead, use && to combine two separate tests: if (0 < amount && amount < 1000) . . .

Another common error, along the same lines, is to write if (ch == ‘S’ || ‘M’) . . . // Error
to test whether ch is ‘S’ or ‘M’. Again, the Java compiler flags this construct as an error. You cannot apply the || operator to characters. You need to write two Boolean expressions and join them with the || operator:
if (ch == ‘S’ || ch == ‘M’) . . .