Operators

1An operator produces a new value

An operator takes one or two values and produces a new value. 200 - 50 is the value 150. It does not change 200 unless you store the result back into a box.

The name on the left of = is the box you write into. The right-hand side is the value you computed.

Figure. 200 - 50 is the value 150. It does not change 200 unless you store the result. Assignment writes that new value into paid.

How it works

1. An operator produces a new valueOperators make a new value. Assignment is what writes that value into a box.

Compute a student fare; leave listPrice alone

int listPrice = 200;
int paid = listPrice - 50;
System.out.println(paid);

2Arithmetic, integer division, and order

+ - * / % work on numbers. Integer / drops the fraction: 5 / 2 is 2, not 2.5. % is the remainder after that kind of division.

Precedence: * / % before + -. Same rank runs left to right. Parentheses first. Write parentheses when a reader would have to hesitate.

No diagram — the idea is carried by the prose or the Java listing.

Coding lab. Compute the student fare runs in the app, with checks on your output.

3++ after the name yields the old value

++ adds one. -- subtracts one. Written before the name (++x) it changes first, and the value of the expression is the new one.

Written after (x++) the expression's value is the old one, then it changes. The trap is using x++ inside a larger expression and expecting the new value.

Figure. seatsLeft++ yields the old 5 into shown, then seatsLeft becomes 6. The trap is expecting the new value from x++ inside a larger expression.

shown is 5; seatsLeft becomes 6

int seatsLeft = 5;
int shown = seatsLeft++;
System.out.println(shown);
System.out.println(seatsLeft);

4Comparisons and short-circuit logic

Relational operators produce boolean: < <= > >= == !=. On primitives, == compares values.

&& is and, || is or, ! is not. && and || stop early: if the left of && is false, the right is not evaluated. That matters when the right side would fail — for example reading args[0] when args is empty.

No diagram — the idea is carried by the prose or the Java listing.

A boolean from two tests

boolean student = true;
int price = 200;
boolean cheapSeat = student && price >= 200;

5Bits are not boolean &&

Bitwise operators work on the bits of integers: & and, | or, ^ xor, ~ not, << and >> shifts.

5 & 3 is 1 because 101 and 011 share only the last 1. These are not the same as && and ||. Do not use & when you mean both conditions.

No diagram — the idea is carried by the prose or the Java listing.

6A short choice between two values

condition ? ifTrue : ifFalse picks one of two values. It is an expression, so it can sit on the right of =.

Use it for a short choice. An if/else is clearer when each branch is more than one value, or when you need statements rather than a value.

Figure. student ? 150 : 200 picks one of two values. It is an expression, so it can sit on the right of =. Here student is true, so paid is 150.

Student fare in one expression

boolean student = true;
int paid = student ? 150 : 200;
System.out.println(paid);