Six Expressions: Write Down the Value First

Six lines, each printing one value. Write all six answers down before you open the reveal — guessing and checking one at a time teaches much less than committing to all six.

java

System.out.println(1 + 2 + "3");
System.out.println("1" + 2 + 3);
System.out.println(9 / 2);
System.out.println(9 % 2);
System.out.println(2 + 3 * 4 - 6 / 2);
int k = 7; System.out.println(k++ + k);

The six values, and the rule behind each one

Each expression, its value, and why
Expression	Value	Why
`1 + 2 + "3"`	`33`	`1 + 2` is two numbers, so it adds; then the string joins
`"1" + 2 + 3`	`123`	The string comes first, so both `+` are concatenation
`9 / 2`	`4`	Two `int` values, so the remainder is discarded
`9 % 2`	`1`	`%` produces the remainder that `/` threw away
`2 + 3 * 4 - 6 / 2`	`11`	`(3*4)` and `(6/2)` bind first: `2 + 12 − 3`
`k++ + k`	`15`	`k++` produces 7 and leaves `k` at 8; then `7 + 8`

The last one is the only one worth arguing with. Both halves read k, and between the two reads the increment has already happened — which is why an expression that uses a variable twice and increments it once is a line to rewrite rather than a line to reason about.