👣 Walkthrough of Expression Problems#
To help you get comfortable with all of these complicated rules, let’s walk through the evaluation of a couple of complex Java expressions step by step.
⭐ Expression 1#
2 + 2 + "hello" + 3 * 5 / 10
() — Parentheses#
There are no parentheses in this particular expression, so we can move on!
% * / — Modulo, Multiplication, Division#
We have a few subexpressions with operators at this level of precedence (specifically the * and /), so we must carefully execute them in order from left to right. In this order, first we deal with the multiplication by evaluating 3 * 5:
2 + 2 + "hello" + 3 * 5 / 10
2 + 2 + "hello" + 15 / 10
Note that because we were dealing with an int times an int, we also get an int result (15, as opposed to 15.0 or "15"). Now, there is only one operation left at this level of precedence—the subexpression 15 / 10:
2 + 2 + "hello" + 15 / 10
2 + 2 + "hello" + 1
Again, we are dealing with two int operands so we produce an int result and we are dealing with int division (or “truncating division”)! So instead of getting a result of 1.5 (that you maybe would have expected from typical mathematics), we get 1 because 10 fits evenly into 15 exactly 1 time. (That 5 remainder would be the result of 15 % 10!)
+ - — Addition, Subtraction#
Finally, we only have + operators left which are all at the same level of precedence. So once again, we will dutifully execute them in order from left to right. Let’s start with the leftmost subexpression 2 + 2:
2 + 2 + "hello" + 1
4 + "hello" + 1
Both operands are int, so we produce an int result which produces a (hopefully unsurprising) 4. Let’s keep going by evaluating the next leftmost subexpression at this level: 4 + "hello"!
4 + "hello" + 1
"4hello" + 1
This time, we have one int operand (4) and one String operand ("hello") so Java converts the int to a String (making it "4" instead of 4) and then we do typical String concatenation, squishing the two Strings together to create "4hello". We’re almost done - just "4hello" + 1 remaining!
"4hello" + 1
"4hello1"
Here, we repeat the exact same logic! One of the operands is an int while the other is a String, so the int operand is converted to a String before we concatenate the two Strings together, and we have our final result! 🥳
Video walkthrough of this problem below (click “Expand”).
Expand
⭐ Expression 2#
177 % 100 % 10 / 2
() — Parentheses#
There are no parentheses in this particular expression, so we can move on!
% * / — Modulo, Multiplication, Division#
All of the operators in this expression are at this level of precedence, so let’s move across the expression one subexpression at a time left to right, starting with 177 % 100.
177 % 100 % 10 / 2
77 % 10 / 2
Remember that the modulo operator gives us the remainder after dividing the left operand by the right operand (so 177 divided by 100 gives us 1, remainder 77). Next up is 77 % 10:
77 % 10 / 2
7 / 2
Same process: 77 divided by 10 gives us 7, with 7 left over! And finally 7 / 2:
7 / 2 3
We return to our very favorite int division which gives us 3 (because 2 divides evenly into 7 three times, with 1 remainder).
+ - — Addition, Subtraction#
No addition or subtraction - we’re done! 🏁
Video walkthrough of this problem below (click “Expand”).
Expand
⭐ Expression 3#
(2.5 + 3.5) / 2
() — Parentheses#
Let’s start with the subexpression in the parentheses: 2.5 + 3.5
(2.5 + 3.5) / 2
6.0 / 2
Notice that we wrote 6.0, not 6. Even though this value could be represented by an int, we must carefully follow Java’s rules about types. Both operands in this subexpression are doubles, so we get a double result! Now, we only have one operator to deal with…
% * / — Modulo, Multiplication, Division#
Now we just need to evaluate 6.0 / 2
6.0 / 2
3.0
Again, you may have expected the result to be 3, but we have double and int expression so Java converts the int to a double before performing double division! Moral of the story: type matters!
+ - — Addition, Subtraction#
No addition or subtraction, we’re done!
Video walkthrough of this problem below (click “Expand”).
Expand
⭐ Expression 4#
(6.7 > 25) && 9 > 2 && !(8.2 < 5)
() — Parentheses#
Let’s start with the subexpression in the first set of parentheses: 6.7 > 25
(6.7 > 25) && 9 > 2 && !(8.2 < 5)
false && 9 > 2 && !(8.2 < 5)
Now we have a subexpression in one more set of parentheses to handle before moving on: 8.2 < 5
false && 9 > 2 && !(8.2 < 5)
false && 9 > 2 && !false
! — Logical Not#
Remember that logical NOT just takes whatever the boolean value is, and flips it to the opposite! So in this case !false becomes true.
false && 9 > 2 && !false
false && 9 > 2 && true
% * / — Modulo, Multiplication, Division#
No mod, multiplication or division!
+ - — Addition, Subtraction#
No addition or subtraction!
< > <= >= — Relational Operators#
We’ve already dealt with some of the relational operators that were in the original expression when we evaluated the parenthesized expressions, but there is one left: 9 > 2
false && 9 > 2 && true
false && true && true
== != — Equality Operators#
No equality operators!
&& — Logical And#
We have multiple subexpressions using && so we evaluate them left to right. Remember, false && true results in false!
false && true && true
false && true
false
|| — Logical Or#
No logical OR, so we’re done!
Video walkthrough of this problem below (click “Expand”).