1.3 Expressions and Output

What you need to know

• Operators: +, -, *, /, %. Normal precedence: parentheses, then * / % left to right, then + - left to right.
• Integer division: when both operands are int, / truncates (drops the decimal). 7 / 2 is 3, -7 / 2 is -3. If either operand is a double, the result is a double: 7 / 2.0 is 3.5.
• % is the remainder: 7 % 2 is 1, 10 % 5 is 0. For negative dividends the result is negative: -7 % 2 is -1.
• Dividing an int by zero throws an ArithmeticException. Dividing a double by zero gives Infinity or NaN, no exception.
• System.out.println(x) prints x and moves to a new line; System.out.print(x) prints without a newline.
• + with a String is concatenation. Evaluation is left to right: "Sum: " + 1 + 2 is "Sum: 12", but "Sum: " + (1 + 2) is "Sum: 3".
• String literals are in double quotes; escape sequences include \n (newline), \" (quote), \\ (backslash).

Worked example

System.out.println(17 / 5);        // 3
System.out.println(17 % 5);        // 2
System.out.println(17 / 5.0);      // 3.4
System.out.println(17.0 / 5);      // 3.4
System.out.println(1 + 2 + "3");   // 33
System.out.println("1" + 2 + 3);   // 123
System.out.println(2 * 3 % 4);     // 2   (6 % 4)

Lines 5 and 6 are the concatenation trap: left-to-right evaluation means the ints are added before the String is reached in line 5, but not in line 6.

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• Integer division truncates toward zero. 7 / 2 is 3; -7 / 2 is −3 (not −4). Truncation, not floor. The exam tests negatives specifically.
• Remainder sign follows the dividend (left operand): -7 % 3 is −1; 7 % -3 is 1. The identity that always holds: a == (a / b) * b + a % b.
• Mixed arithmetic promotes to double: if either operand is a double, the whole operation is done in double. 1 / 2 * 2.0 is 0.0 (integer division first: 0, then 0 * 2.0). 1 / 2.0 * 2 is 1.0. Order matters because promotion happens per operation, not per expression.
• String concatenation is left-to-right. Once a String is on the left of a +, everything after is concatenated. 1 + 2 + "3" + 4 + 5 is "3345": 1 + 2 = 3, then "3" + "3" = "33", then + 4 → "334", + 5 → "3345".
• System.out.println() with no argument prints a blank line. print vs println matters for exact-output questions: with print, consecutive outputs run together on one line.
• Printing a double prints the full value: System.out.println(10.0 / 4) shows 2.5; 10.0 / 5 shows 2.0, not 2. The .0 is part of the answer.
• Escape sequences: \n newline, \t tab, \" literal quote, \\ literal backslash. The exam uses \n and \" most.

Mistakes that cost points

• Rounding integer division. 7 / 2 is 3, never 3.5 or 4. Two ints in, int out, fraction dropped.
• Flooring negative division. -7 / 2 is −3. Java truncates toward zero.
• Adding before concatenating. "Total: " + 5 + 10 is "Total: 510". The String came first, so no addition happens.
• Dropping the .0 on doubles. If the result is a double, the output shows a decimal point. 4.0, not 4.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.