Expressions and Output

Overview

Day 1 is output: what System.out.println and System.out.print do to the cursor (1.3.A.1), what a literal and a string literal are (1.3.B.1, 1.3.B.2), and the three escape sequences \", \\, and \n (1.3.B.3). Day 2 is arithmetic: expressions built from values, variables, and the five operators, and the rule that decides whether a result is an int or a double (1.3.C.1, 1.3.C.2), then integer division (1.3.C.3) and the remainder operator (1.3.C.4). Day 3 is compound expressions: operator precedence, left-to-right order, and parentheses (1.3.C.5), and why dividing an int by the int zero compiles but throws an ArithmeticException (1.3.C.6).

Students have seen System.out.println display text (Topic 1.1) and have declared int, double, and boolean variables (Topic 1.2). Stay inside the topic boundary. Every program only displays values; students do not join text and numbers in one statement (that is Topic 1.15), so a label and a value always take a print followed by a println. Variables are declared with literal starting values, as in Topic 1.2; say that the full rules for = are Topic 1.4. Do not convert between int and double (Topic 1.5), use += (Topic 1.6), or declare String variables. Two exclusions from the CED: % is used only with a first operand that is not negative and a positive second operand, and dividing by zero is shown only with two int values.

Show MenuBoard.java in the IDE without running it. Students write the exact output, one console line per line of paper. Most will write three lines; ask what line 7, System.out.println(); with nothing inside the parentheses, does. Take two or three answers (“nothing”, “an error”, “a blank line”), then run it: Today's lunch:, Veggie tacos, an empty line, Enjoy!.

1. Goals (slide 3). Goals 1 and 2 are today’s; goals 3–5 are Days 2 and 3.
2. The console and the cursor (1.3.A.1). Output goes to the console, the IDE’s output window. A marker called the cursor shows where the next character will be written. Every display starts at the cursor.
3. println does two jobs, in order (slide 5): display the value inside the parentheses, then move the cursor to the start of the next line. Slide 5 is the warm-up program with a cursor table: line 7 had nothing to display, but still moved the cursor, so an empty line appeared.
4. Predict: ReadySetGo. Two print statements, then two println statements. Students predict, then run it: Ready set go! on one line and Race over. on the next. print displays its value and leaves the cursor right after it. The only spaces in the output are the ones inside the quotes.
5. Labels and values (slide 6). Show LabelThenValue: print for the label, println for the value, so each pair ends its own line. "Level:" has no space, so 3 lands against the colon. Joining text and a number in one statement waits for Topic 1.15.
6. Check: where is the cursor? (slide 7). Students write the output alone, then compare with a partner: XY, Z, W. The empty println() does not make an empty line here, because the cursor was in the middle of a line; it just ends Z.

1. Predict: LiteralKinds (slide 9 afterward). Five println statements: 42, 3.75, true, "42", and "Room 101, 2nd floor!". Students predict, then run it. Lines 1 and 4 look the same on the console.
2. Literals (1.3.B.1, 1.3.B.2). A literal is the code for a fixed value. The way it is written tells Java its type: no decimal point is an int, a decimal point is a double, true and false with no quotes are boolean, and characters in double quotes are a string literal. The quotes mark where the text starts and stops and are not displayed. "42" is text, the character 4 followed by the character 2; Java calculates with 42 but only displays "42".
3. How a double literal is displayed (slide 9). Run DoubleLiterals: 2.50 displays as 2.5, but 7.0 keeps its .0. A double always shows at least one digit after the decimal point. Inside a string literal every character, including every space, is kept; "" is the empty string.
4. Predict: QuoteInside.java. "She said "hi" to me." Ask whether it compiles. Run it: errors such as ')' or ',' expected (the wording varies between Java versions). The second quote ended the string literal.
5. Escape sequences (1.3.B.3, slide 10). Inside a string literal, a backslash \ starts an escape sequence: two characters that stand for one special character. The course uses three: \" (a double quote), \\ (one backslash), and \n (a new line). It is the backslash, not the forward slash /. Run EscapeQuotes, then NewlineEscape: one println can display two lines, and a print whose text ends in \n still leaves the cursor on a new line.

Write the target output on the board or project it from this plan. Students create FairPoster.java in the IDE and write a program that displays it exactly, using exactly four statements, at least one print, and at least one \n:

Spring "Code" Fair
Files go in C:\fair\projects
Room 101

Bring a friend!

Students run their program and compare it with the target character by character. Debrief by asking two or three students how they made the empty line and how they displayed the quotes and the backslashes. There is more than one correct program; FairPoster.java is one.

Hand out the puzzle worksheet. Questions 2, 6, and 8 (be the JVM with print and println, the escape-sequence match-up, and a one-token fix to a line break) are homework. Students keep the worksheet for Days 2 and 3.

Ask: “After System.out.print("A"); runs, where is the cursor? After System.out.println("A");?” Students answer on a sticky note or in their notes: right after the A; at the start of the next line.

Show slide 2. Students write a guess for each question: what Java says for 7 / 2 when a calculator says 3.5, and how many full cartons and leftover eggs 50 eggs make in cartons of 12. Take a few guesses without confirming them; the closure (slide 22) comes back to both.

1. Vocabulary (1.3.C.1). An expression is code that Java evaluates to produce a single value. An arithmetic expression is built from numeric values (literals and variables) and arithmetic operators. The values an operator works on are its operands (the word from Topic 1.2).
2. The five operators (1.3.C.2, slide 13). Show the operator table and run OperatorTable: +, -, * (there is no × key), /, and %. Division and remainder get their own segments today.
3. Calculate or display? Run TicketMath: 3 * 12 displays 36, and "3 * 12" displays the characters 3 * 12. The quotes turn anything into text that is displayed, not calculated. Slide 13 shows both lines.
4. Predict: ResultTypes (slide 13). 6 * 2, 6 * 2.0, 1.5 + 2.5, 10 - 0.25. Run it: 12, 12.0, 4.0, 9.75. The rule: two int operands give an int; at least one double operand gives a double. Java decides from the types, not from whether the answer happens to be whole, so 1.5 + 2.5 is 4.0.
5. Variables as operands. Run TicketVariables. Java uses the value the variable holds when the expression is evaluated. Evaluating an expression does not change the variable: after the three lines, tickets still holds 3.

1. Seventeen cookies, five friends. Cookies cannot be broken: each friend gets 3. Predict and run CookieShare: 3, 3, 0, 3.4, 3.4.
2. The rule (1.3.C.3). When both operands are int, the result is only the integer part of the quotient. 19 / 5 is 3.8 in math and 3 in Java: no rounding. When the first int is smaller than the second, the result is 0. When at least one operand is a double, the result is the full quotient as a double, and it does not matter which side the double is on.
3. Same numbers, different types (slide 15). AverageTwo: total / 2 gives 87, total / 2.0 gives 87.5. HalfCup: 1 / 2 gives 0, a logic error that compiles and runs. The fix in this topic is to write one operand as a double literal, 1.0 / 2 (HalfCupFixed); other ways of converting types are Topic 1.5.

1. What is left over (1.3.C.4). Run CookieRemainder: 17 / 5 is 3 and 17 % 5 is 2. a % b is what is left after taking as many whole groups of b out of a as possible. Write the check on the board: 17 = 5 × 3 + 2. / counts the whole groups; % counts what is left.
2. Hours and minutes (slide 14). A 135-minute movie: 135 / 60 is 2, 135 % 60 is 15, and 135 / 60.0 is 2.25. Check it: 135 = 60 × 2 + 15.
3. Patterns. Run RemainderCases and fill in the table together: 20 % 5 is 0 (divides evenly), 3 % 5 is 3 (a smaller first number is all left over), 1234 % 10 is 4 (the last digit), 125 / 60 and 125 % 60 are hours and minutes, and % 2 is 0 for even numbers and 1 for odd ones.
4. Scope. In this course the first operand is never negative and the second is always positive.

Show one expression at a time. Students write the value and the type on a mini whiteboard, then hold it up on your signal. Discuss any split before moving on. Start with the six expressions on slide 16 (47 / 10, 47 % 10, 47 / 10.0, 6 % 8, 6 / 8, 10 % 5), then continue with the table below as time allows.

Finish with a word problem: 70 students ride buses that hold 30 each. Which expression gives the number of full buses, and which gives the students left over? (70 / 30 is 2; 70 % 30 is 10.)

Puzzle worksheet questions 1, 3, and 7 (values of expressions, sticker-sharing code tiles, and the divide-versus-remainder face-off) are homework. The last row of question 1, 2 + 6 / 2, previews Day 3’s precedence rules; tell students to try it and check it on Day 3.

Return to the warm-up with slide 22. Students answer it in Java (50 / 12 is 4 full cartons, 50 % 12 is 2 eggs left over), then finish the sentence starter: “7 / 2 is 3 because both operands are int, so Java keeps only the integer part.” Look for “the types of the two operands,” not “whether the answer is whole.”

Write 2 + 3 * 4 on the board. Students vote: 14 or 20? Ask one student from each side to explain. A program cannot be ambiguous, so Java needs a fixed rule. Run the first line of PrecedenceSteps to settle it: 14. Then check the last row of worksheet question 1 (2 + 6 / 2 is 5).

1. Compound expressions and precedence (1.3.C.5, slide 18). An expression with more than one operator is a compound expression. Operator precedence decides which operators are grouped with their operands first: parentheses first, then *, /, and % (one level), then + and - (a lower level). Operators with the same precedence go left to right.
2. When it happens. The compiler groups the values with operators when the program is compiled, as if adding invisible parentheses: 2 + 3 * 4 means 2 + (3 * 4). The calculation happens when the program runs.
3. Predict: PrecedenceSteps. Work the five lines step by step on the board: 14, 20, 10, 12, 6. Slide 18 shows the first four with their first and second steps. Stress 20 - 6 - 4 is 10, not 18; in 24 / 4 * 2, * does not go before /; they share a level, so the left one goes first.
4. Parentheses change the grouping. Run AverageParens: quiz1 + quiz2 / 2 is 130, higher than either quiz. (quiz1 + quiz2) / 2 is 87, and / 2.0 keeps the .5: 87.5. Show PerPerson (31) and PerPersonFixed (9) as a second example.
5. The type is decided one operation at a time. Predict and run MixedOrder: 1 / 2 * 4.0 is 0.0, because 1 / 2 was already integer division by the time Java reached 4.0. 1.0 / 2 * 4 and 4.0 * 1 / 2 are both 2.0.
6. Precedence check (slide 20). Students find each value alone and say which operation happens first: 10 - 2 * 3 is 4, (10 - 2) * 3 is 24, 18 / 3 / 2 is 3, 7 + 8 % 3 is 9.

1. Predict: TeamSplit.java (1.3.C.6; slide 19 shows the same pattern with 30 slices and 0 guests). snacks holds 24 and teams holds 0. Ask: “Does it compile? What is displayed?” Run it: Splitting snacks..., then java.lang.ArithmeticException: / by zero naming line 8. Done! never appears.
2. Why it compiles. snacks / teams follows every rule of Java. The compiler checks how the code is written, not what the values will be; the divisor is only known to be zero when the line runs. This is the run-time error students saw in Topic 1.1, now explained: no int is the answer to 24 ÷ 0, so Java throws an ArithmeticException instead of making up a number.
3. Only the divisor matters. Run ZeroDivisor: 0 / 24 is 0 (fine), and 24 / (3 - 3) throws, because the parentheses produce a zero divisor. Finished never appears.
4. Scope. The course covers dividing by zero only when both values are int. Dividing by zero when a double is involved behaves differently and is not tested.

Students type PizzaParty.java (or open the copy you made) and write their predicted output before running it. Run it and check: *** "Pizza Party" Planner ***, Total slices: 24, Slices each: 4, Left over: 4, Total cost: $37.5, Cost each: $7.5, an empty line, Thanks for planning!.

Then, one at a time, students make each change, predict, run, record, and undo:

1. On line 14, add parentheses: pizzas * (slicesPerPizza / friends).
2. On line 7, change 5 to 0.
3. On line 21, change "\nThanks for planning!" to "\\nThanks for planning!".
4. On line 18, change println to print.

For each, students record what was displayed and which rule explains it. Debrief: (1) Slices each: 3, because 8 / 5 is now done first as integer division (1), then 3 * 1. (2) The first two lines appear, then Slices each: and an ArithmeticException at line 14; nothing after it runs. (3) The last line becomes \nThanks for planning! with no empty line before it, because \\ is a backslash and the n is an ordinary letter. (4) Total cost: $37.5Cost each: $7.5 on one line, because print leaves the cursor after 37.5.

Puzzle worksheet questions 4, 5, 9, and 10 (be the compiler, the quiz-average snippet pool, two truths and a lie about 1 / 2 * 4.0, and who am I) are homework. Question 9 settles the Think First prediction from question 2.

Show slide 21 (vocabulary) while students settle. Give the exit ticket (about five minutes). Question 1 checks output, escape sequences, and string literals (1.3.A, 1.3.B), question 2 precedence and the result type (1.3.C.2, 1.3.C.5), and question 3 dividing by zero (1.3.C.6). Re-teach the weakest one at the start of Topic 1.4.