Increment, Decrement, Shorthand Operators, Precedence, new and Dot in Java (ICSE Class 9)

Understand Increment, Decrement, Shorthand Operators, Precedence, new and Dot

Increment, Decrement, Shorthand Operators, Precedence, new and Dot: key facts

Prefix and postfix

• •++a: change, then use
• •a++: use, then change
• •alone, both add 1

The same rule holds for --.

Shorthand

• •a += b means a = a + b
• •x -= y + 1 means x = x - (y + 1)
• •right side first

Includes -= /= %= and the multiply form.

Precedence ladder

• •() . postfix
• •prefix and unary
• •multiply, divide, remainder
• •add, subtract
• •relational
• •== !=
• •&&
• •||
• •?:
• •assignment

Brackets override all.

Joining text

• •number + number adds
• •once a String appears, + joins
• •work left to right

4 + 6 + "x" gives 10x.

How to answer Increment, Decrement, Shorthand Operators, Precedence, new and Dot questions

1. 1List every variable with its starting value.
2. 2Mark the precedence: postfix, prefix, then * / %, then + -, and so on.
3. 3Work left to right; for a++ note the old value then update a, for ++a update a then note the value.
4. 4After the statement, write the final value of every changed variable.
5. 5For printing, evaluate each part in order and decide whether + adds or joins.

Increment, Decrement, Shorthand Operators, Precedence, new and Dot in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Increment, Decrement, Shorthand Operators, Precedence, new and Dot

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Pre = change first

  ++a changes before you look; a++ lets you look and then changes.

  Use it when: Prefix versus postfix.

• Equals goes last

  Assignment has the lowest precedence, so everything on its right is solved before storing.

  Use it when: Shorthand operators.

• Number, number, then text

  Numbers add until the first String; after it, everything joins.

  Use it when: Output questions with + and strings.

Increment, Decrement, Shorthand Operators, Precedence, new and Dot: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

int a = 6, b = 2;
int c = a++ * b-- + --a;
System.out.println(a + " " + b + " " + c);

int r = 5 + 18 / 4 % 3 * 2 - 7;
boolean s = r == 0 || r++ > 3;
System.out.println(r + " " + s);

int bill = 500, items = 2;
bill += 120;
items++;
bill -= bill % 100;
items *= 2;
System.out.println(bill + " " + items + " " + bill / items);

int n = 14;
String t = n % 2 == 0 ? (n > 10 ? "big even" : "small even") : "odd";
System.out.println(t + " " + (n > 10 ? n / 4 : n * 4));

Where students lose marks in Increment, Decrement, Shorthand Operators, Precedence, new and Dot

The slips examiners see most often, and what to write instead.

• ✗Mistake: Using the new value for a postfix

  Postfix is read as prefix.

  Wrong working

  b = a++ with a = 5 gives b = 6

  Right working

  b = a++ with a = 5 gives b = 5, a = 6

  ✓Do this: Write "use, then change" next to every a++.

  Usually costs: 1 mark

• ✗Mistake: Shorthand without brackets

  The right side is split.

  Wrong working

  x = 4; x *= 3 + 2 gives 14

  Right working

  x = 4; x *= 3 + 2 gives 20

  ✓Do this: Rewrite as x = x * (3 + 2) first.

  Usually costs: 1 mark

• ✗Mistake: Adding after a String has appeared

  + has two meanings.

  Wrong working

  "Sum: " + 4 + 6 gives Sum: 10

  Right working

  "Sum: " + 4 + 6 gives Sum: 46

  ✓Do this: After the first String, everything joins.

  Usually costs: 1 mark

Practice questions on Increment, Decrement, Shorthand Operators, Precedence, new and Dot

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which of these has the highest precedence?
   (a) + (b) % (c) && (d) =

   Hint

   Think of the ladder from the top.

   Answer

   (b) %

2. Q2 · 2 marks · Very short answerBoard level

   Rewrite each with a shorthand operator: (i) s = s - 3; (ii) p = p * (q + 1); (iii) r = r % 5;

   Hint

   The whole right side stays together.

   Answer

   (i) s -= 3; (ii) p *= q + 1; (iii) r %= 5;

3. Q3 · 2 marks · Short answerHarder board level

   Find the value of a:

   int a = 3;
   a = a++ + ++a;
   System.out.println(a);

   Hint

   The first a++ gives 3 and a becomes 4.

   Answer

   8 (a++ gives 3, a is 4; ++a makes a 5 and gives 5; 3 + 5 = 8)

4. Q4 · 3 marks · Short answerHarder board level

   Find the output:

   int k = 8;
   k -= --k - k--;
   System.out.println(k);

   Hint

   The old k (8) is kept for the -=.

   Answer

   8 (the saved k is 8; --k gives 7; k-- gives 7 and k becomes 6; 7 - 7 = 0; k = 8 - 0 = 8)

5. Q5 · 2 marks · Short answerHOTS / competency

   Why does System.out.println("Total: " + 4 + 6); print Total: 46 while System.out.println(4 + 6 + " is total"); prints 10 is total?

   Hint

   Where does the first String appear?

   Answer

   In the first, "Total: " is already a String, so 4 and 6 are joined one after the other. In the second, 4 + 6 is calculated first because both are numbers; the String comes after, so 10 is joined to the text.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What does a += b mean?Show answer

   a = a + b

2. 2.Value used by b = a++ when a is 5?Show answer

   5

3. 3.Which operator creates an object?Show answer

   new

4. 4.Which operator reaches a member of an object?Show answer

   The dot operator.

5. 5.Associativity of assignment?Show answer

   Right to left.

6. 6.Does println() move to a new line?Show answer

   Yes, after printing.

Increment, Decrement, Shorthand Operators, Precedence, new and Dot: frequently asked questions