Wrapper Class Methods, Character Tests and Autoboxing in Java (ICSE Class 10)

Understand Wrapper Class Methods, Character Tests and Autoboxing

Wrapper Class Methods, Character Tests and Autoboxing: key facts

Wrapper classes

int	Integer
long	Long
float	Float
double	Double
char	Character
boolean	Boolean
byte	Byte
short	Short

Capital first letter; Integer and Character are spelt in full.

Parse methods

Integer.parseInt	String
Long.parseLong	String
Float.parseFloat	String
Double.parseDouble	String

Invalid text gives NumberFormatException at run time.

Character tests

• •isDigit
• •isLetter
• •isLetterOrDigit
• •isLowerCase
• •isUpperCase
• •isWhitespace (each takes a char, returns boolean)

Called as Character.isDigit(ch).

Character conversion and boxing

• •toLowerCase(ch)
• •toUpperCase(ch)
• •Autoboxing: primitive to object
• •Unboxing: object to primitive

Non-letters are unchanged by toUpperCase.

How to answer Wrapper Class Methods, Character Tests and Autoboxing questions

1. 1Decide whether the argument is a String (parse methods) or a char (Character methods).
2. 2Evaluate the wrapper call and note its return type (int, double, boolean or char).
3. 3Remember that + joins when a String is involved and adds when both sides are numbers.
4. 4For char arithmetic, convert to the ASCII number first.
5. 5If the text is not a valid number, state the run-time NumberFormatException.

Wrapper Class Methods, Character Tests and Autoboxing 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 Wrapper Class Methods, Character Tests and Autoboxing

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

• parse + Type

  parseInt, parseLong, parseFloat, parseDouble: the type name follows parse.

  Use it when: Choosing a parse method.

• is... asks, to... changes

  Methods starting with is answer true or false; methods starting with to return a changed char.

  Use it when: Return types of Character methods.

• Box in, box out

  Autoboxing puts a primitive in a box (object); unboxing takes it out.

  Use it when: Definitions.

Wrapper Class Methods, Character Tests and Autoboxing: solved examples

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

String s = "25";
int n = Integer.parseInt(s) + 5;
String t = s + 5;
double d = Double.parseDouble("2.5") * 2;
System.out.println(n + " " + t + " " + d);

String w = "A1b2 c3!";
int L = 0, D = 0, W = 0, O = 0;
for (int i = 0; i < w.length(); i++) {
    char c = w.charAt(i);
    if (Character.isLetter(c)) L++;
    else if (Character.isDigit(c)) D++;
    else if (Character.isWhitespace(c)) W++;
    else O++;
}
System.out.println(L + " " + D + " " + W + " " + O);

String s = "Java 8";
String r = "";
for (int i = 0; i < s.length(); i++) {
    char c = s.charAt(i);
    if (Character.isUpperCase(c))
        r = r + Character.toLowerCase(c);
    else if (Character.isLowerCase(c))
        r = r + Character.toUpperCase(c);
    else
        r = r + c;
}
System.out.println(r);
Integer a = 10;
int b = a + 3;
System.out.println(b);

Where students lose marks in Wrapper Class Methods, Character Tests and Autoboxing

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

• ✗Mistake: Writing "25" + 5 as 30

  The text "25" is a String, so + joins.

  Wrong working

  "25" + 5 = 30

  Right working

  "25" + 5 = "255"; Integer.parseInt("25") + 5 = 30

  ✓Do this: Parse before adding.

  Usually costs: 1 mark

• ✗Mistake: Calling the method on a variable instead of the class

  The wrapper methods are called with the class name.

  Wrong working

  ch.isDigit()

  Right working

  Character.isDigit(ch)

  ✓Do this: Write Character. or Integer. first.

  Usually costs: 1 mark

• ✗Mistake: Answering parseInt("4.5") as 4

  Decimal text is not a valid int; the program stops with NumberFormatException.

  Wrong working

  Integer.parseInt("4.5") = 4

  Right working

  Integer.parseInt("4.5") throws NumberFormatException

  ✓Do this: Use parseDouble for decimal text.

  Usually costs: 1 mark

Practice questions on Wrapper Class Methods, Character Tests and Autoboxing

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

   The wrapper class for the primitive type char is:
   (a) Char (b) Character (c) String (d) Charac

   Hint

   Spelt in full.

   Answer

   (b) Character

2. Q2 · 1 mark · Very short answerBoard level

   Which method converts the String "3.75" to a double value?

   Hint

   Wrapper class plus parse.

   Answer

   Double.parseDouble("3.75")

3. Q3 · 2 marks · Short answerHarder board level

   Write the output:

   System.out.println(Character.toUpperCase('b') + 1);
   System.out.println(Character.isLetter('5') + " " + Character.isLetterOrDigit('5'));

   (ASCII code of A is 65.)

   Hint

   A char plus an int is an int.

   Answer

   67
   false true

4. Q4 · 3 marks · Short answerHarder board level

   Define autoboxing and unboxing with one example each. What does the following display?
   Integer a = 7;
   int b = a + 3;
   System.out.println(b);

   Hint

   Which direction is each?

   Answer

   Autoboxing is the automatic conversion of a primitive to its wrapper object, e.g. Integer a = 7;. Unboxing is the automatic conversion of a wrapper object to its primitive, e.g. int c = a;. In the code a is unboxed in a + 3, so b = 10 and 10 is displayed.

5. Q5 · 1 mark · Assertion-reasonHOTS / competency

   Assertion (A): Integer.parseInt("3.5") causes a run-time exception.
   Reason (R): The text "3.5" is not a valid representation of an int.
   (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.

   Hint

   Which exception, and why?

   Answer

   (a) Both are true and R explains A (NumberFormatException).

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 is the wrapper class of boolean?Show answer

   Boolean

2. 2.What does Character.isWhitespace(' ') return?Show answer

   true

3. 3.Which method converts a String to long?Show answer

   Long.parseLong(String)

4. 4.What is autoboxing?Show answer

   Automatic conversion of a primitive value to its wrapper object.

5. 5.What does Character.toUpperCase('5') return?Show answer

   '5' (unchanged)

6. 6.Which exception is thrown by parseInt("abc")?Show answer

   NumberFormatException

Wrapper Class Methods, Character Tests and Autoboxing: frequently asked questions