Interesting Facts About YOU

1
Interesting Facts About YOU!

• You each made an average of thirty errors
  compiling each exercise last term
• For every error per person in the morning, there
  were 2.5 errors in the afternoon

2
The Moral

• You are morning people after all (unlike me)
• If I can make the TUEs and THUs at 900 so can
  you!
• Come in early and do lots of programming

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How to Avoid Over 80 of Your Mistakes
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The Most Common Mistakes -

• The most common error is missing a semicolon or a
  comma
• String s Every line ends with a
• Fred.method(arg1, 12) // needs comma

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Arguments and Operators

• The next most common error is missing a between
  strings or similar
• System.Out.println(root is r)
• People often put
• System.Out.println(root is , r)
• The number of errors went up a lot when methods
  were taught

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Undefined variables

• The most common semantic error is using a
  variable which has not been defined or to a
  lesser extent not initialised
• Think carefully about types and your style of
  declaration

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Static Confusion

• non-static variable in a static context
• Oh Dear! The result of writing Fortran in Java
• Avoid static except for main()
• Keep main() SHORT

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Use of in Conditionals

• If (x 2) something
• In C or C this compiles
• is assignment
• is the comparison operator

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Right Hand Side ! LHS

• All variables have a type
• All expressions return a type
• These MUST be the same
• Well you can use a cast
• int i 0
• Double b 10.2
• i (int) b

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1. Compile and run

• public class Fred
• public static void main()
• System.out.println(Hello)

What Happens?
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2. After Execution

• String types new String20

• What will the first element of the array contain?
• An empty String?
• The null value?
• Zero?

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3. What will be Printed?

• Integer x1 new Integer(5)
• Integer x2 x1
• Integer x3 new Integer(5)
• if(x1 x2) System.out.println(x1 x2)
• if(x1 x3) System.out.println(x1 x3)

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4. What will be printed?

• String s1 yes
• String s2 yes
• if(s1 s2) System.out.println(same)
• if(s1 ! s2) System.out.println(different)
• (very trick question!)

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5. What is printed out?

• int counts
• System.out.println(counts counts)

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6. What is printed out?

• int counts
• counts new int10
• System.out.println(counts counts)

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7. What is printed out?

• int ids
• ids new int5
• System.out.println(ids2)

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8. What is printed out?

• String names
• names new String5
• System.out.println(names2)

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9. String to int

• String age 97
• int freds_age

How can I get freds_age from age?
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10. int to String

• String age
• int freds_age 99

How can I turn freds_age into a String?
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11. What will happen?

• public class Example
• public Boolean flags new Boolean4
• public static void main(String args)
• Example E new Example()
• System.out.println(Flag 1 is
  E.flags1)

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12. What will be the result?

• public int adder(int N)
• return 0x100 N

In n of calling n adder(2)
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13. How many String Objects are created here?

• String A, B, C
• A new String(1234)
• B A
• C A B

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• 14. What keyword is used to derive once class
  from another?
• 15. What keyword is used to state that a certain
  set of methods must be implemented by a class?

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• 16. A variable in a class is defined as
• protected int age
• Why does the keyword protected make age more
  visible!?

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17. This is Student.java

• Public class Student
• String name
• int age
• boolean male
• Module subjects
• public class Module
• String module_code
• int Semester
• int mark

What message is generated by javac?
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18. Which line will cause a compilation failure?

• double fred
• boolean bill
• int 2fred
• String _lucy
• float ron

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19. Define the following

• A primitive
• An Object Reference
• Which can have the value zero?
• Which can have the value null?