Chapter 1: Advanced Class Design

Question 1

How does Java match overloaded methods

Answer 1

When matching overloaded methods, Java checks for:

• Exact match
• Match by superclass
• Converting to a larger primitive
• Converting to autoboxed
• Varargs

Question 2

When is instanceof true?

Answer 2

a instanceof b. True if A is an instance of B, a subclass of B, or a class which implements the B interface (directly or indirectly)

Question 3

Instanceof causing a compilation error?

Answer 3

If the compiler knowns that A can never be B. (Except when B is an interface. Even if your class A is final, you can still check instanceof interface. This is because you could have a subclass which implements the interface and this only gets checked at runtime)

Question 4

Virtual method invocation

Answer 4

This is when we invoke regular non-static methods. The reason it virtual method invocation is because Java can look for overriden methods rather than the ones in the class the compiler says we have.

Question 5

@Override

Answer 5

For @Override to work correctly, the method must be doing one of these:
* Implementing from an interface
* Overriding a superclass method
* Overriding hashCode, equals, toString()
Otherwise there will be a compilation error

Question 6

Equals

Answer 6

Signature is public boolean equals(Object obj). If you use anything other than Object it is an overloaded method (not overriden).

Question 7

Rules for overriding Equals

Answer 7

• x.equals(x) == true
• x.equals(y) y.equals(x) both true or both false
• transitive x.eq(y) y.eq(z) then z.eq(x)
• must be consistent, so you can do it multiple times with the same result
• x.eq(null) == false. Note: Shouldn’t throw NPE!

Question 8

Overriding hashCode

Answer 8

A hashCode is a number which puts instances of a class into a finite number of categories. (Makes it easier to sort in a HashMap).

• Common not to include Boolean or ‘char’ values
• Within the same program, the result of a hashCode must not change. (i.e. don’t include variables which are likely to change)
• Can use a subset of variables which equals() uses
• hashCode() results don’t need to be unique when called on unequal results
• Common to multiply by a prime number to make the hashCode more unique

Question 9

Enum values

Answer 9

.values() returns an array of all the values

Question 10

Enum ordinal

Answer 10

.ordinal() is the int value that corresponds to the order the enum was declared. But you can’t compare enum to int, so watch out for this.

Question 11

Enum valueOf

Answer 11

.valueOf(“”) returns the enum value of the exact match. Note it is case-sensitive. If it can’t find the enum value it will throw an IllegalArgumentException.

Question 12

Enums in a switch statement

Answer 12

Needs to be just the ENUM value, can’t compare to int ordinal, or String values

Question 13

Semi-colon in enums

Answer 13

Semi-colon is needed at the end of the enums if there is anything other than just the enum values

Question 14

Enum constructors

Answer 14

The constructor is called once when you first call any Enum value.

Question 15

Adding abstract methods to enums

Answer 15

Can add an abstract method into an enum, but then all enums must override this otherwise there will be a compilation error.

Question 16

Member inner classes

Answer 16

• Defined at the same level as methods, instance vars and constructors
• No restrictions on accessibility (can be public or private)
• Can be abstract or final
• Cannot declare static fields or methods
• Can access private members of outer class
• You need an instance of the outer class to be able to instantiate the inner class

Question 17

Same variable names in inner class

Answer 17

A > B > C nesting with all having a variable x
If you’re calling from deepest level C:
* this.x will be C.x
* B.this.x will be B.x
* A.this.x will be A.x.
If you’re trying to get the x from C, but you’re in in B, this won’t work

Question 18

Instantiating member inner classes

Answer 18

A a = new A();
A.B b = a.new B(); (Note: could have used the reference B here since B is a member of A)
A.B.C c = b.new C(); (Note: Here we have to reference the full path to C since C is not a member of A).

Question 19

Inner interfaces

Answer 19

Can be private

e.g. Can only be referred to from outer class

Question 20

Local inner classes

Answer 20

• Nested class defined within a method.
• Can only be accessed within that method (and doesn’t have an access modifier)
• Cannot be declared static, or have any static fields or methods
• Does not exist until a method is invoked
• Has access to all fields and methods of the enclosing class.
• Does not have access to local variables of the method, unless they are final or effectively final (Since it will exist in a separate file. But if variables are final or effectively final they will be passed in the constructor to the class when it is instantiated)

Question 21

Effectively final

Answer 21

• We don’t change the reference to the object, even if a change occurs in the state of the referenced object
• It doesn’t have to be initialised in the same line it is declared.
• Just make sure it doesn’t change reference after it’s been initialised.

Question 22

Anonymous inner classes

Answer 22

• Extend an existing class or implement an interface
• Short implementation not used anywhere else
• You can just override the methods you need directly
• Note that interface implementations must be public
• Can define them in a parameter of another method

Question 23

Static nested classes

Answer 23

• Defined at member level
• Don’t need an instance of out class to use
• Can import statically or non-statically
• Enclosing class can refer to methods + fields of static class