12 - AP CSA - random ap review

Animal = new Dog();

if the method catch() is in the Dog class and the Animal class, which class does it run the method from?

The Dog method

If using super() to call a parent class constructor, where do you put it in the subclass constructor?

It has to be the first line

Do you have to initialize instance variables that are primitive data types?

No, if you forget to initialize them in a constructor, because they are primitive, they will set themselves to a default value

Can you use methods to initialize instance variables to a value in a constructor? (The methods and constructor are part of the same class)

yes

Do subclasses inherit the parent class’s constructor?

no

What kind of methods are equals(), lessThan() and toString()?

Accessor method

System.out.println(this) == toString() ?

yes

What is a NullPointer Exception?

runtime exception, when an object does not point to anything (null)

When are two objects considered aliases?

when. they both reference the same object

what are algorithms?

 step by step process that describes how to solve a problem in a way that always gives a correct answer

What is the running part of a class called?

public void static main(String[] args)

for ap cs a, classes and constructors are private/public and only instance variables are private/public

public, private

Can you call Math.sqrt(x) on x if it is a Double (wrapper class) instead of double (primitive)? Can it be called on an Integer or int?

No, it has to be called on a double, the wrapper class doesn’t count. It can be called on an int, because it will implicitly convert it but not the Integer wrapper class.

How do you get the primitive value of a Wrapper class object?

.intValue() or the wrapper class equivalent

How would you box the Double 3.0 to a double?

double x = Double.doubleValue();

Double x = 3.4;

double y = 2.3;

Is System.out.println(x) == System.out.println(y);

yes, it prints the same thing

If you add 1 to an Integer at its max or subtract 1 from an Integer at its min, what happens?

The int will wrap around to the min or max

Autoboxing

Automatic conversion that the Java compiler makes between primitive types and their corresponding object wrapper classes (ex: int → Integer)

Unboxing

Automatic conversion that the Java compiler makes between object wrapper classes and their corresponding primitive types (ex: Integer → int)

When does Java apply Autoboxing?

1. When primitive types are psased as parameters of their corresponding wrapper class

2. when primitives are assigned to a variable of the type of their corresponding wrapper class

When does Java apply unboxing?

1. When wrapper class types are passed as parameters of their corresponding primitive type

2. when wrapper class types are assigned to a variable of their corresponding primitive type

Reference types only store ________

references (ex: String, ArrayList, Integer)

Pair p1 = new Pair(2, 3)

Pair p2 = new Pair(2, 3)

p1.equals(p2) = t/f?

false
.equals() has to be overridden for the Pair class, otherwise it just checks to see if p1 and p2 reference the same thing

Overloaded constructor

set default values to something else, this constructor has parameters

What happens if you don’t write a constructor for a class?

Java will write a no-argument constructor for it

What are the standard Java default values if the instance variables are not initialized?

int = 0

double = 0.0

boolean = false

Strings and other objects = null

What if an accessor or mutator method tries to run on a String or other object that hasn’t been initialized?

might be a nullPointer exception for accessor, but def for mutator

What is the difference between methods that return primitive types vs. methods that return reference types?

primitive types: returns a copy, so the original is not changed

reference types: Java returns a copy of the object reference, not the value itself (saves space)

What are the 5 parts of a method header? Name an example.

Access level (public/private), return type (primitive/reference/void), identifier (name of method), parameter list

ex: public void main (String[] args)