chapter 2 Flashcards
How do you create an instance of a class?
write "new" before the class name and parentheses after the class name. e.g. Foo f = new Foo();
What do constructor methods do?
Create new objects.
What are two characteristics of constructor methods?
Their name matches the name of the class, and they do not have return types.
What is the purpose of a constructor method?
To initialize fields in an instance of a class.
What keyword is required anytime a constructor is used?
new
What is a code block?
The lines of code between {}.
What are instance initializers?
Blocks of code that run outside of a method.
What order are fields and initializer blocks run?
The order in which they appear in the file.
When does the constructor run in relation to fields and initializer blocks?
After all fields and instance initializer blocks have run.
What are the two data types Java applications contain?
primitive and reference types.
How many and what are the built-in primitive data types Java has?
8 types.
boolean: true or false
byte: 8-bit integral value
short: 16-bit integral value
int: 32-bit integral value
long: 64-bit integral value
float: 32-bit floating-point value
double: 64-bit floating-point value
char: 16-bit Unicode value
What is and what is not a primitive data type in Java?
A single value in memory, not an object.
What does a float value require following the number in Java?
f
Are all numeric types signed in Java?
Yes.
What does it mean when a numeric type is signed?
The type reserves one bit to cover a negative range.
What are the ranges of values possible for each of the 6 primitive numeric types in Java?
byte: -128 to 127
short: -32,768 to 32,767
int: -2,147,483,648 to 2,147,483,647
long: -9,223,372,036,854,775,808 to 9,223,372,036,854,775,80
float: 7 decimal digits
double: 16 decimal digits
What are the similarities and differences between short and char?
Both are stored as integral types with the same 16-bit length. Short is signed, char is not.
How are floating point numbers stored in most computer systems?
Scientific notation.
What is a number called when it is present in code?
A literal.
What does Java assume as the default type for a literal?
an int.
How can a numeric literal be made a long type?
Append an L (or an l) to the literal.
What can be added to numeric literals to allow easier reading?
underscores _
Where are underscores NOT allowed to be placed in a numeric literal?
The beginning or end of a literal.
Right before or after the decimal point.
What are the two ways a reference type can be assigned to a value?
To another object of the same or compatible type.
To a new object using the new keyword.
What are some differences between primitive and reference types?
Reference types: 1. Can be assigned null 2. Can be used to call methods Primitive types: 1. Have lowercase type names
What is a variable?
A name for a piece of memory that stores data.
What is required in variable declaration?
A type and a name.
What is an identifier?
The name for a variable, method, class, interface, or package.
What are the four rules for legal identifiers?
- must begin with a letter, $, or _
- can include numbers but not start with them
- single underscore is not a valid identifier
- cannot use a Java reserved word
What is camelCase and snake_case?
camelCase:
convention where the first letter of each word is capitalized.
snake_case:
convention where an underscore separates each word.
What is the rule for declaring multiple variables in the same declaration (inline)?
All the variables must be the same type.
What is a local variable?
A variable defined within a constructor, method, or initializer block.
What is true about initializing local variables?
They do not have a default value and must be initialized before use.
What is the name of variables that are passed to constructors or methods?
parameters.
What is an instance variable?
A value defined within a specific instance of an object.
What is a class variable?
A variable that is defined at the class level and shared among all instances of the class.
How can you tell a variable is a class variable?
It has the keyword static in front of it.
What types of variables do not require initialization before using?
instance and class variables. They are assigned default values at declaration.
Define the default values for each type.
boolean: false byte, short, int, long: 0 float, double: 0.0 char: '\u0000' (NULL) all object refs: null
When was the var keyword introduced?
Java 10.
What types of variables can you use keyword var with?
local variables.
What is the formal name of the var feature?
local variable type reference.
How is the type determined when keyword var is used to initialize a local variable?
The compile infers it from the value.
When is the type associated with a variable declared with var determined?
At compile time.
When using var, does the variable need to be assigned a value on the same line as the declaration in order to compile?
yes
Can null be assigned to a variable that has been declared using the var keyword?
no
What does it mean when a word is a reserved type name?
It cannot be used to define a type, such as a class, interface, or enum.
Is var a reserved type name?
Yes
Is var a reserved word?
No
Where can var not be used to declare a variable?
Parameters for constructors and methods, instance variables, and class variables.
Can the type of a var change?
no
Is var permitted in a multiple-variable declaration?
no
What is the largest possible scope for a local variable?
The method they are defined in.
What is the scope of an instance variable?
From declaration until object is available for garbage collection.
What is the scope of a class aka static variable?
From declaration until the program ends.
Where are all Java objects stored?
In your programs memory heap (free store).
What is garbage collection?
The process of freeing memory on the heap by deleting objects that are no longer reachable in your program.
What is the built-in method to help support garbage collection that can be called at any time?
System.gc();
When is System.gc(); guaranteed to run?
never.
When is an object available for garbage collection?
- The object no longer has any references pointing to it.
2. All references to the object have gone out of scope.
What gets garbage collected, the object or the reference?
The object.
