memory-types-variables
Last updated
Was this helpful?
Last updated
Was this helpful?
Was this helpful?
Types -
Primitive
size
https://en.wikibooks.org/wiki/Java_Programming/Primitive_Types
types
```byte/short/int/long/char/boolean/float/double````
Reference/Objects
Casting
Can apply to objects or primitives
The boolean type cannot be cast to/from any other primitive type.
casting floating point primitives (float, double) to whole number primitives, the number is rounded down.
implicit Casting
Going from smaller range to larger range
byte byteVar = 42; short shortVar = byteVar;
Explicit Casting
Going from larger range to smaller
`double doubleVar = 42.0d; float floatVar = (float) doubleVar;
Explicit casting has to be done when the source type is extended or implemented by the target type (casting to a subtype).
promotion
example
instanceOf()
will check if object is instance of some class or superclass (abstract/interface)
Variables
Types
non-static/instance fields - naming lowercase snake case
static/class fields
Constants - naming uppercase camel case
local variable
declared and used in methods
parameters
methods
replaced with arguments when method is called
wrappers
https://stackoverflow.com/questions/20697868/why-we-need-wrapper-class
Defining/Declaring
Types/primitives - char/int/float etc
Objects/reference types
initialization of variables
primitive types with default initiliazation
primitives variables needs to declared and set
default values
null for objects
for primitive values
Naming conventions
lower case camel case for variables
capital snake case for Constants
CRUD (Create/Read/Update/Delete)
Although we do not delete variables
Pointers for objects
equality
pointers/reference
if two objects are different instances of the same class, they will by default not be equal.
If two objects newed separately but with same state should be equal, then need to override the equal() method.
object equality
Definitions
A primitive data type specifies the size and type of variable values, and it has no additional methods.
Defaults is some value
Wrapper classes provide a way to use primitive data types (int, boolean, etc..) as objects.
Have identities separate from their value
Default is always null if not assigned
Problems
A Boolean variable should only be true or false, but in fact has third state of null
If int variable has default, but this can never be true
If some type is null by default can lead to NPE
Good practice
use primitives in local scope
Effective java suggest to use primitives over wrapped
Use boxed for elements, keys and values in collections
https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value
https://dzone.com/articles/java-copy-shallow-vs-deep-in-which-you-will-swim
https://www.baeldung.com/java-deep-copy
List
https://stackoverflow.com/questions/23607881/returning-a-private-collection-using-a-getter-method-in-java
https://blog.frankel.ch/back-to-basics-encapsulating-collections/
references to data (generally reflective) will not be garbage collected until the JVM is low on memory
Dereferencing datatypes
https://www.developer.com/java/data/autoboxing-and-unboxing-in-java.html
http://www.learnjavaonline.org/en/Variables_and_Types
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/variables.html
https://www.tutorialspoint.com/java/java_variable_types.htm
https://www.tutorialspoint.com/java/java_modifier_types.htm
http://tutorials.jenkov.com/java/variables.html
http://tutorials.jenkov.com/java/data-types.html
https://www.javaworld.com/article/2077424/learn-java/does-java-pass-by-reference-or-pass-by-value.html
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
http://www.baeldung.com/java-primitives
example
can produce a runtime exception (ClassCastException) when the object being cast is not of the target type (or the target's subtype).
example
Classes - ie String, ArrayList
String aString = "Hello"
same as String aNewedUpString = new String("Hello") but is redundent but correct way as String is not a primitive type
interfaces
Declaring a list of string using the interface rather than the implementation: List<String> aStringList = new ArrayList<>();
use created
ie SomeNewType instanceOfNewType = new SomeNewType()
modifers (public/private/protected/default)
final
Where the value 10 is stored in i
reference types with null as default
ie SomeNewType instanceOfNewType; instanceOfNewType will have value of null
ie SomeNewType instanceOfNewType = new SomeNewType(); will have a reference to the object which is an instance of SomeNewType.
Example
Object is a reference type.
obj is the variable in which to store the new reference.
Object() is the call to a constructor of Object.
What happens
Space in memory is allocated for the object.
The constructor Object() is called to initialize that memory space.
The memory address is stored in obj, so that it references the newly created object.
Use of Database models, can have null values, so need to use wrappers
Generics can only use Types and not primitives
prevents use of collections libraries
Memory issues, primitives use less than type (regarding time and space)
Comparasion bugs when using == and equals
Doing multiple boxing and unboxing, espeically in loops can lead to increased time taken
short short1 = 1, short2 = 2;
int int1 = 1;
char char1 = 1;
int1 = short1 + short2; // short is promoted to int.
int1 = char1 + short2; // both char and short promoted to int.Object obj = Calendar.getInstance();
obj instanceof Object;
obj instanceof Calendar;
obj instanceof Serializable;
obj instanceof Date; // false, but IDE will tell you thisString s = "str";
Object obj= s;Object o = "str";
String str = (String) o;int i = 10;Object obj = new Object();