memory-types-variables

Memory

  • 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;

        • Implicit casting happens when the source type extends or implements the target type (casting to a superclass or interface).

          • example

          String s = "str";
Object obj= s;

      • 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).

          • can produce a runtime exception (ClassCastException) when the object being cast is not of the target type (or the target's subtype).

          • example

          Object o = "str";
String str = (String) o;

      • promotion

        • example

        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.

      • instanceOf()

        • will check if object is instance of some class or superclass (abstract/interface)

        Object obj = Calendar.getInstance();
obj instanceof Object;
obj instanceof Calendar;
obj instanceof Serializable;
obj instanceof Date; // false, but IDE will tell you this

  • 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

        • Common - Char, Integer, Float

        • 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

    • initialization of variables

      • primitive types with default initiliazation

        • primitives variables needs to declared and set

        • Example

        int i = 10;

        • 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 obj = new Object();

      • 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.

    • 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

boxing and unboxing

Wrappers vs primitives

  • 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

    • 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

  • Good practice

    • use primitives in local scope

    • Effective java suggest to use primitives over wrapped

    • Use boxed for elements, keys and values in collections

Pass by value

  • 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/

Soft Reference

  • references to data (generally reflective) will not be garbage collected until the JVM is low on memory

Other

  • Dereferencing datatypes

Links

  • https://www.developer.com/java/data/autoboxing-and-unboxing-in-java.html

Links

  • 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

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