1, the computer's memory unit
The data in the computer is represented by 0 and one, and one of the 0 or 1 is called a bit.
8 bits are called a byte (byte), two bytes are called a word (word), and four bytes are called two-word (Dword).
1byte=8bit, 1kb= 1024 Byte, 1m=1024 KB.
2, Java has four categories of 8 basic types
A, 4 kinds of integral type
BYTE 1 byte
Short 2 bytes
int 4 bytes
Long 8 bytes
B, 2 kinds of floating-point types
Float 4 bytes
Double 8 bytes
C, 1 kinds of truth types
Boolean
D, 1 types of characters
Char
3, Java three types of reference
A, Classes class reference
First, the class
Class
Reference
Can be we created, here I do not say much, mainly to explain a few
Java
Classes in the Library
Object
:
Object
is a very important class,
Object
is the root class of the class hierarchy, and each class makes the
Use
Object
As a superclass, all objects (including arrays) implement methods of this class. Use
Object
Can
To define all the classes
Such as:
Object object= New Integer (1);
To define a
Interger
Class
Integer i= (integer) object;
Are here to put this
Object
Cast into
Interger
Class
String
:
String
class represents a String,
Java
All string literal values in the program, such as
"ABC"
) are
Implemented as an instance of this class. Check the individual characters of the sequence, compare strings, search strings, extract
Child
Word
Character
String
、
Invasive
Built
Word
Character
String
Deputy
This
、
In
The
Deputy
This
In
、
The
Yes
Of
Word
Character
are
Be
Turn
Change
For
Big
Write or lowercase form.
Date
:
Date
Represents a specific moment, accurate to milliseconds.
Date
The classes are generally now
Calendar
And
GregorianCalendar
All replace
Void
:
Void
A class is a placeholder class that is not instantiated, and it maintains a pair representing
Java
The essential
Word
void
Of
Class
The reference to the object.
Class classes reference can be created by us, here I do not say much, mainly to explain a few Java library class Object:object is a very important class, Object is the class hierarchy of the root class, each class uses Object as a superclass, All objects, including arrays, implement methods of this class. Object can be used to define all classes such as: Object object= new Integer (1); To define an Interger class integer i= (Integer) object; To cast this object into the Interger class String:string class represents a string, all string literals in the Java program (such as "ABC") are implemented as instances of this class. Checks a single character of a sequence, compares a string, searches a string, extracts a substring, creates a copy of a string, and in that copy, all characters are converted to uppercase or lowercase. Date:date represents a specific moment, accurate to milliseconds.
The class of date is generally now all replaced by calendar and GregorianCalendar. The Void:void class is a placeholder class that is not instantiated, keeping a reference to a class object that represents the Java keyword Void.
also has a corresponding class such as: Integer long boolean byte character double float short& nbsp &NBSP;B, Interface interface reference Interface interface Reference can be we created, here I don't say much, The main is to explain several Java Library interface interface list<e>: List, users of this interface can accurately control the insertion position of each element in the list. The user can access the element based on the integer index of the element (position in the list) and search for the elements in the list. The List interface provides two ways to search for a specified object. From a performance standpoint, these methods should be used with care. In many implementations, they perform high overhead linear searches. The list interface provides two ways to efficiently insert and remove multiple elements at any location in the list. add (): Inserts the specified element in the list. remove (): Removes the element from the specified position in the list. get (int index): Returns the element at the specified position in the list. MAP<K,V>: K-the type of the key maintained by this mapping V-the type of the mapped value maps the key to the value's object. A map cannot contain duplicate keys, and each key can be mapped to at most one value. Put (K key,v value): Associates the specified value with the specified key in this map (optional action). If this mapping previously contained a mapping relationship for the key, the old value is replaced with the specified value (if and only if true is returned, the mapping m contains the mappings of the key K). remove (Object key) If a mapping relationship exists for a key, it is removed from this map (optional operation). Rather, if this map contains a mapping relationship from key k to value V satisfying (key==null k==null:key.equals (k)), the mapping relationship is removed. (This mapping can contain at most one such mapping relationship.) Get (Object Key): Returns the value mapped by the specified key, or null if the mapping does not contain a mapping relationship for the key. Here we mainlyis a string List Map Object is the most commonly used number arraylist<e> arrays can be traced to the JDK API These classes and interfaces in Java.lang: providing access to Java programming languages The basic class for programming. Java.unit: Contains the collection framework, legacy collection classes, event models, date and time facilities, internationalization, and various utility classes (string tag builder, random number generator, and bit array).