Basic Java Knowledge

1, enter

Scanner Sc=new scanner (systom.in);

Sc.next (); Accept String Type data

Sc.nextint ();//Accept int type data

2, variable

In essence, a piece of storage space in memory (used to store data)

Naming rules:

You can only use numbers, letters, underscores, and $ components. Cannot start with a number, cannot use a reserved word

To the hump-type naming rules: To distinguish between words, the initial letter is smaller, followed by the first letter of the word capital

such as: Stumanagesystem

3, constant

Final

4, data type

Integer: Byte byte short integer int shaping long long shaped shape

Float type: float, double

Character type: Char

Boolean Type: Boolean

Note: In cases where high accuracy is required, do not use float,double, use Java.util.BigDecimal

Reference data types: string, array, interface, enumeration, class

5, array

int[] aa=new int[length];

6, Construction method

With and without parameters: used to initialize

7, access modifier

Public (can be used throughout the project)

Protected protected (can be used in the same package, can also be used with inheritance)

Private (can only be used in the current class)

Default (Friendly) defaults (in the same package)

8, rewrite

Return values, parameter names, method names are the same, content in the method body can be different

9, overload

Different parameters of the same life

10, interface

Can not be instantiated, only through his implementation class to operate, implement

11, abstract class (Abstrart)

cannot be instantiated and can only be manipulated by inheritance

12, Time

Date De=new date ();

Simpiedateformat sd=new Simpiedateormat ("Yyyy/mm/dd hh:mm:ss E");

Sd.format (DE)

13, random number

Random rd=new random ();

Rd.nextint ();

Generates random numbers from 3 to 30: Rd.nextint (27) +3;

Ran.nextfloat ()

Ran.nextdouble ()

14, set

The container used to store the data

The difference between a set and an array: the length must be certain, the data type must be orderly, the set

Collection is the root interface of the collection

Common methods:

The number of elements in the size () collection

IsEmpty () Determines whether the collection is empty (True is an empty collection)

Contains (Object o) determines whether the specified element is included

Iterator () iterator (no enhanced for loop is included in the loop before jdk1.5, only iterators are used)

ToArray () converts a collection into an array

Add (Object o) adds a collection element

Remove (Object o) removes the collection element

Clear () Removes all elements from the collection

Sub-interface; List ordered data is not unique. Set unordered, data unique map key value pairs stored in the form of data

The implementation class for list:

Arra ' ylist find data quickly, add, modify, delete slower

Find data quickly, add, modify, delete slower

List is ordered, data is not unique

Set unordered data unique

Basic Java Knowledge