Basic problems of Java

1、long(8) and float(4)Who is the largest value range?

becauselongyes8Bytes,floatyes4It’s a byte. As a rule, we can’t move up. But becausefloatIt is a floating-point type, and its calculation method is different from that of shaping

ⅠFirst of alllongCan be automatically converted tofloat;

For example:

Long number=88888;//88888

Float newNumber=number;//88888.0

Ⅱ、longRange of values indicated:2^63-1

floatRange of values indicated:3.4 * 10^38

longandfloatAll of the storage structures are inconsistent:

1） longIs an integer type, directly1，08Bit storage;

2） floatFloating point type, binary conversion with decimals, stores significant digits (scientific counting method)

   

2The characteristics of XOR operators are as follows

^Features: one data bit XOR twice to another data bit, the number itself does not change.

​int a = 1;

int b = 2;

a = a ^ b;

b = a ^ b;//b = 1

a = a ^ b;//a = 2

The essence of XOR operator isConvert the decimal system to binary, and then the same bit is the same0,The difference is1.

   

   

3, bit operation efficiency is the fastest, its core is binary

For example:2 * 8 = 2 * 2^3 = 2 << 3

   

4、switchsupportbyte/lang/String(1.8).

   

5、continue、break 、return What’s the difference?

Continue ：Jump out of this loop and only use it in the loop.

Break : ends the current loop. It can only be used in the loop.

Return ：Terminate the current method, not just the loop.

   

6Three characteristics of object-oriented

Encapsulation, inheritance and polymorphism

   

7, array

One dimensional array initialization:

①String[] strs = new String[x]. xIt’s a fixed lengthJust a statementstrsCount RegxString array of

②String[] strs = {strs1,strs2…}; Declare and initialize

Initialization of two-dimensional array: a two-dimensional array is a collection of one-dimensional arrays.

①String[][] strs = {arr1,arr2…}; arr1:Index group

②String[][] strs = new String[4][3]; 4: index group length,3: refers to the maximum length (tested) that contains each array,If you assign a value to a two-dimensional array directly, you will report an error. It’s useless at other times.

③String[][] strs = new String[4][];[]The value assigned to represents lengthening.

   

8, inherit execution order

   

   

   

   

   

Execution sequence:father_static > son_static >father_codePiece>father_comnstructor>son_codePiece>son_constructor

Note: the static code block is called only once, and the construction code block is executed once before the construction method is executed.

   

9Overloading and rewriting of methods

Overload Overload: overload can change the type of return value. The overload of method only depends on the different parameter list.

Override Rewriting: generally, the subclass rewrites the parent class method, and the rewriting is only related to the return value type. The return value of a subclass method can only be a subclass of the return value of a parent class.

   

10Abstract methods and interfaces

   

11、finalandstatic

Final:

Modify class, class cannot be inherited

Modify method. Method cannot be overridden

Modify a variable, and the variable becomes a constant, which can only be assigned once.

Naming conventions for constants:

Public static final String STRING_NAME = “zhangsan”;

Modify the variable of the reference class:

 

static:

Load as class loads

Prior to object existence

Shared by all objects of the class

·If a member variable is shared by all objects, it can be defined as static.

   

① The static method can be used directlyclassName.staticFunction()call

You can also use objects.staticFunction()call

   

②Static members cannot access regular members, but regular members can access static members

   

③ There are no static methodsthisKeywords, becausethisKeywords exist as objects are created

If a class is full of static members, it is recommended to set the constructor of the class toprivate.

   

12、package 、 import 、class The order relation of

order package > import > class

Package : The package of the current class has and must only have1individual

Import : other classes introduced can havenindividual

Class:Modifying the current class. Must have.

   

13, outer class and inner class

   

   

14, anonymous inner class

   

15、StringThe characteristics of the system.

Cannot be inherited because it is inheritedfianlModify, and there are no subclasses.

   

16、==andequalsThe difference between

== ：The comparison basic data type is literal quantity, and the comparison reference data type is memory address

Equals: Only reference data types can be compared if the object is not overriddenequalsMethods, then and==There is no difference (literal quantity).

After rewriting, it generally compares every attribute in the object.

17, abnormal

Try…catchandthrowsThe differences between them are as follows:

Try…catchThe program will not be interrupted, andthrowsWill interrupt the program. Usually you can handle it yourselftry…catch,Use it if you can’t handle itthrows.

   

throwandthrowsThe differences between them are as follows:

theowIn generalcatch() inside

Throws-Move behind the method

18、JVMIs it multithreading?

yes

1, main programmainMethod start

2、finalizaThe garbage collection mechanism will also be activated

   

19, adapter mode（adapter）

Overview: without modifying the original2In this case, they can cooperate through transformation.

Definition: transform the interface of a class into the interface expected by customers.

The configuration modes are3Species:

Target: Expected interface（USB）

Adaptee:Adapted interface(PS2)

AdapteeSon:Implement the adapted interface

Adapter ：adapter class

①Class Adapter

Interface Target:

>> void usb();//There is no method body here

Interface Adaptee:

>>void ps2();//There is no method body here

AdapteeSon implement Adaptee:

>>void ps 2(){};

Adapter extends Adaptee implements Target :

>>void usb(){

Super.ps2(); //Implementation interfaceyesDirect useImplementation method of parent class (configured)

}

②object adapter

Target:

>>void usb(){};

Adaptee:

>>void ps2(){};

Adapter extends Target:

Adaptee adptee;

Public Adaptee(Adapter adapter){this.adapter = adapter}

@overload

Void usb(){

Adapter.ps2();

}

③ Interface adapter

In short, it isThere are two kinds of interfacesnThere are three ways

And it happens that you’re going to use it2、3There are three ways

At this point, the implementation of the interface is to implement all the methods in the interface, which is certainly not ideal.

thereforeThere is the interface adapter:

Define aabstrctClass, implementing implementsClass. But only implement what you need to use2、3There are two ways.

Define an adapterAdapterClasses, inheritingabstrctClass. That’s itokIt’s too late.