Java interview knowledge point

First, let's talk about the differences between final, finally, and finalize.

Final
It is used to declare attributes, methods, and classes, indicating that the attributes are unchangeable, methods cannot be overwritten, and classes cannot be inherited. Finally is a part of the structure of the exception handling statement, indicating that it is always executed. Finalize is
A Method of the object class. This method is called when the garbage collector is executed. It can overwrite this method to collect other resources during garbage collection, such as closing files.

Second, can anonymous inner class (anonymous internal class) be extends (inherited) other classes, or implements (implemented) interface (Interface )?

It can inherit other classes or complete other interfaces. This method is often used in swing programming.

Third, the difference between static nested class and inner class is that the more you say, the better (the more general the interview questions are ).

Static nested class is an internal class declared as static. It can be instantiated without relying on external class instances. In general, internal classes must be instantiated before they can be instantiated.

Fourth, the difference between & and.

& Is a bitwise operator that represents bitwise and operation, & is a logical operator that represents logic and (and ).

Fifth, the difference between hashmap and hashtable.

Hashmap is a lightweight Implementation of hashtable (non-thread-safe implementation). They all complete the map interface. The main difference is that hashmap allows null key values ), because of non-thread security, the efficiency may be higher than that of hashtable.

Sixth, the difference between collection and collections.
　　
Collection is the upper-level interface of the Collection class. Its inherited interfaces include set and list. collections is a help class for collection classes. It provides a series of static methods for searching, sorting, thread security, and other operations on various sets.

7. When to use assert.

　
1.4 new keywords (syntax) are used to test the Boolean expression status and can be used to debug programs. How to Use assert
If the expression is true, the following statement is executed; otherwise, assertionerror is thrown. In addition, assert <
Boolean expression>: indicates that if the expression is true, the subsequent expression is ignored. Otherwise, the value of the subsequent expression is used as the construction parameter of assertionerror. Note that
Add the-source 1.4 parameter; otherwise, an error is returned.] Add the-ea parameter during running; otherwise, the assert row is ignored.

Eighth, what is GC? Why does GC exist?
　　
　
GC is the meaning of garbage collection (gabage
Collection), memory processing is a problem that programmers are prone to. Memory collection that is forgotten or incorrect may cause instability or even crashes of the program or system. The GC function provided by Java can
The Java language does not provide a display operation to release allocated memory because the automatic monitoring object exceeded the scope to automatically recycle the memory.

Ninth, string S = new string ("XYZ"); how many string objects are created?

Two

10. How much is math. Round (11.5? Math. Round (-11.5) and so on?

Math. Round (11.5) = 12math. Round (-11.5) =-11round method returns a long integer closest to the parameter. After the parameter is added to 1/2, ask for its floor.

11th, short S1 = 1; S1 = S1 + 1; what is the error? Short S1 = 1; S1 + = 1; what is the error?

Short S1 = 1; S1 = S1 + 1; (S1 + 1 is an int type and requires forced conversion) Short S1 = 1; S1 + = 1; (can be compiled correctly)

12th. What is the difference between sleep () and wait?

　
Sleep is a thread method, which causes the thread to suspend the execution for a specified time and give the execution opportunity to other threads. However, the monitoring status remains unchanged and will be automatically restored after the time. Call
Sleep does not release the object lock. Wait is an object method. Calling the wait method for this object causes this thread to discard the object lock and enter the waiting lock pool for this object. Only
This thread enters the object lock pool only after sending the notify method (or notifyall) to prepare for obtaining the object lock and entering the running state.

13th. Does Java have a goto?

There are no 13 questions. If any interview asks this question, I suggest you stay away from the company.

14th, does the array have the length () method? Does string have the length () method?

The array does not have the length () method. It has the Length attribute.
String has the length () method.

15th, the difference between overload and override. Can the overloaded method change the type of the returned value?

　
Overriding and overloading are different manifestations of Java polymorphism. Overriding is a manifestation of the polymorphism between the parent class and the Child class,
Overload Overloading is a manifestation of polymorphism in a class. If a subclass defines a method with the same name and parameter as its parent class, we say this method is overwritten.
(Overriding ). When a subclass object uses this method, it calls the definition in the subclass. For it, the definition in the parent class is "blocked. If multiple vertices with the same name are defined in a class
Method, which may have different numbers of parameters or different parameter types, is called overloading ). The overloaded method can change the type of the returned value.
16th. The elements in the set cannot be repeated. How can we identify whether the elements are repeated or not? Is = or equals () used ()? What are their differences?

The elements in the set cannot be repeated, so the iterator () method is used to identify whether the elements are repeated or not. Equals () is used to determine whether two sets are equal.
The equals () and = Methods Determine whether the reference value points to the same object equals () is overwritten in the class, in order to return the true value when the content and type of the two separated objects match.

17th. Give me the most common runtime exception.

Refer to the following:
Arithmeticexception,
Arraystoreexception,
Bufferoverflowexception,
Bufferunderflowexception,
Cannotredoexception,
Cannotundoexception,
Classcastexception,
Cmcmexception,
Concurrentmodificationexception,
Domexception,
Emptystackexception,
Illegalargumentexception,
Illegalmonitorstateexception,
Illegalpathstateexception,
Illegalstateexception,
Imagingopexception,
Indexoutofboundsexception,
Missingresourceexception,
Negativearraysizeexception,
Nosuchelementexception,
Nullpointerexception,
Profiledataexception,
Providerexception,
Rasterformatexception,
Securityexception,
Systemexception,
Undeclaredthrowableexception,
Unmodifiablesetexception,
Unsupportedoperationexception

18th, what is the difference between error and exception?

Error indicates that recovery is not a serious problem that is impossible but difficult. For example, memory overflow. It is impossible to expect the program to handle such a situation.
Exception indicates a design or implementation problem. That is to say, it indicates that if the program runs normally, it will never happen.

19th, list, set, and map are inherited from the collection interface?

List, set is
Map is not

20th. What is the difference between abstract class and interface?

　
The class that declares the existence of a method rather than implementing it is called an abstract class (Abstract
Class), which is used to create a class that reflects certain basic behaviors and declare methods for this class, but cannot implement this class in this class. Abstract cannot be created
Class. However, you can create a variable whose type is an abstract class and point it to an instance of a specific subclass. Abstract constructors or abstract static methods are not allowed. Abstract
Class subclasses provide implementation for all abstract methods in their parent class, otherwise they are also abstract classes. Instead, implement this method in the subclass. Other classes that know their behavior can implement these methods in the class.

　
An interface is a variant of an abstract class. All methods in the interface are abstract. Multi-inheritance can be achieved by implementing such an interface. All methods in the interface are abstract, and none of them
There is a program body. The interface can only define static
Final member variable. The implementation of an interface is similar to that of a subclass, except that the implementation class cannot inherit behaviors from the interface definition. When a class implements a special interface, it defines (to be given by the program body) all the methods of this interface.
Then, it can call the interface method on any object that implements the interface class. Because there is an abstract class, it allows the interface name as the type of the referenced variable. Normally, dynamic Association editing will take effect. References can be converted
The instanceof operator can be used to determine whether the class of an object implements the interface.

21st can abstract methods be both static, native, and synchronized?

None

22nd. Can an interface inherit an interface? Can an abstract class implement the (implements) interface? Can an abstract class inherit a concrete class )?

Interfaces can inherit interfaces. Abstract classes can be implemented (implements) interfaces, and whether abstract classes can inherit object classes, provided that object classes must have clear constructors.

23rd. Is run () or start () used to start a thread ()?

When a thread is started, the START () method is called to make the virtual processor represented by the thread in a running state, which means that it can be scheduled and executed by JVM. This does not mean that the thread will run immediately. The run () method can generate the exit sign to stop a thread.

24th. Can constructor be overwritten?

Constructor cannot be inherited, so overriding cannot be overwritten, but overloading can be overloaded.

25th. Can I inherit the string class?

The string class is a final class, so it cannot be inherited.

26th. After a thread enters a synchronized method of an object, can other threads access other methods of this object?

No. One Synchronized Method of an object can only be accessed by one thread.
27th, there is a return statement in try {}, so will the code in finally {} following this try be executed? When will it be executed, before or after return?

Will be executed, before return.

28th. Programming question: how many equals 2x8 in the most efficient way?

Programmers with a C background are particularly fond of asking such questions.
2 <3

29th, the two objects have the same value (X. Equals (y) = true), but different hash codes are available, right?

No. It has the same hash code.

30th. After an object is passed as a parameter to a method, this method can change the attributes of the object and return the changed result, so is it a value transfer or a reference transfer?

Is the value transfer. The Java programming language only transmits parameters by values. When an object instance is passed as a parameter to a method, the parameter value is a reference to the object. The object content can be changed in the called method, but the object reference will never change.

31st. Does swtich work on byte, long, and string?

In switch (expr1), expr1 is an integer expression. Therefore, the parameters passed to the switch and case statements should be int, short, Char, or byte. Long and string cannot apply to swtich.

32nd programming question: Write a singleton.

The Singleton mode ensures that only one instance of a class exists in a Java application.
The Singleton mode generally has several forms:

The first form: defines a class. Its constructor is private. It has a static private class variable. When the class is initialized, use a public getinstance method to obtain its reference, and then call the method.

Public class Singleton {
Private Singleton (){}
// Define your own instance internally. Isn't it strange?
// Note that this is private for internal calls only
Private Static Singleton instance = new Singleton ();
// Here is a static method for external access to this class, which can be accessed directly.
Public static Singleton getinstance (){
Return instance;
}
}

Second form:

Public class Singleton {
Private Static Singleton instance = NULL;
Public static synchronized Singleton getinstance (){
// This method is better than above. You don't need to generate objects every time. It's just the first time.
// Generate instances during use, improving efficiency!
If (instance = NULL)
Instance = new Singleton ();
Return instance ;}
}

Other forms:
Defines a class. Its constructor is private and all methods are static.
It is generally considered that the first form is more secure.

There are also some problems. I hope you can continue to add them.

　
1. It is best to introduce the final variable modification situation (separate the basic type variables and reference type variables ).
2. I don't know how to describe it well. The anonymous internal class itself is a class that implements a specific interface or inherits other classes.
3. Non-static classes cannot have static methods. For more information, see the Java collections framework code.
4. & it is both a bit operator and a logical operator.
& And & are logical operators. The difference between them is that & performs short-circuit computation.
6. For more information, see the document.
11. The specific reason may be better.
13. Java has the goto keyword, but it is not used yet.
15. Overload
There is no relationship with polymorphism. Polymorphism is the basic feature of object-oriented, and is related to inheritance and overriding. I thought that overload was a kind of polymorphism from C ++. Later
I flipped through the basic C ++ classic book, and stressed that polymorphism is only related to inheritance, virtual functions, and overriding, so I don't know where the wrong idea came from.
16. Public
Interface set <E> extends collection <E>
Collection. More specifically, set does not contain elements that meet e1.equals (E2) on E1 and E2, and contains a maximum of null values.
Element. As its name implies, this interface imitates the set abstraction in mathematics.
Check the document.
22. "Whether the abstract class can inherit the object class, but the premise is that the object class must have a clear constructor. "What does it mean? Who defined it?
27. This is complicated, for example

Public class main {

Public static void main (string [] ARGs ){
System. Out. println (test ());
}

Public static Boolean test (){
Boolean B = false;
Try {

Return B = 4> 3;
} Finally {
System. Out. println (B );
// Return false;
}
}
}
In this example, we can see that the expression after return has been computed, and the Return Statement is suspended. If return is not found in finally, the return statement will be returned.
If return exists in finally, return is returned in finally.
29. No one can meet a noncompliant programmer.

31. What about enumeration?
32. Notes are incorrect.