Java study _ Java interview questions

First, let's talk about the differences among final, finally, and finalize. Second, can Anonymous Inner Class (Anonymous internal Class) be extends (inherited) other classes, or implements (implemented) interface (interface )? 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 ). Fourth, the difference between & and. Fifth, the difference between HashMap and Hashtable. Sixth, the difference between Collection and Collections. 7. When to use assert. 8. What is GC? Why does GC exist? Ninth, String s = new String ("xyz"); how many String objects are created? 10. How much is Math. round (11.5? Math. round (-11.5) and so on? 11th, short s1 = 1; s1 = s1 + 1; what is the error? Short s1 = 1; s1 + = 1; what is the error? 12th. What is the difference between sleep () and wait? 13th. Does Java have a goto? 14th, does the array have the length () method? Does String have the length () method? 15th, the difference between Overload and Override. Can the Overloaded method 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? 17th. Let me know the most common runtime exception. 18th. What is the difference between error and exception? 19th, List, Set, and Map are inherited from the Collection interface? 20th. What is the difference between abstract class and interface? 21st can abstract methods be both static, native, and synchronized? 22nd. Can an interface inherit an interface? Can an abstract class implement the (implements) interface? Can an abstract class inherit a concrete class )? 23rd. Is run () or start () used to start a thread ()? 24th. Can Constructor be overwritten? 25th. Can I inherit the String class? 26th. After a thread enters a synchronized method of an object, can other threads access other methods of this object? 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? 28th. Programming question: how many equals 2x8 in the most efficient way? 29th, the two objects have the same value (x. equals (y) = true), but different hash codes are available, right? 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? 31st. Does swtich work on byte, long, and String? 32nd programming question: Write a Singleton. Answer: First, let's talk about the differences between final, finally, and finalize. Final? Modifier (keyword) If a class is declared as final, it means that it cannot generate a new subclass and cannot be inherited as a parent class. Therefore, a class cannot be declared both abstract and final. Declare variables or methods as final to ensure that they are not changed during use. Variables declared as final must be declared with an initial value, which can only be read and cannot be modified in future references. Methods declared as final can only be used, but cannot be used to reload finally? Finally blocks are provided for troubleshooting. If an exception is thrown, the matched catch clause is executed, and the control enters the finally block (if any ). Finalize? Method Name. Java technology allows you to use the finalize () method to clear objects from the memory before the Garbage Collector clears them. This method is called by the garbage collector when it determines that this object is not referenced. It is defined in the Object class, so all classes inherit it. Subclass overwrites the finalize () method to sort system resources or perform other cleanup tasks. The finalize () method is called before the Garbage Collector deletes an object. Second, can Anonymous Inner Class (Anonymous internal Class) be extends (inherited) other classes, or implements (implemented) interface (interface )? An anonymous internal class is an internal class without a name. It cannot be extends (inherited) other classes, but an internal class can be used as an interface and implemented by another internal class. 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 ). Nested Class (generally C ++) and Inner Class (generally JAVA ). The biggest difference between Java internal classes and C ++ Nested classes is whether there are external references. For details, see http: // www.frontfree.net/articles/services/view.asp? Id = 704 & page = 1 Note: The static internal Class (Inner Class) means that 1 creates an object of the static internal Class and does not need an external Class object, 2. You cannot access an external class object from an object of a static internal class. The forth, & and & are different. & Is a bitwise operator. & Is a Boolean logical operator. Fifth, the difference between HashMap and Hashtable. All belong to the Map interface class, which maps the unique key to a specific value. The HashMap class is not classified or sorted. It allows a null key and multiple null values. Hashtable is similar to HashMap, but does not allow null keys and null values. It is also slower than HashMap because it is synchronized. Sixth, the difference between Collection and Collections. Collections is a java. util class that contains various static methods related to set operations. Collection is an interface under java. util. It is the parent interface of various Collection structures. 7. When to use assert. An assertion is a statement that contains a Boolean expression. When executing this statement, it is assumed that the expression is true. If the expression is calculated as false, the system reports an AssertionError. It is used for debugging purposes: assert (a> 0); // throws an AssertionError if a <= 0 assertions can be in two forms: assert Expression1; assert Expression1: Expression2; expression1 should always generate a Boolean value. Expression2 can be any expression that generates a value. This value is used to generate a String message that displays more debugging information. Assertions are disabled by default. To enable assertions during compilation, use the source 1.4 flag: javac-source 1.4 Test. java to enable assertions at runtime, use the-enableassertions or-ea flag. To disable assertions during running, use the-da or-disableassertions flag. To enable assertions in the system class, you can use the-esa or-dsa tag. You can also enable or disable Assertion Based on the package. You can place assertions on any location that is not expected to arrive normally. Assertions can be used to verify parameters passed to private methods. However, assertions should not be used to verify the parameters passed to the public method, because public methods must check their parameters whether or not assertions are enabled. However, you can use assertions to test the post-condition either in a public method or in a non-public method. In addition, assertions should not change the state of the program in any way. Eighth, what is GC? Why does GC exist? (Basic ). GC is the garbage collector. Java Programmers don't have to worry about memory management, because the Garbage Collector automatically manages. To request garbage collection, you can call one of the following methods: System. gc () Runtime. getRuntime (). gc () Ninth, String s = new String ("xyz"); how many String objects are created? Two objects, one being "xyx" and the other being referenced object s pointing to "xyx. 10. How much is Math. round (11.5? Math. round (-11.5) and so on? Math. round (11.5) returns (long) 12, Math. round (-11.5) returns (long)-11; 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; error: s1 is short type, s1 + 1 is int type, cannot be converted to short type explicitly. It can be changed to s1 = (short) (s1 + 1 ). Short s1 = 1; s1 + = 1 is correct. 12th. What is the difference between sleep () and wait? The favorite sleep () method for thread execution is to stop the thread for a period of time. After the sleep interval expires, the thread may not resume execution immediately. This is because at that time, other threads may be running and not scheduled to give up the execution, unless (a) the "Awake" thread has a higher priority, (B) the running thread is blocked for other reasons. When wait () is a thread interaction, if the thread sends a wait () call to a synchronization object x, the thread will pause the execution and the called object will enter the waiting state, wait until the wake-up or wait time is reached. 13th. Does Java have a goto? Goto? Reserved Words in java, which are not currently used in java. 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, and Overloading is a manifestation of the 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 ). 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 methods with the same name are defined in a class, they may have different numbers of parameters or different parameter types, it is called Overloading ). The Overloaded method can change the type of the returned value by 16th, and the elements in the Set cannot be repeated. So what method can be used to distinguish whether 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. ArithmeticException, ArrayStoreException, except, failed, CannotRedoException, CannotUndoException, ClassCastException, cmcmexception, warn, DOMException, EmptyStackException, failed, ImagingOpException, failed, MissingRes OurceException, exceptions, exceptions, NullPointerException, ProfileDataException, ProviderException, RasterFormatException, SecurityException, SystemException, UndeclaredThrowableException, UnmodifiableSetException, UnsupportedOperationException 18th, error and exception are different? 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 not 20th. What is the difference between abstract class and interface? The class that declares a method rather than implementing it is called abstract class. It is used to create a class that reflects some basic behaviors and declare a method for this class, however, this class cannot be implemented in this class. You cannot create an abstract instance. 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. The subclasses of Abstract classes provide implementation for all Abstract methods in their parent classes. 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 have a program body. The interface can only define static final member variables. 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. The reference can be converted to the interface type or from the interface type. 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 of them can be 22nd. CAN interfaces inherit interfaces? 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. Generally, Singleton has several forms: the first one :?? 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 only for internal calls. private static Singleton instance = new Singleton (); // a static method is provided for external access to this class, you can directly access public static Singleton getInstance () {return instance ;}} In the second form: public class Singleton {private static Singleton instance = null; public static synchronized Singleton getInstance () {// This method is better than above. You do not need to generate objects every time. It is only the first time you use this method to generate an instance, which improves the efficiency! If (instance = null) instance = new Singleton (); return instance ;}} other forms: Define a class, its constructor is private, and all methods are static. It is generally considered that the first form is more secure. 33rd Hashtable and HashMap Hashtable inherit from the Dictionary class, hashMap is an implementation of the Map interface introduced by Java1.2. HashMap allows null to be used as the key or value of an entry, but Hashtable does not allow it. HashMap removes the contains method of Hashtable, change to containsvalue and containsKey. The contains method is easy to misunderstand. The biggest difference is that the Hashtable method is Synchronize, but HashMap is not. When multiple threads access Hashtable, they do not need to implement synchronization for their own methods, hashMap must provide external synchronization for it. The hash/rehash algorithms used by Hashtable and HashMap are roughly the same, so there is no big difference in performance. Part 2 (J2EE) Basic Q & A: 1. Which of the following classes can be inherited? 2. Differences between abstract classes and interfaces 3. Principles of Hashtable, and differences between HashMap and Hashtable 4. Differences between forward and redirect 5. What is Web container? 6. Explanation of the following J2EE terms 7. which technologies are implemented based on EJB? Let's talk about the differences between SessionBean and EntityBean. Differences between StatefulBean and StatelessBean. 8. XML Parsing Method 9. What is Web Service? Basic Q & A 1. Which of the following classes can be inherited? Java. lang. thread (T) java. lang. number (T) java. lang. double (F) java. lang. math (F) java. lang. void (F) java. lang. class (F) java. lang. classLoader (T) 2. differences between abstract classes and interfaces (1) interfaces can be multiple implements. abstract classes can only be defined by a single extends (2) interface, and abstract classes can be defined and implemented (3) the default field definition of the interface is public static final. The default field of the abstract class is "friendly" (visible in this package). 3. hashtable principle, and the difference between HashMap and Hashtable HashTable principle: Determine the storage location of the node through the key code of the node, that is, the key code k of the given node, obtain the function value H (k) by using a certain function relation H (number of Hash Functions). The value is interpreted as the storage location of the node. Address. hashMap is similar to Hashtable, but HashMap is non-synchronous (unsynchronizded) and can take null as the key code. 4. forward and redirect difference forward: an internal transfer in servlet redirect: Redirection, there are two requests, 2nd requests will lose the first attributs/parameters, etc. 5. what is a Web container? Implement the web Protocol application in the J2EE specification. this Protocol defines the runtime environment of web programs, including concurrency, security, and lifecycle management. 6. the following J2EE terms (1) JNDI: Java Naming & Directory Interface, JAVA Naming Directory service. the main function is to provide a directory system that allows applications from other places to leave their own indexes on it to quickly find and locate distributed applications. (2) JMS: Java Message Service, JAVA Message Service. communication between applications is mainly implemented. including point-to-point and broadcast. (3) JTA: Java Transaction API, JAVA Transaction service. provides various distributed transaction services. the application only needs to call the provided interface. (4) JAF: Java Action FrameWork, JAVA security authentication FrameWork. provides some security control frameworks. allows developers to customize their individual security through various deployments Full Control Policy. (5) RMI: Remote Method Interface, Remote Method call 7. which technologies are implemented based on EJB? The differences between SessionBean and EntityBean are also described. The differences between StatefulBean and StatelessBean are described. EJB includes Session Bean, Entity Bean, and Message Driven Bean. It is implemented based on technology such as JNDI, RMI, and JAT. sessionBean is used in J2EE applications to perform some server-side business operations, such as accessing the database and calling other EJB components. entityBean is used to represent the data used in the application system. for clients, SessionBean is a non-persistent object that implements some business collections running on servers. EntityBean is a persistent object, it represents an object view of an object stored in persistent storage, or an object implemented by an existing enterprise application. session Bean can be further subdivided into Stateful Session Bean and Stateless Session Bean. ses All sion beans can execute the system logic in method. The difference is that Stateful Session Bean can record the status of callers. Therefore, generally, A user will have a corresponding Stateful Session Bean entity. although Stateless Session Bean is also a logical component, it is not responsible for recording the user status, that is, when the user calls Stateless Session Bean, the EJB Container does not find the entity of the specific Stateless Session Bean to execute this method. in other words, when several users execute a Stateless Session Bean's methods, they will execute the same Bean Instance. in terms of memory, Stateful Session Bean and Stateless Session Bean In comparison, Stateful Session Bean consumes a lot of memory on the J2EE Server. However, the advantage of Stateful Session Bean is that it can maintain the user's status. 8. XML parsing Methods: Sax, DOM, JDOM 9. what is Web Service? Web Service is an interface proposed to enable communication and sharing between isolated sites. Web services use unified and open standards on the Internet, such as HTTP, XML, SOAP (Simple Object Access Protocol), and WSDL, therefore, Web services can be used in any environment that supports these standards (Windows and Linux. Note: The SOAP protocol (Simple Object Access Protocal) is an XML-based communication protocol used for network information exchange in distributed and distributed environments. Under this protocol, software components or applications can communicate through the standard HTTP protocol. Its design goal is simplicity and scalability, which facilitates the interoperability between a large number of heterogeneous programs and platforms, so that existing applications can be accessed by a wide range of users. Advantages: (1) cross-platform. (2) the SOAP protocol is based on industry standards such as XML and HTTP and is supported by all important companies. (3 ). because SOAP is used, data is transmitted in ASCII text instead of binary, which facilitates debugging. In this way, its data is easily transmitted through the firewall, the firewall does not need to create a "Vulnerability" for the program ". (4). In addition, the technical difficulty of WebService implementation is much lower than that of CORBA and DCOM. (5). To achieve B2B integration, EDI is relatively complete and complex, while WebService can be implemented at a low cost, and can be used by small companies. (6) In the C/S program, WebService can deal with the server and retrieve the number of webpages without overall refreshing. Disadvantages: (1) WebService uses XML to encapsulate data, which may cause a large amount of data to be transmitted over the network. Part 3: You have 4 bottles of medicine. The weight of each pill is fixed, but one of the bottles is contaminated, the weight of the pill changes, and each pill adds a little weight. How did you suddenly determine which bottle of medicine was contaminated? Java code: [java] package cn. nx. majinze; public class GetMedicine {public static void main (String [] args) {int [] T = {12,17, 12,12}; getTrue (T );} public static void getTrue (int [] T) {int flag = 0; for (int I = 0; I <T. length; I ++) {if (flag = 0) {for (int j = 0; j <T. length; j ++) {if (flag = 0) {for (int k = 0; k <T. length; k ++) {if (flag = 0) {if (I! = J & j! = K & I! = K) {if (T [I] = T [j] & T [j] = T [k]) {System. out. println ("qualified products have been found:" + I + j + k); flag = 1; break; Part 4 (IBMJava interview questions) 1. what is oracle. 2. what is major differenece oracle8i and oracle9i. 4. tell me some thing ur self. 5. please tell me about oops. 6. what is single inheritance. 7. what is multiple inheritance. 8.can java support multiple inheritance. 9. what is interface. 10. what is differenec between abstract Class and interface. 11.how to u prove that has Ace class cannot instantiate directly. 12. what is differenece between string and stringbuffer. 13. what is immutable 14.how to write a program using sort program. 15 how to write a program using unsort program. 16. what is legacy. 17. what is legacy api 18. what is legacy interface. 19. what is main difference hashmap and hastable 20. what is main differe Nce between arraylist and vector. 21. what is struts framework. 22. what are distributed techonologies. 23. what is advantage and disadvantage of distributed techonologies. 24. what is main difference between jsp and servlets. 25. what is difference between procedure and functions. 26. what is jdbc. 27. what are types of drivers. 28. what is type 4 driver. 29.how to collect requuirements form u r client. 3 0. which process use in ur project. 31. what is deployment descriptor. 32. what is heirarchy of files in struts. 33. please draw struts frame wrok. 34. please draw j2ee architecture. 35. please draw mvc-2 architecture. 36. please draw that how design op module. 37.how to find a file on linux. 38.how to configure weblogic8.1 on linux. 39.why you use struts framework in ur project. 40. what is platfrom inde Pendent 41. what is awt and swing. 42. what is heavy wieght components. 43. what is feature of weblgoic8.1. 44.why you choose application server on linux and database server on aix. 45. please tell me about ur project. 46. what is major concepts in oops. 47.why u choose mvc-2 architecture. 48. what is implicit object. 49.how does implicit objects in jsp 50.why choose weblogic8.1 other than any applicati Onserver. 51. what is water fall model vs sdlc 52. what is use of dataflowdiagrams 53.wha t is ip in ur project. 54. what about partition module answer: 1. oracle is an RDBMS product with DDL and DML from a company called Oracle Inc. 2. difference between 8i and 9i is given in the Oracle site 3. question not available 4. something 5. oops is Object Oriented Programming 6. what is single inheritance. ans: o Ne class is inherited by only other one class 7. what is multiple inheritance. ans: One class inheriting more than one class at atime 8.can java support multiple inheritance. ans: No 9. what is interface. ans: Interface has only method declarations but no defn 10. what is differenec between abstract class and interface. ans: In abstract class some methods may contain in definition, but In interface every met Hod shoshould be abstract 11.how to u prove that has Ace class cannot instantiate directly. ans: As they dont have constructor they cant be instantiated 12. what is differenece between string and stringbuffer. ans: Strings are immutable where as string buffer can be modified 13. what is immutable ans: Which cant be changed 14.how to write a program using sort program. 15 how to write a program using unso Rt program. ans: Both can be done using javascript This is for Sort function SelectTextSort (obj) {// sort by text var N = obj. options. length; for (var I = 0; I <> for (var j = I + 1; j <> if (obj. options [I]. text> obj. options [j]. text) {var i1 = (obj. options [I]. selected = true )? True: false var j1 = (obj. options [j]. selected = true )? True: false var q1 = obj. options [j]. text; var q2 = obj. options [j]. value; obj. options [j]. text = obj. options [I]. text; obj. options [j]. value = obj. options [I]. value; obj. options [I]. text = q1; obj. options [I]. value = q2; obj. options [I]. selected = (j1 & true )? True: false obj. options [j]. selected = (i1 & true )? True: false }}return true} 16. what is legacy. 17. what is legacy api 18. what is legacy interface. ans: legacy is something that is old in terms of technology/system 19. what is main difference hashmap and hastable ans: Hash table is synchronised 20. what is main difference between arraylist and vector. ans: Vector is synchronised 21. what is struts framework. 22. what are distributed techonologies. distributed technologies means any technology/s/w program that are having component in multiple environments that interact with each other depending on the functional requirements and design. 23. what is advantage and disadvantage of distributed techonologies. language is avoided. application can be built flexible to meet requirements. division of labor is possible. best of all the technologies and platforms can be optimally utilized. complexity of requirements can be already CED. 25. what is difference between procedure and functions. ''ans: Fuctions can return value, procedures cant return value 26. what is jdbc. ans: Connecting to DB from java program requires JDBC 27. what are types of drivers. type1, 2, 3, 4 29.how to collect requuirements form u r client. is not a job of a technical person. it is better for a BA to do it. 30. which process use in ur project. generally u can say: Project related process: Analysis, Design, Sign-off Documents, Implementation, Integration, Testing, UAT Work related process: Technical Design, Work Allocation, Code Review Checklist, unit Test Form will be prepared by the Project Lead and given to the developer. developer prepares the Unit Test Case Implements Code, Performs Test Submits Code through CVS/VSS Submits documents along with Release Checklist to the tester/leader. 31. what is deployment descriptor. ans: Which contains the infrnmation like which file to be used 40. what is platfrom independent ans: A language is said to be platform independent if it can be run on any machine with out modifying code 41. what is awt and swing. ans: AWT are heavy weight components and swings are light weight components 46. what is major concepts in oops. ans: Unknown action, polymorphism, encapsulation, inheritance 47.why u choose mvc-2 architecture. ans: In MVC-2 controller is servlet rather than JSP which makes it efficient 48. what is implicit object. ans: Implicit objects are a set of Java objects that the JSP Container makes available to developers in each page 49.how does implicit objects in jsp ans: out, page, session, request, response, application, page context, config