Eleventh, short s1 = 1; S1 = S1 1; What is wrong? Short S1 = 1; S1 = 1; What is wrong? SHORT S1 = 1; S1 = S1 1; wrong, S1 is Short S1 1 is an INT type, which cannot be explicitly converted to a Short type. Can be modified to S1 = (Short) (S1 1). Short S1 = 1; S1 = 1 is correct.
Chapter 12, SLEEP () and WAIT () What is the difference? The favorite SLEEP () method of the thread is to stop the thread to stop a period of time. After the SLEEP time interval is full, the thread does not necessarily resume it immediately. This is because at that moment, other threads may be running and are not scheduled to give up execution, unless (a) "wake up" thread has higher priority (b) that is running because of other reasons. Wait () is a thread interaction if the thread issues a wait () call, the thread is suspended, the object is adjusted to the waiting state until it is awake or waiting time.
Thirteenth, there is a reserved word in Goto? Goto? Java in Java, and now not use in Java.
Fourteenth, there is a length () method for arrays? String has a length () method? The array does not have Length () method, with the properties of the length. String has a length () method.
Fifteenth, the difference between overload and override. OverLoaded method can change the type of return value? Override Overriding and overload overloading is a different manifestation of Java polymorphism. Overriding Overriding is a manifestation of polymorphism between parent class and subclasses, and overloading overloading is a manifestation in a class. If a method is defined in the subclass with the same name and parameters as its parent class, we say that this method is overriddled. When the subject of subclass uses this method, the definition of the subclass will be called. For it, the definition in the parent class is like "shielded". If a plurality of the same names are defined in a class, they or have different parameters or different parameter types, called overloading. The way OVERLOADED is to change the type of return value.
The elements in the SET are not repeated, then what method is used to distinguish whether it is repeated? Is it == or equals ()? What is the difference between them? The elements in Set cannot be repeated, then use Iterator () Method to distinguish whether it is repeated. Equals () is the judgment of whether two SETs are equal. Equals () and == Method Decide whether the reference value points to the same object Equals () override in the class, which is the true value of the content and type of two separate objects.
Seventest, give me a Runtime Exception you most often. ArithmeticException, ArrayStoreException, BufferOverflowException, BufferUnderflowException, CannotRedoException, CannotUndoException, ClassCastException, CMMException, ConcurrentModificationException, DOMException, EmptyStackException, IllegalArgumentException, IllegalMonitorStateException, IllegalPathStateException, IllegalStateException, ImagingOpException, IndexOutOfBoundsException, MissingResourceException, NegativeArraySizeException, NoSuchElementException, NullPointerException, ProfileDataException, ProviderException, RasterFORMatException, SecurityException, What is the difference between systemException, undeclaredthrowableException, unmodifiablesetexception, unsupportedopertyXception, error, and exception? Error indicates a serious problem that recovery is not impossible but very difficult. For example, memory is overflow. It is impossible to expect the program to handle this situation. Exception represents a design or implementation problem. That is, it means that if the program runs normally, it will never occur.
Ninth, List, SET, MAP inherit from the Collection interface? List, SET is
MAP is not
Ten, what is the difference between Abstract Class and Interface? The presence of a declaration method does not implement its class called abstract class, which is used to create a class that reflects certain basic behavior and for this class. Declaration methods, but cannot implement this class in this class. An instance of the Abstract class cannot be created. However, you can create a variable, which is an abstract class and let it point to an instance of the specific subclass. There is no abstract constructor or abstract static method. Subclasses of the Abstract class provide implementation for all abstraction methods in their parent class, otherwise they are also abstract classes. Instead, this method is implemented in the subclass. Other classes that know their behavior can be implemented in the class. Interface is a variant of an abstract class. In the interface, all methods are abstract. Multiple inheritance can be obtained by achieving such an interface. All methods in the interface are abstract, no probabilities. The interface can only define the Static Final member variable. The implementation of the interface is similar to the subclass, except that the implementation class cannot inherit behavior from the interface definition. When the class implements a special interface, it defines the method of all such an interface (that is, the program body is given). Then, it can call the interface to call the interface on any object of the class of the interface. Because there is an abstract class, it allows the interface name as the type of reference variable. The usual dynamic cable will take effect. The reference can be converted to an interface type or a slave interface type, and the InstanceOf operator can be used to determine whether an object is implemented.
Second Eleventh, whether the method of Abstract can be static at the same time, whether it can be native at the same time, is it simultaneous to be synchronized?
Typographic Abstract classes can be implemented, whether abstract classes can inherit physical classes, but provided that the entity class must have a clear constructor. Second, the thirteenth, starting a thread is Run () or start ()? Starting a thread is calling the start () method so that the virtual processor represented by the thread is running, which means it can be scheduled by JVM and carried out. This doesn't mean that the thread will run immediately. The Run () method can generate a flag that must be exited to stop a thread.
Chapter 24. Whether the constructor can be can't be inherited by the Override® constructor constructor, so Overriding cannot be overridden, but Overloading can be overloaded.
The second fifteenth, can you inherit the String class? The String class is not inherited by Final.
Twentydays, when a thread enters an SYNCHRONIZED method of an object, other methods of other threads can enter this object? No, one Synchronized method of an object can only be accessed by one thread.
27. There is a Return statement in Try {}, so the code in finally {} after this try is not executed, when is executed, before returnome or after it will be executed, in Return Pre-execution.
Chapter 28. Program: Two multiplier with the most efficient method is equal to a few? There are C backgrounds, special programs like to ask this question.
2 << 3
The second nine, the two object values are the same (x.equals (y) == True, but there can be different Hash Code, this sentence is wrong? No pair, there is the same haveh code.
Thirty, when an object is passed as a parameter to a method, this method can change the properties of this object, and can return the resulting result, then here is the value delivery or reference delivery? Is a value transfer. The Java programming language is only passed by the value. When an object instance is transmitted to the method as a parameter, the value of the parameter is a reference to the object. The content of the object can be changed in the called method, but the reference to the object will never change.
Thirty-first, whether SWTICH can act on Byte, whether it can act on long, can it work on String? Switch (expr1), expr1 is an integer expression. Therefore, the parameters passing to the Switch and the CASE statement should be int, short, char or byte. Long, String can not work on SWTICH.
Article 32. Program: Write a Singleton. The main role of the Singleton mode is to ensure that only one example exists in a Java application. General Singleton mode usually has several forms: the first form: Define a class, its constructor is private, it has a state of static private, instantoneation, and GetInstance through a public Method Get a reference to it, then calls it. Public class singleton {private singleton ()} // define one instance in itself, is it very strange? / Note This is private only for internal calling private static singleton instance = new singleton (); // here provides a static method for external access to this Class, you can directly access public static singleton getInstance () {return instance;}} The second form: public class singleleton {private static singleleton instance = null; public static synchronized singleton getInstance () {// This method is improved above, do not have to generate an object each time, only the first time // Generate an instance and improve efficiency! IF (instance == null) instance = new start;}} Other forms: Define a class, its constructor is private, all methods are static. It is generally considered that the first form should be more secure to inherit from the Dictionary class, while HashMap is an implementation of Map Interface introduced by Java1.2.
Hashmap allows null as a key or value of Entry, and hashtable is not allowed
Also, Hashmap removes the HashTable's Contains method, changed to ContainsValue and Containskey. Because the Contains method is easy to cause misunderstandings.
The biggest difference is that the HashTable method is Synchronize, and HashMap is not, when accessed HashTable, does not need to be synchronized for its method, and HashMap must provide external synchronization.
Hash / rehash algorithms used by HashTable and Hashmap are probably, so performance will not have a big difference.
