Java Novice Faq__java

Directory:
Q1.1 What is Java, Java2, JDK. What happened to the 1.3, 1.4 version number behind the JDK.
Q1.2 what is jre/j2re.
Q1.3 Learn what Java tools are better.
Q1.4 Learn what good reference books Java has.
Q1.5 Java and C + + which is better.
Q1.6 what is j2se/j2ee/j2me.
Q2.1 I wrote the first Java program, how it should be compiled/run.
Q2.2 I did what you said, but what happened? "' Javac ' is not an internal or external command, it is not a running program or a batch file. "。
Q2.3 environment variable how to set.
Q2.4 I show what "unreported exception java.io.IOException;" When I javac Xxx.java.
Q2.5 Javac Xxx.java successfully passed, but the Java XXX show what "noclassdeffounderror".
Q2.6 I show "Exception in Thread" main "Java.lang.NoSuchMethodError:main" when I am in Java XXX.
Q2.7 displays "Exception in thread" main "java.lang.NullPointerException" when Java XXX.
What is the meaning of Q2.8 package. How to use.
Q2.9 I didn't declare any package what would happen.
Q2.10 How to use other classes in a class.
Q2.11 I used the package to show "Noclassdeffounderror", but I took all the package out of the normal operation.
Q2.12 I want to compile Java into EXE file, how to do.
What does it mean to Q2.13 what "deprecated API" I encountered at compile time?
Q3.1 How do I add a startup parameter to a Java program, just like dir/p/w.
Q3.2 How do I enter a int/double/string from the keyboard.
Q3.3 How do I output a int/double/string.
Q3.4 I've found that some books are typed directly with system.in, much simpler than you.
Q3.5 How do I enter a int/double/string from a file.
Q3.6 I want to read and write to the specified location of the file, what to do.
Q3.7 How to judge that the document to be read has come to an end.
Q4.1 Java inside how to define macros.
Q4.2 Java cannot use Const.
Q4.3 Java inside also can not use Goto.
Q4.4 Java inside can overload operator.
Q4.5 I have a new object, but I can't delete it.
Q4.6 I want to know why the main method must be declared as public static. Why you cannot call a non-static member in the Main method.
What is the difference between Q4.7 throw and throws?
Q4.8 what is an exception.
Q4.9 final and finally what is different.
What is the difference between Q5.1 extends and implements?
Q5.2 Java How to implement multiple inheritance.
Q5.3 abstract is what.
What is the difference between Q5.4 public,protected,private.
What is the difference between Q5.5 override and overload?
Q5.6 I inherited a method, but now I want to invoke the method defined in the parent class.
Q5.7 I want to invoke the constructor method of the parent class in the constructor method of the subclass.
Q5.8 I've defined several constructs in the same class and want to call another in one of the constructor methods.
Q5.9 I didn't define the construction method.
Q5.10 I called the parameterless construction method failed.
Q5.11 How do I define a destructor similar to C + + (destructor).
Q5.12 I want to convert a parent class object to a subclass object.
Q5.13 In fact, I'm not sure if A is a B instance, can be dealt with in a separate case.
Q5.14 I modified the value of an object in the method, but after exiting the method I found that the object's value did not change.
Q6.1 Java can dynamically allocate arrays.
Q6.2 How do I know the length of the array.
Q6.3 I also want to have the length of the array automatically changed to add/remove elements.
Q6.4 What is a linked list. Why should there be ArrayList and linkedlist two kinds of list.
Q6.5 I want to use the queue/stack.
Q6.6 I hope not to have duplicate elements.
Q6.7 I want to traverse the collection/map.
Q6.8 I also want to be able to sort.
Q6.9 but I want to sort the array.
Q6.10 I want to sort by different ways.
What's the use of Q6.11 map?
The Q6.12 set method is fine, but the Get method returns object.
What is the difference between Q6.13 ArrayList and vector? What is the difference between HashMap and Hashtable.
Q6.14 I'm going to get a random number.
Q6.15 me to compare two string is always false, but they are obviously "abc".
Q6.16 I want to modify a string but the edit method is not found in the string class.
Q6.17 I want to deal with date/time.
First, prepare the article
Q1.1 What is Java, Java2, JDK. What happened to the 1.3, 1.4 version number behind the JDK.
A: Java is a generic, concurrent, strongly typed, object-oriented programming language (excerpted from the second edition of the Java specification). The JDK is a free Java development kit that is distributed by Sun, and is officially named J2SDK (JAVA2 Software Develop kit). Includes a basic Java toolkit and a standard class library. Until now (July 2003), Java has 3 major versions, namely 1.0,1.1,2.0;JDK 1.0, 1.1,1.2,1.3,1.4 five versions. From JDK1.2, Sun felt that Java changes were large enough to raise the Java language version number to 2.0. The different JDK is mainly provided by different class libraries. As a study you can download the latest JDK1.4.2. Real development should consider forward compatibility, such as 1.3. Download please go to http://java.sun.com. JDK1.5 is expected to launch in 2004, which will include a number of new features.
Q1.2 what is jre/j2re.
A: J2re is JAVA2 Runtime environment, the Java Runtime Environment, sometimes referred to as JRE. If you only need to run a Java program or applet, download and install it. If you want to develop your own Java software, please download the JDK. The JRE is included with the JDK. Note Since Microsoft's support for Java is incomplete, do not run the Applet with a virtual machine from IE, make sure
Install a JRE or JDK.
Q1.3 Learn what Java tools are better.
A: The authors recommend that you use the jdk+ text editor first, which helps you understand the following basic concepts: Path,classpath,package and familiarity with basic commands: Javac and Java. and download the same API help as your JDK version. If you are unsure of the use of a class or function, consult the API first instead of posting for help. When you are familiar with Java, you can consider starting with an IDE. The author recommends eclipse, download URL http://www.eclipse.org. Because the Eclispe is free, plug-in. The main disadvantage of ECLISPE is the lack of a visual desktop development tool, and fortunately IBM has donated some of the code to the Eclipse Organization in November 2003, which can be expected to be quickly remedied. In any case, do not use Microsoft's vj++. It is well known that Microsoft has never seriously supported
Java. Last but not least, have a good reference book, and English to pass.
Q1.4 Learn what good reference books Java has.
A: The author first recommended thinking in Java, Chinese name "Java programming thought", with Chinese version. The current version is the third edition. The English version can be downloaded for free in http://64.78.49.204. The first chapter of the book introduced a lot of object-oriented programming ideas, as a novice should read carefully. In addition, the books O ' relly and Wrox publishers are good. The author does not like the books of the mainland authors. Maybe you think English is too difficult, but most of the information on the Internet is in English. In addition, you need to check the API frequently, which is also in English.
Q1.5 Java and C + + which is better.
A: This question is a very inappropriate question. You should ask: Java and C + + which is more suitable for my project. Java is as obvious as its pros and cons. Cross-platform is the main advantage of Java, but the cost is the decline in the speed of operation. VC and Windows platforms are well integrated and fast enough, but can only be limited to Windows platforms. Java is faster than C + +, and developers don't run into a lot of error-prone features. But VB programmers even need to assemble the module on it.
Q1.6 what is j2se/j2ee/j2me.
A: J2SE is the general Java. J2ME is for embedded devices, such as Java-enabled handsets, and it has its own JRE and SDK. Java EE is a set of specifications and class libraries for enterprise program development that uses the J2SE JRE.

Ii. Order Articles
Q2.1 I wrote the first Java program, how it should be compiled/run.
A: First, please save the program as a Xxx.java file, and note that you may need to modify the file suffix name. Then in the DOS window using the javac Xx.java command, you will find that the directory more than a Xxx.class file, and then use the Java XXX command, your Java program began to run.
Q2.2 I did what you said, but what happened? "' Javac ' is not an internal or external command, it is not a running program or a batch file. "。
A: You're having a path problem. The operating system searches for javac.exe within a certain range (path), but cannot be found. Please edit your operating system environment variable, add a Java_home variable, set the installation directory for your JDK, edit the path variable, and add a%java_home%/bin. Then save and open a new DOS window, and you can use the Javac and Java commands.
Q2.3 environment variable how to set.
A: Please consult the people around you.
Q2.4 I show what "unreported exception java.io.IOException;" When I javac Xxx.java.
A: See Q4.8 to learn about the exception mechanism in Java.
Q2.5 Javac Xxx.java successfully passed, but the Java XXX show what "noclassdeffounderror".
Answer: 1. You're having a classpath problem. The Java command searches within a certain range (Classpath) for a class file that you use directly or indirectly, but cannot be found.
First make sure you are not wrong to knock into Java Xxx.class, second, check your CLASSPATH environment variables, you do not need to set the variable, but if you set the variable and do not contain the. (Represents the current directory), you will encounter this problem. Please add an item to your CLASSPATH environment variable. Or simply erase the variable.
2. If you use a standard package that is not a JDK, such as a javax.servlet.* package, you will also experience this problem, please add the appropriate jar file to the classpath.
3. If you have defined package in the Java source file, see Q2.11.
Q2.6 I show "Exception in Thread" main "Java.lang.NoSuchMethodError:main" when I am in Java XXX.
A: First of all, in your program each Java file has and can only have a public class, the class name must be the same as the file name of the exact same case. Second, there is and only one public static void main (string[] args method in the class you want to run, and this is your main program.
Q2.7 displays "Exception in thread" main "java.lang.NullPointerException" when Java XXX.
A: In a program where you try to invoke a method on an object variable that is NULL, check your program to make sure that your object is properly initialized.
See Q4.8 for exception mechanisms in Java.
What is the meaning of Q2.8 package. How to use.
A: In order to uniquely identify each class and group, Java uses the concept of package. Each class has a full name, for example, the full name of string is java.lang.String, where Java.lang is the package name and string is the short name. According to the Java naming convention, the package name is all lowercase, and the first letter of the class name is uppercase. So, if you define a class string of the same name yourself, you can put it in mypackage by using the full name MyPackage. String and java.lang.String to differentiate between these two classes. At the same time, placing logically related classes in the same package can make the program structure clearer. In order to define the package, all you have to do is add a line "package MyPackage" to the beginning of the Java file. Note that packages are not nested or contain relationships, mypackage packages and mypackage.mysubpackage packages are and
A column of two packages (although developers may imply inclusion of relationships).
Q2.9 I didn't declare any package what would happen.
A: Your class is considered to be placed in the default package. The full name and the short name are consistent.
Q2.10 How to use other classes in a class.
A: If you use a Java.lang package or a class in the default package, you don't have to do anything. If your class is in a MyPackage package and you want to invoke other classes in the same package, you don't have to do anything. If you use a class in another package, after the package declaration, use the import Otherpackage1 before the class declaration. Class1; or import therpackage2.*; Here. * Represents the introduction of all the classes in this package. Then you can use the short name of the other class in the program. If there is a duplicate name conflict between the short names, you must use the full name to differentiate.
Note that when using classes in other packages, you can use only public classes and interfaces, see Q5.4.
Q2.11 I used the package to show "Noclassdeffounderror", but I removed all package
Time to run normally.
A: Organize your Java files by package name. For example, your working directory is/work, and your class is package1. Class1, then store it as/work/package1
/class1.java. If the package is not declared, put it directly under/work. Execute Javac Package1/class1.java under/work, and then execute Java package1.class1, and you'll find everything's fine. Also, if you have more than the number of classes you need to use to organize the package, you can consider starting to use
Ide.
Q2.12 I want to compile Java into EXE file, how to do.
A: JDK can only compile Java source files into class files. A class file is a Cross-platform byte code that must be run by a platform-dependent JRE. Java in order to achieve the cross
Platform Nature. Some development tools can compile Java files into EXE files. The author rejects this practice because it cancels the cross-platform nature. If you are sure that your software is running on only the Windows platform, you can consider using c++/c# to program.
What does it mean to Q2.13 what "deprecated API" I encountered at compile time?
A: The so-called deprecated refers to the outdated, but still retained for the sake of forward compatibility. These methods may cancel support at a later time. You should switch to a newer method. The API will show you how you should replace it.
Third, I/O article
Q3.1 How do I add a startup parameter to a Java program, just like dir/p/w.
A: Remember the public static void main (string[] args). The args here is your startup parameter. At runtime you enter the Java package1.class1 arg1 Arg2,args there will be two strings, the first is Arg1, and the second is arg2.
Q3.2 How do I enter a int/double/string from the keyboard.
A: Java I/O operations are more complex than C + +. If you want to enter from the keyboard, the sample code is as follows:
BufferedReader cin = new BufferedReader (new InputStreamReader (system.in));
String s = cin.readline ();
So you get a string, if you need a number to use it again:
int n = integer.parseint (s); or double D = double.parsedouble (s) to convert the string "534" to an int or double.
Q3.3 How do I output a int/double/string.
Answer: Use SYSTEM.OUT.PRINTLN (n) or System.out.println ("Hello") and so on.
Q3.4 I've found that some books are typed directly with system.in, much simpler than you.
A: Java uses Unicode, which is double-byte. The system.in is a single byte stream. If you want to enter double-byte text such as Chinese, please use the author's approach.
Q3.5 How do I input/output a int/double/string from a file.
A: Similar to the input from the keyboard, just change to bufferedreader fin = new BufferedReader (New FileReader ("myFileName")); PrintWriter fout = new PrintWriter ("myFileName"), and if you have not yet downloaded the API, start downloading and reading the contents of the Java.io package.
Q3.6 I want to read and write to the specified location of the file, what to do.
Answer: Java.io.RandomAccessFile can meet your needs.
Q3.7 How to judge that the document to be read has come to an end.
A: In reader's Read method, it is clear that return-1 indicates the end of the stream.
Iv. Key Words and articles
Q4.1 Java inside how to define macros.
A: Java does not support macros because macro substitution does not guarantee type safety. If you need to define a constant, you can define it as a static inal member of a class. See Q4.2 and Q4.6.
Q4.2 Java cannot use Const.
Answer: You can use the final keyword. For example final int m = 9. A variable declared final cannot be assigned again. The only exception is the so-called blank final, as shown in the following example:
public class MyClass1 {
Private final int a = 3;
private final int b; Blank final
Public MyClass1 () {
A = 5; Not valid, final variable cannot be assigned again.
b = 4; Legally, this is the first time that B is assigned a value.
b = 6; Illegal, b cannot be assigned again.
}
}
Final can also be used to declare a method or class, and a method or class declared as final cannot be inherited. Note that const is a reserved word for Java to expand.
Q4.3 Java inside also can not use Goto.
A: Even in a process-oriented language, you can use no goto at all. Please check that your program flow is reasonable. If you need to jump out of a multi-layer loop quickly, Java enhances the break and continue functions (compared to C + +), supporting label.
For example:
Outer:
while (...)
{
Inner:
For (...)
{
... break inner; ...
... continue outer; ...
}
}
As with const, Goto is also a reserved word for Java to expand.
Q4.4 Java inside can overload operator.
Answer: No. The + number of the string is the only built-in overloaded operator. You can actually do this by defining interfaces and methods.
Now similar functions.
Q4.5 I have a new object, but I can't delete it.
A: Java has automatic memory recovery mechanism, that is, the so-called Garbarge Collection. You don't need to delete objects. You'll
Also do not worry about pointer errors, memory overflow.
Q4.6 I want to know why the main method must be declared as public static. Why you cannot call a non-static member in the Main method.
A: Declaring public is for this method to be called externally, see Q5.4 for details. Static is to associate a member variable/method with a class (class) rather than an instance (instance). You do not need to create an object to directly use the static members of this class, so you cannot call a static member in a static member because the latter is associated to an object instance (instance). Static members that call Class B in Class A can use B.staticmember. Note that the static member variables of a class are unique and are shared by all of these objects, and are especially cautious in multithreaded design. The static members of a class are initialized the first time the class is loaded by the JRE.
You can use a non-static member by using the following methods:
public class A
{
private void SomeMethod ()//non-static member
{}
public static void Main (String args)
{
A A = new A (); Create an object instance
A.somemethod (); Now you can use the non static method.
}
}
What is the difference between Q4.7 throw and throws?
A: Throws is used in a method declaration to declare which exceptions a method throws. The throw is actually performing an action that throws an exception in the method body. If you throw an exception in the method, but you do not declare it in the method declaration, the compiler will make an error. Note that the error and runtimeexception subclasses are exceptions and do not require a special declaration.
Q4.8 what is an exception.
A: The exception was first introduced in the ADA language to dynamically handle errors and recover in the program. You can intercept the underlying exception and handle it in the method, or you can throw it to a higher level module to handle it. You can also throw your own exception indicating that something abnormal has occurred. Common blocking processing code is as follows:
Try
{
...//The following is a code that may have an exception
...//exception is thrown by you or low-level APIs, execute process interrupt and turn to intercept code.
......
}
catch (Exception1 E)//If Exception1 is a subclass of Exception2 and needs to be handled specifically, it should be in the front
{
Intercepted by this segment when Exception1 occurs
}
catch (Exception2 e)
{
Intercepted by this segment when Exception2 occurs
}
Finally//This is optional
{
Executes this code regardless of whether the exception occurred
Even if a new exception,finally segment is thrown out of the catch segment, it is executed.
}
Q4.9 final and finally what is different.
A: final please see Q4.2. Finally for the exception mechanism, see Q4.8.
V. Object-oriented articles
What is the difference between Q5.1 extends and implements?
A: For class, extends is used to (single) Inherit a class, and implements is used to implement an interface (interface).
Interface is introduced to partially provide multiple inheritance functions. In interface, you simply declare the method header and leave the method body to the implemented class. The instances of the class that are implemented can be treated as interface instances. The relationship between interface can also be declared as extends (multiple inheritance). Note that a interface can extends multiple other interface.
Q5.2 Java How to implement multiple inheritance.
Answer: Java does not support explicit multiple inheritance. Because in an explicit, more inherited language such as C + +, a subclass is forced to declare the ancestor virtual base class constructor, which violates the object-oriented encapsulation principle. Java provides interface and implements keywords to partially implement multiple inheritance. See Q5.1.
Q5.3 abstract is what. A: A method declared abstract does not need to give the method body, leaving the subclass to implement it. And if there is an abstract method in a class, then the class must also be declared abstract. A class declared abstract cannot be instantiated, although it can define a constructor method for use by subclasses.
What is the difference between Q5.4 public,protected,private.
A: These keywords are used to declare the visibility of classes and members. Public members can be accessed by any class, protected members are restricted to themselves and subclass access, and private members are limited to their own access. Java also provides a fourth kind of default visibility, commonly referred to as package private, when there are no public,protected,private modifiers, members
is visible within the same package. A class can be decorated with public or by default.
What is the difference between Q5.5 override and overload?
A: Override refers to the inheritance of methods between the parent class and the subclass, which have the same name and parameter type. Overload refers to the relationship between different methods in the same class, which can be defined in subclasses as well as in the parent class, which have the same name and different parameter types.
Q5.6 I inherited a method, but now I want to invoke the method defined in the parent class.
A: Use Super.xxx () to invoke the parent class method in the subclass.
Q5.7 I want to invoke the constructor method of the parent class in the constructor method of the subclass.
A: Call Super (...) in the first line of the subclass construction method. Can.
Q5.8 I've defined several constructs in the same class and want to call another in one of the constructor methods.
A: Call this (...) in the first line of the constructor method.
Q5.9 I didn't define the construction method.
A: Automatically obtain a method of constructing without parameters.
Q5.10 I called the parameterless construction method failed.
A: If you define at least one construction method, there is no longer an automatically supplied parameterless construction method. You need to explicitly define an argument-free construction method. Another possibility is that your construction method or class is not public, see Q5.4 to understand the visibility in Java.
Q5.11 How do I define a destructor similar to C + + (destructor).
Answer: Provide a void Finalize () method. This method is called when the object is reclaimed by Garbarge collector. Note that it's really hard to tell when an object will be recycled. The author never felt the need to use the method.
Q5.12 I want to convert a parent class object to a subclass object.
A: Force type conversions. Such as
public void meth (a)
{
b b = (b) A;
}
If a is not actually an instance of B, it throws a classcastexception. So make sure that a is really an instance of B.
Q5.13 In fact, I'm not sure if A is a B instance, can be dealt with in a separate case.
Answer: You can use the instanceof operator. For example
if (a instanceof B)
{
b b = (b) A;
}
Else
{
...
}
Q5.14 I modified the value of an object in the method, but after exiting the method I found that the object's value did not change.
A: It is very likely that you have assigned an incoming parameter to a new object, such as the following code that causes this error:
The public void Fun1 (a)//a is a local parameter that points to an external object.
{
A = new A (); A points to a new object, decoupling from the external object. If you want a as an outgoing variable, do not write this sentence.
A.setattr (attr)//modified the value of the new object, the external object has not been modified.
}
This is also the case with the base type. For example:
public void fun2 (int a)
{
A = 10;//only for this method, the outside variables will not change.
}
Six, Java.util article
Q6.1 Java can dynamically allocate arrays.
Answer: Yes. for example int n = 3; language[] mylanguages = new Language[n];
Q6.2 How do I know the length of the array.
Answer: Use the length attribute. As in the example above, the Mylanguages.length is 3.
Q6.3 I also want to have the length of the array automatically changed to add/remove elements.
Answer:--java.util.list interface with sequential table. You can choose to use ArrayList or LinkedList, the former is an array implementation, the latter is a linked list implementation. For example: List list = new ArrayList (); or the list list = new LinkedList ();.
Q6.4 What is a linked list. Why should there be ArrayList and linkedlist two kinds of list.
A: Please cram data structure.
Q6.5 I want to use the queue/stack.
Answer: Use Java.util.LinkedList.
Q6.6 I hope not to have duplicate elements.
Answer: Use the Set--java.util.set interface. For example: Set set = new HashSet ().
Q6.7 I want to traverse the collection/map.
Answer: Use Java.util.Iterator. See API.
Q6.8 I also want to be able to sort.
Answer: Use Java.util.TreeSet. For example: Set set = new TreeSet (). The elements that are put in are sorted automatically. You need to implement the comparable interface for the elements, and you may also need to provide the Equals () method, the CompareTo () method, the Hashcode () method.
Q6.9 but I want to sort the array.
A: The Java.util.Arrays class contains practical methods such as sort.
Q6.10 I want to sort by different ways.
A: Define a sort class that implements the interface comparator for each method and use it with arrays or treeset.
What's the use of Q6.11 map?
A: Store key-value keyword-value pairs, you can quickly access the corresponding value by keyword.
The Q6.12 set method is fine, but the Get method returns object.
A: Force types to be converted to the type you need. See Q5.12.
What is the difference between Q6.13 ArrayList and vector? What is the difference between HashMap and Hashtable.
A: ArrayList and HashMap are multi-threaded, and accessing the same ArrayList object in multiple threads may
Can cause conflicts and cause errors. Vector and Hashtable are multi-threaded and secure, even if the same vector object is accessed simultaneously in multiple threads
will not cause mistakes. It looks like we should use vectors and hashtable, but the vectors and Hashtable are really bad, so if you're not using multiple threads, you should use ArrayList and HashMap.
Q6.14 I'm going to get a random number.
Answer: Use the Java.util.Random class.
Q6.15 me to compare two string is always false, but they are obviously "abc".
A: Compare string must use Equals or Equalsignorecase method, do not use = =. = = Compares whether two references (variables) point to the same object, rather than comparing its contents.
Q6.16 I want to modify a string but the edit method is not found in the string class.
Answer: Use the StringBuffer class. String str = "..."; The string to be processed stringbuffer buffer = new StringBuffer (str); Initializes a stringbuffer buffer.append ("...") using the string; Call StringBuffer's associated API to edit the string str2 = Buffer.tostring (); Gets the edited string. In addition, if you need to connect multiple strings, try to avoid using the + number to connect directly, but instead use the
String buffer.append () method.
Q6.17 I want to deal with date/time.
Answer: Use the Java.util.Date class. You can use the Java.text.SimpleDateFormat class to convert between string and date.
SimpleDateFormat formatter = new SimpleDateFormat ("Yyyy-mm-dd HH:mm:ss"); Date format specified
Date = Formatter.parse ("2003-07-26 18:30:35"); Converts a string that matches a format to date
String s = formatter.format (date); Converts date to a format-compliant string
For more information about defining date formats, see the API.