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Java keywords

忆风 — Tue, 16 Dec 2008 16:09:00 GMT

In the Java programming language, a keyword is one of 50 reserved words which have a predefined meaning in the language; because of this, programmers cannot use keywords as names for variables, methods, classes, or as any other identifier.^[1] Due to their special functions in the language, most integrated development environments for Java use syntax highlighting to display keywords in a different color for easy identification.^{[citation needed]}

The following is a list of the keywords in Java, along with brief descriptions of their functions:^[1]

abstract: The abstract keyword is used to declare a class or method to be abstract. An abstract method has no implementation; all classes containing abstract methods must themselves be abstract, although not all abstract classes have abstract methods. Objects of a class which is abstract cannot be instantiated, but can be extended by other classes. All subclasses of an abstract class must either provide implementations for all abstract methods, or must also be abstract.^[2]

assert: The assert keyword, which was added in J2SE 1.4,^[1] is used to make an assertion—a statement which the programmer believes is always true at that point in the program. If assertions are enabled when the program is run and it turns out that an assertion is false, an AssertionError is thrown and the program terminates. This keyword is inteded to aid in debugging.^[3]

boolean: The boolean keyword is used to declare a field that can store a boolean value; that is, either true or false.^[4] This keyword is also used to declare that a method returns a value of type boolean.^[5]

break: Used to resume program execution at the statement immediately following the current enclosing block or statement. If followed by a label, the program resumes execution at the statement immediately following the enclosing labeled statement or block.

byte: The byte keyword is used to declare a field that can store an 8-bit signed two's complement integer.^[4] This keyword is also used to declare that a method returns a value of type byte.^[5]

case: The case keyword is used to create individual cases in a switch statement; see switch.^[6]

catch: Defines an exception handler—a group of statements that are executed if an exception is thrown in the block defined by a preceding try keyword. The code is executed only if the class of the thrown exception is assignment compatible with the exception class declared by the catch clause.

char: The char keyword is used to declare a field that can store a 16-bit Unicode character.^[4] This keyword is also used to declare that a method returns a value of type char.^[5]

class: A type that defines the implementation of a particular kind of object. A class definition defines instance and class fields, methods, and inner classes as well as specifying the interfaces the class implements and the immediate superclass of the class. If the superclass is not explicitly specified, the superclass is implicitly Object.

const: Although reserved as a keyword in Java, const is not used and has no function.^[1]

continue: Used to resume program execution at the end of the current loop body. If followed by a label, continue resumes execution at the end of the enclosing labeled loop body.

default: The default can optionally be used in a switch statement to label a block of statements to be executed if no case matches the specified value; see switch.^[6]

do: The do keyword is used in conjunction with while to create a do-while loop, which executes a block of statements assocated with the loop and then tests a boolean expression assocated with the while. If the expression evaluates to true, the block is executed again; this continues until the expression evaluates to false.^[7]

double: The double keyword is used to declare a field that can hold a 64-bit double precision IEEE 754 floating-point number.^[4] This keyword is also used to declare that a method returns a value of type double.^[5]

else: The else keyword is used in conjunction with if to create an if-else statement, which tests a boolean expression; if the expression evaluates to true, the block of statements associated with the if are evaluated; if it evaluates to false, the block of statements associated with the else are evaluated.^[8]

enum (as of J2SE 5.0): A Java keyword used to declare an enumerated type. Enumerations extend the base class Enum.

extends: Used in a class declaration to specify the superclass; used in an interface declaration to specify one or more superinterfaces. Class X extends class Y to add functionality, either by adding fields or methods to class Y, or by overriding methods of class Y. An interface Z extends one or more interfaces by adding methods. Class X is said to be a subclass of class Y; Interface Z is said to be a subinterface of the interfaces it extends.; Also used to specify an upper bound on a type parameter in Generics.

final: Define an entity once that cannot be changed nor derived from later. More specifically: a final class cannot be subclassed, a final method cannot be overridden, and a final variable can occur at most once as a left-hand expression. All methods in a final class are implicitly final.

finally: Used to define a block of statements for a block defined previously by the try keyword. The finally block is executed after execution exits the try block and any associated catch clauses regardless of whether an exception was thrown or caught, or execution left method in the middle of the try or catch blocks using the return keyword.

float: The float keyword is used to declare a field that can hold a 32-bit single precision IEEE 754 floating-point number.^[4] This keyword is also used to declare that a method returns a value of type float.^[5]

for: The for keyword is used to create a for loop, which specifies a variable initialization, a boolean expression, and an incrementation. The variable initialization is performed first, and then the boolean expression is evaluated. If the expression evaluates to true, the block of statements assocated with the loop are executed, and then the incrementation is performed. The boolean expression is then evaluated again; this continues until the expression evaluates to false.^[9]

As of J2SE 5.0,^{[citation needed]} the for keyword can also be used to create a so-called "enhanced for loop", which specifies an array or Iterable object; each iteration of the loop executes the associated block of statements using a different element in the array or Iterable.^[9]

goto: Although reserved as a keyword in Java, goto is not used and has no function.^[1]

if: The if keyword is used to create an if statement, which tests a boolean expression; if the expression evaluates to true, the block of statements associated with the if statement is executed. This keyword can also be used to create an if-else statement; see else.^[8]

implements: Included in a class declaration to specify one or more interfaces that are implemented by the current class. A class inherits the types and abstract methods declared by the interfaces.

import: Used at the beginning of a source file to specify classes or entire Java packages to be referred to later without including their package names in the reference. Since J2SE 5.0, import statements can import static members of a class.

instanceof: A binary operator that takes an object reference as its first operand and a class or interface as its second operand and produces a boolean result. The instanceof operator evaluates to true if and only if the runtime type of the object is assignment compatible with the class or interface.

int: The int keyword is used to declare a field that can hold a 32-bit signed two's complement integer.^[4] This keyword is also used to declare that a method returns a value of type int.^[5]

interface: Used to declare a special type of class that only contains abstract methods, constant (static final) fields and static interfaces. It can later be implemented by classes that declare the interface with the implements keyword.

long: The long keyword is used to declare a field that can hold a 64-bit signed two's complement integer.^[4] This keyword is also used to declare that a method returns a value of type long.^[5]

native: Used in method declarations to specify that the method is not implemented in the same Java source file, but rather in another language.

new: Used to create an instance of a class or array.

package: A group of types. Packages are declared with the package keyword.

private: The private keyword is used in the declaration of a method, field, or inner class; private members can only be accessed by other members of their own class.^[10]

protected: The protected keyword is used in the declaration of a method, field, or inner class; protected members can only be accessed by members of ther own class or that class's subclasses.^[10]

public: The public keyword is used in the declaration of a class, method, or field; public classes, methods, and fields can be accessed by the members of any class.^[10]

return: Used to finish the execution of a method. It can be followed by a value required by the method definition that is returned to the caller.

short: The short keyword is used to declare a field that can hold a 16-bit signed two's complement integer.^[4] This keyword is also used to declare that a method returns a value of type short.^[5]

static: Used to declare a field, method or inner class as a class field. Classes maintain one copy of class fields regardless of how many instances exist of that class. static also is used to define a method as a class method. Class methods are bound to the class instead of to a specific instance, and can only operate on class fields. (Classes and interfaces declared as static members of another class or interface are actually top-level classes and are not inner classes.)

strictfp (as of J2SE 1.2): A Java keyword used to restrict the precision and rounding of floating point calculations to ensure portability.

super: Used to access members of a class inherited by the class in which it appears. Allows a subclass to access overridden methods and hidden members of its superclass. The super keyword is also used to forward a call from a constructor to a constructor in the superclass.; Also used to specify a lower bound on a type parameter in Generics.

switch: The switch keyword is used in conjunction with case and default to create a switch statement, which evaluates a variable, matches its value to a specific case, and executes the block of statements associated with that case. If no case matches the value, the optional block labelled by default is executed, if included.^[6]

synchronized: Used in the declaration of a method or code block to acquire the mutex lock for an object while the current thread executes the code. For static methods, the object locked is the class' Class. Guarantees that at most one thread at a time operating on the same object executes that code. The mutex lock is automatically released when execution exits the synchronized code. Fields, classes and interfaces cannot be declared as synchronized.

this: Used to represent an instance of the class in which it appears. this can be used to access class members and as a reference to the current instance. The this keyword is also used to forward a call from one constructor in a class to another constructor in the same class.

throw: Causes the declared exception instance to be thrown. This causes execution to continue with the first enclosing exception handler declared by the catch keyword to handle an assignment compatible exception type. If no such exception handler is found in the current method, then the method returns and the process is repeated in the calling method. If no exception handler is found in any method call on the stack, then the exception is passed to the thread's uncaught exception handler.

throws: Used in method declarations to specify which exceptions are not handled within the method but rather passed to the next higher level of the program. All uncaught exceptions in a method that are not instances of RuntimeException must be declared using the throws keyword.

transient: Declares that an instance field is not part of the default serialized form of an object. When an object is serialized, only the values of its non-transient instance fields are included in the default serial representation. When an object is deserialized, transient fields are initialized only to their default value.

try: Defines a block of statements that have exception handling. If an exception is thrown inside the try block, an optional catch block can handle declared exception types. Also, an optional finally block can be declared that will be executed when execution exits the try block and catch clauses, regardless of whether an exception is thrown or not. A try block must have at least one catch clause or a finally block.

void: The void keyword is used to declare that a method does not return any value.^[5]

This keyword is also used as a nonfunctional statement.^{[citation needed]}

volatile: Used in field declarations to specify that the variable is modified asynchronously by concurrently running threads. Methods, classes and interfaces thus cannot be declared volatile.

while: The while keyword is used to create a while loop, which tests a boolean expression and executes the block of statements associated with the loop if the expression evaluates to true; this continues until the expression evaluates to false. This keyword can also be used to create a do-while loop; see do.^[7]

忆风 2008-12-17 00:09 发表评论

使用Java Generics简化数据库存取类DAO开发

忆风 — Sat, 23 Aug 2008 21:39:00 GMT

本文介绍怎么样使用Java Generics来简化数据库存取类DAO的开发，并给出了一个完整的例子，大家可以把代码用在实际的开发应用中。

在系统开发中，为了降低耦合性，一般把跟数据库有关的操作封装在被叫做DAO（Dao Access Object）的类里。一般DAO的封装有以下几个原则：

1、一个表操作封装成一个DAO。例如：操作表User的DAO封装为UserDao.java，操作UserRole的DAO封装为UserRole.java等

2、具有良好的接口定义。为了使用简单以及不至于被错误调用，DAO接口必须有具体的型定义。例如：可以直接
User user = userDao.get(userId);取得指定userId的User，而不必进行强制类型转换 User user = (User)userDao.get(userId); [X]
为了避免误操作，userDao.delete(UserRole);应该在编译期就报错。

为了对DAO进行良好的封装，JDK1.5之前，DAO开发是一个大量重复代码的体力过程；JDK1.5开始引入了Generics概念，我们可以利用JDK1.5的Generics特性大大简化DAO开发。

Generics概念

Java的Generics就是Java的泛型，类似于C++的模板概念，Generics介绍起来比较复杂，简单说来，泛型从语言特性的底层上支持，可以让你为某些相似功能定义一个具有统一接口的通用模板，该模板可以接收任意类型并应用到类成员变量的类型，方法的参数类型，方法的返回值类型等；并可以在编译期支持类型的安全转换。举例说：
java.util.List是一个泛型，一个定义为List userList;的变量userList，可以通过User user = userList.get(0);便可直接得到User，不用经过类型的强制转换了。

Generics更详细的介绍请参考相关文章，这里不做详述。

下面举例我们给出2种DAO的封装方法，一种是不使用Java Generics特性的一般封装方法；一种是使用泛型的封装方法。通过比较，我们会发现Java的Generics特性到底为我们简化了哪些工作。

使用Java Generics特性封装DAO

文件名	说明
IDao.java	dao的顶层抽象接口
AbstractDao.java	IDao的实现类，实现了get/update/delete等基本操作
User.java	USER表对应的entity定义
IUserDao.java	USER表操作DAO接口定义
UserDao.java	USER表操作DAO实现
HelloGenerics.java	测试类

IDao.java

// super dao interface
public interface IDao extends Serializable> {
    public T get(PK pk);
    public List getAll();
    public PK save(T entity);
    public void update(T entity);
    public void saveOrUpdate(T entity);
    public void delete(T entity);
}

AbstractDao.java

//super abstract dao class
public abstract class AbstractDao extends Serializable>implements IDao {
    private Class clz;

    public AbstractDao(Class clz) {
        this.clz = clz;
    }

    public T get(PK pk) {
        return (T)getSession().get(clz, pk);
    }
    public List getAll() {
        return getSession().createCriteria(clz).list();
    }
    public PK save(T entity) {
        return (PK)getSession().save(entity);
    }
    public void update(T entity) {
        getSession().update(entity);
    }
    public void saveOrUpdate(T entity) {
        getSession().saveOrUpdate(entity);
    }
    public void delete(T entity) {
        getSession().delete(entity);
    }

    private Session session;
    protected Session getSession() {
        //wrap session in a class such as HibernateUtils, then you can use  HibernateUtils.getCurrentSession() for getting a session.
        return session;
    }
}

User.java

//persist entity
public class User {
    private String id;
    private String name;

    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

    //... setter/getter HERE
}

IUserDao.java

//User Dao interface
public interface IUserDao extends IDao {
// all are empty
}

UserDao.java

//UserDao Implementation
public class UserDao extends AbstractDao implements IUserDao {
    public UserDao() {
        super(User.class);
    }
    // or use the following constructor
    //public UserDao(Class type) {
    //    super(type);
    //}
}

HelloGenerics.java

public class HelloGenerics {

    public static void main(String[] args) {
        IUserDao userDao = new UserDao();
        User user = userDao.get("1");
        if (user != null) {
            System.out.println(user.getName());
        }

        List userList = userDao.getAll();
        for (User u:userList) {
            System.out.println(user.getName());
        }
    }
}

我们可以看到IUserDao.java与UserDao.java非常简单。

不使用Java Generics特性DAO的封装

同样，我们给出不使用Java Generics特性DAO的封装。该封装也是一个具有良好结构性的封装。

文件列表2：

文件名	说明
DaoUtils.java	dao通用工具类，提供get/update/delete等基本操作
User.java	USER表对应的entity定义
IUserDao.java	USER表操作DAO接口定义
UserDao.java	USER表操作DAO实现
HelloGenerics.java	测试类

DaoUtils.java

public class DaoUtils {

    public static Object get(Class clz, Serializable pk) {
        return getSession().get(clz, pk);
    }
    public static List getAll(Class clz) {
        return getSession().createCriteria(clz).list();
    }
    public static Serializable save(Object entity) {
        return getSession().save(entity);
    }
    public static void update(Object entity) {
        getSession().update(entity);
    }
    public static void saveOrUpdate(Object entity) {
        getSession().saveOrUpdate(entity);
    }
    public static void delete(Object entity) {
        getSession().delete(entity);
    }

    private static Session session;
    protected static Session getSession() {
        //wrap session in a class such as HibernateUtils, then you can use  HibernateUtils.getCurrentSession() for getting a session.
        return session;
    }
}

User.java

IUserDao.java

//User Dao interface
public interface IUserDao {
    public User get(String pk);
    public List getAll();
    public String save(User entity);
    public void update(User entity);
    public void saveOrUpdate(User entity);
    public void delete(User entity);
}

UserDao.java

//UserDao Implementation
public class UserDao implements IUserDao {

    public User get(String pk) {
        return (User)DaoUtils.get(User.class, pk);
    }

    public List getAll() {
        return DaoUtils.getAll(User.class);
    }

    public String save(User entity) {
        return (String)DaoUtils.save(entity);
    }

    public void update(User entity) {
        DaoUtils.update(entity);
    }

    public void saveOrUpdate(User entity) {
        DaoUtils.saveOrUpdate(entity);
    }

    public void delete(User entity) {
        DaoUtils.delete(entity);
    }
}

HelloGenerics.java

我们注意到IUserDao.java与UserDao.java的实现比使用Java Generics特性的情况要复杂多了。

忆风 2008-08-24 05:39 发表评论