访问权限控制的等级，从最大权限到最小权限：public proteced 包访问权限 private
6.1 包：库单元
public class FullQualification {
public static void main(String[] args) {
java.util.ArrayList list = new java.util.ArrayList<>();
}
}import java.util.ArrayList;
public class SingleImport {
public static void main(String[] args) {
ArrayList list = new ArrayList();
}
}编译单元 .java 每个编译单元只能有一个public类
6.1.1 代码组织
库名 小写
6.1.2 创建独一无二的包名
sudo cd /usr/libexec
./java_home
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home
6.1.3定制工具库
import java.io.PrintStream;
public class Print {
// Print with a newline
public static void print(Object obj){
System.out.println(obj);
}
//Print a newline by itself
public static void print(){
System.out.println();
}
//Print with no line break
public static void printnb(Object obj){
System.out.print(obj);
}
//The new Java SE5 printf() (from C)
public static PrintStream printf(String format,Object... args){
return System.out.printf(format,args);
}
}
import static chapter6access.Print.print;
public class PrintTest {
public static void main(String[] args) {
print("avaliable from now on!");
print(100);
}
}6.1.4 用import改变行为
c语言解决跨平台的条件编译，由于java跨平台，没有必要
java 条件编译的调试用户，修改被导入的package()方法如：调试版本-发布版
6.2 Java访问权限修饰词
public proteced 包访问权限(不提供任何访问权限修饰词，默认) private
6.2.1包访问权限
6.2.2public 接口访问权限
package chapter6access.subfolder; //处于子目录下
public class Cookie {
public Cookie(){
System.out.println("Cookie constructor");
}
void bite(){
System.out.println("bite");
}
}
package chapter6access;
import chapter6access.subfolder.Cookie;
public class Dinner {
public static void main(String[] args) {
Cookie x = new Cookie();
// x.bite(); //处于子目录下，can't access，若不是子目录，同目录下，则享有包管理权限。
}
}6.2.3 private:你无法访问
private 多线程 重要（21章）
class Sundae {
private Sundae(){
}//不能通过构造器创建Sundae对象，必须调用makeASundae方法达到目的。
//会阻碍对此类对继承
static Sundae makeASundae(){
return new Sundae();
}
}
public class IceCream{
public static void main(String[] args) {
// Sundae x = new Sundae();
Sundae x = Sundae.makeASundae();
}
}
别名机制 类引用是private,其他对象也可以拥有该对象public引用6.2.4 protected: 继承访问权限
package chapter6access.subfolder;
public class Cookie {
public Cookie(){
System.out.println("Cookie constructor");
}
protected void bite(){ //如果没有 protected ,即 默认包管理权限，则继承的非同级包，无法使用。
System.out.println("bite");
}
}
import chapter6access.subfolder.Cookie;
public class ChocolateChip extends Cookie {
public ChocolateChip(){
System.out.println("ChocolateChip constructor");
}
public void chomp(){
bite(); //can't access bite
}
public static void main(String[] args) {
ChocolateChip x = new ChocolateChip();
x.chomp();
}
}
main 是成员方法，因此可以访问一切
package chapter6access.subfolder;
public class E05 {
public int a;
private int b;
protected int c;
int d;//package access
public void f1(){}
private void f2(){}
protected void f3(){}
void f4(){} ////package access
public static void main(String[] args) {
E05 e05 = new E05();
e05.a =1;
e05.b =1;
e05.c =1;
e05.d =1;
e05.f1();
e05.f2();
e05.f3();
e05.f4();
}
}
在当前包下创建单独的类，则不能访问私有成员
package chapter6access.subfolder;
public class E05Class {
public E05Class(){
E05 e05 = new E05();
e05.a =1;
// e05.b =1; //can't access :private
e05.c =1;
e05.d =1;
e05.f1();
// e05.f2(); //can't access :private
e05.f3();
e05.f4();
}
}
在不同的包下，或使用包引入语句，只能访问 公有成员
package chapter6access;
import chapter6access.subfolder.E05;
public class E05Class {
public E05Class(){
E05 e05 = new E05();
e05.a =1;
// e05.b =1; //can't access :private
// e05.c =1; //can't access :protected
// e05.d =1; //can't access :package
e05.f1();
// e05.f2(); //can't access :private
// e05.f3(); //can't access :protected
// e05.f4(); //can't access :package
}
}
package chapter6access.subfolder;
class WithProtected {
protected int a; //also means package access 在相同的包内，可以访问protected
}
public class E06Class{
public static void main(String[] args) {
WithProtected wp = new WithProtected();
wp.a = 47;
System.out.println("wp.a = " + wp.a);
}
}6.3 接口和实现
具体实现的隐藏
封装
public protected 包访问权限 private 创建类的形式
类的访问权限
类既不能是private 也不是protected.(内部类除外)，只有包访问权限 或 public
class Soup1{
private Soup1(){} //必须编写默认构造器，系统不会自动创建默认构造器
public static Soup1 makeSoup(){
return new Soup1();//返回soup1对象的一个引用，可以做额外工作，如计数等
}
}
class Soup2{
private static Soup2 ps1 = new Soup2();//单例模式 private static创建， 有且仅有一个，且通过access 访问
public static Soup2 access(){
return ps1;
}
public void f(){}
}
public class Lunch {
void testPrivate(){
// Soup1 soup = new Soup1();
}
void testStatic(){
Soup1 soup = Soup1.makeSoup();
}
void testSingleton(){
Soup2.access().f();
}
}这个蛮有意思
import chapter6access.connection.Connection;
import chapter6access.connection.ConnectionManager;
public class E08_ConnectionManager {
public static void main(String[] args) {
Connection c = ConnectionManager.getConnection();
while (c != null) {
System.out.println(c);
c.doSomething();
c = ConnectionManager.getConnection();
}
}
}
package chapter6access.connection;
public class ConnectionManager {
private static Connection[] pool = new Connection[10];
private static int counter = 0;
static {
for (int i =0;i<pool.length;i++){
pool[i] = new Connection();
}
}
public static Connection getConnection(){
if(counter < pool.length)
return pool[counter++];
return null;
}
}
package chapter6access.connection;
public class Connection {
private static int counter = 0;
private int id = counter++;
Connection(){}
public String toString(){
return "Connection " + id;
}
public void doSomething(){}
}