你强任你强，我干我本行——Java基础(下)

八、流与文件

8.1 File类

8.2 流

8.2.1 简介

8.2.2 InputStream与OutputStream

8.2.3 Reader与Writer

处理流(运用结构型设计模式:装饰模式)相对应:

Reader与Writer层次结构:

8.3 示例

8.3.1 文件操作

8.3.1.1 读文件

 public static String readFile(String fileName) { File file = new File(fileName).getAbsoluteFile(); if (!file.isFile()) { return ""; } StringBuilder sb = new StringBuilder(); try (InputStreamReader isr = new InputStreamReader(new 
 FileInputStream(file), "UTF-8"); BufferedReader br = new BufferedReader(isr)) { String str; while ((str = br.readLine()) != null) { sb.append(str).append("\r\n"); } } catch (IOException e) { e.printStackTrace(); } return sb.toString(); } 复制代码

8.3.1.1 写文件

 public static void writeFile(String fileName, String msg) { File file = new File(fileName); try { BufferedWriter bf = null; try { bf = new BufferedWriter(new FileWriter(file, true)); bf.write(msg); bf.newLine(); bf.write(msg); } finally { if (bf != null) { bf.flush(); bf.close(); } } } catch (IOException e) { e.printStackTrace(); } } 复制代码

8.3.2 内存操作

 public static void memoryOpt(){ String msg = "hello world !!!"; try(InputStream in = new ByteArrayInputStream(msg.getBytes()); OutputStream out = new ByteArrayOutputStream();){ int temp = 0; while((temp = in.read()) != -1){ out.write(Character.toUpperCase(temp)); } System.out.println(out); } catch (IOException e) { e.printStackTrace(); } } 复制代码

8.3.3 管道流

 static class Writer implements Runnable { PipedOutputStream out; public Writer(PipedOutputStream out) { this.out = out; } @Override public void run() { try { String message = "I'm K^Joker"; try { out.write(message.getBytes()); } finally { out.close(); } } catch (IOException e) { e.printStackTrace(); } } } static class Reader implements Runnable { PipedInputStream in; public Reader(PipedInputStream in) { this.in = in; } @Override public void run() { byte data[] = new byte[1024]; try { try { int len = in.read(data); System.out.println(new String(data, 0, len)); } finally { in.close(); } } catch (IOException e) { e.printStackTrace(); } } } public static void main(String[] args) { PipedOutputStream out = new PipedOutputStream(); PipedInputStream in; try { in = new PipedInputStream(out); Thread read = new Thread(new Reader(in)); Thread write = new Thread(new Writer(out)); read.start(); write.start(); } catch (IOException e) { e.printStackTrace(); } } 复制代码

8.3.4 对象流

 public class Person implements Serializable { private String name; private transient Integer age; private final Integer height = 180; private static Integer weight = 160; private transient final String nickName = "取毛名"; private transient static String phone = "110"; public Person(String name, Integer age) { this.name = name; this.age = age; } @Override public String toString() { return "Person{" + "name='" + name + '\'' + ", age=" + age + ", height=" + height + ", nickName='" + nickName + '\'' + '}'; } public static void serialize() { File file = new File("E:" + File.separator + "serializable.txt"); try (ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(file))) { out.writeObject(new Person("K^Joker", 23)); } catch (IOException e) { e.printStackTrace(); } } public static void deserialize() { File file = new File("E:" + File.separator + "serializable.txt"); try(ObjectInputStream in = new ObjectInputStream(new FileInputStream(file));){ Object obj = in.readObject(); System.out.println(obj); } catch (IOException e){ e.printStackTrace(); } catch (ClassNotFoundException e) { e.printStackTrace(); } } public static void main(String[] args) throws IOException { // serialize(); deserialize(); } } 复制代码

九、枚举

9.1 特性

① 枚举经常用来表示一组相同类型的常量，例如性别、学历、日期。
② 枚举是一种特殊的类，它与普通类一样，不过构造器访问修饰符只能private，enum关键字定义，默认继承了 java.lang.Enum 类，故不能继承其他类，并实现了 java.lang.Seriablizable 和 java.lang.Comparable 两个接口。 ③ 定义为枚举类后编译器会加上final声名，所以该类无法被继承 ④ 所有的枚举值都是 public static final 的，且必须在第一行定义 ⑤ 若在枚举中定义了抽象方法，那所有枚举变量必须实现其抽象方法

9.2 实现机制

我们先定义一个枚举:

 public enum ColorEnum { RED(), YELLOW(), BLUE(); } 复制代码

我们可以看到枚举其实就是一个类，并且该类被声明为final，继承了Enum类，枚举定义的变量被声明为public static final，另外多出了静态代码块，和两个静态方法values()和valueOf()。Enum类是一个抽象类，主要有name(枚举变量的名称)和ordinal(枚举变量的位置索引)两个属性，在多出的静态代码块中做了这样的操作:

 RED = new ColorEnum("RED", 0); YELLOW = new ColorEnum("YELLOW", 1); BLUE = NEW ColorEnum("BLUE", 2); $VALUES = new ColorEnum[]{RED, YELLOW, BLUE}; 复制代码

values()方法是返回$VALUES数组的复制，valueOf方法是根据传入的变量名称返回对应枚举实例。从下面那张图我们可以大致看出这些。

9.3 枚举使用

① 枚举可以实现接口，具有抽象方法

 public enum ColorEnum implements ColorInterface{ RED("红色"){ @Override void say(){ System.out.println("我是红色"); } }, YELLOW("黄色"){ @Override void say(){ System.out.println("我是黄色"); } }, BLUE("蓝色"){ @Override void say(){ System.out.println("我是蓝色"); } }; //必须第一行 private String color; private ColorEnum(String color){ this.color = color; } @Override public String getColor(){ return color; } abstract void say(); public static void main(String[] args) { System.out.println(ColorEnum.RED); //RED System.out.println(ColorEnum.RED.getColor()); //红色 System.out.println(ColorEnum.RED.name()); //RED System.out.println(ColorEnum.RED.ordinal()); //0 for(ColorEnum color : ColorEnum.values()){ System.out.println(color); //RED YELLOW BLUE } ColorEnum.RED.say(); //我是红色 } } 复制代码

② 接口中使用枚举

 public interface Food { enum Appetizer implements Food{ SALAD, SOUP, SPRING_ROLLS; } enum Dessert implements Food{ FRUIT, TIRAMISU, GELATO } } 复制代码

十、注解

10.1 基本注解

10.2 自定义注解

10.2.1 元注解

10.2.2 规则

10.2.3示例

 public class Person { @MyValidation(nullable = false, message = "姓名不能为空") private String name; public Person(){ } public Person(String name) { this.name = name; } } @Target(ElementType.FIELD) @Retention(RetentionPolicy.RUNTIME) public @interface MyValidation { boolean nullable() default true; String message() default ""; } public class Validation { public void validate(Object object) throws Exception { Class obj = object.getClass(); Field[] fields = obj.getDeclaredFields(); for (Field field : fields) { field.setAccessible(true); verify(field, object); field.setAccessible(false); } } private void verify(Field field, Object object) throws Exception { MyValidation mv = field.getAnnotation(MyValidation.class); if (mv != null && !mv.nullable()) { Object name = field.get(object); if("".equals(name) || name == null){ throw new Exception(mv.message()); } } } public static void main(String[] args) throws Exception { Validation v = new Validation(); // v.validate(new Person("")); v.validate(new Person()); } } 复制代码

十一、结语