Effective Java学习笔记---------类和接口

使类和成员的可访问性最小化

包含公有可变域的类通常不是线程安全的

在仔细设计了一个最小的公有api之后，应该防止把散乱的类、接口或者成员变成api的一部分

除了公有静态final域的特殊情形之外（此时它们充当常量），公有类都不应该包含公有域，并且要确保公有静态final域所引用的对象都是不可变的

要在公有类中使用访问方法而非公有域

package codeTemplate.effectiveJava.bean;
 
import lombok.Getter;
import lombok.Setter;
 
import java.util.Arrays;
import java.util.List;
 
@Setter
@Getter
public class CommonBean {
    private String name = "123";
    //Make this final field static too.
    private final String finalName = "123";
    private static String staticName = "123";
    //常量【finalStaticName】命名应全部大写并以下划线分隔
    private static final String FINAL_STATIC_NAME = "123";
 
    private List names = Arrays.asList("123", "234");
    private final List finalNames = Arrays.asList("123", "234");
    private static List staticNames = Arrays.asList("123", "234");
    //常量【finalStaticNames】命名应全部大写并以下划线分隔 
    private static final List FINAL_STATIC_NAMES = Arrays.asList("123", "234");
 
    private PhoneArea phoneArea;
    private PhoneArea phoneAreaNew = new PhoneArea("China");
    //Variable 'finalPhoneArea' might not have been initialized
    private final PhoneArea finalPhoneArea;
    //Make this final field static too.
    private final PhoneArea finalPhoneAreaNew = null;
    private static PhoneArea staticPhoneArea;
    //Variable 'staticFinalPhoneAreaNew' might not have been initialized
    private static final PhoneArea staticFinalPhoneArea;
    //常量【staticFinalPhoneAreaNew】命名应全部大写并以下划线分隔
    private static final PhoneArea STATIC_FINAL_PHONE_AREA_NEW = null;
 
 
    //Make publicName a static final constant or non-public and provide accessors if needed.
    public String publicName = "123";
    //Make this final field static too.
    public final String publicFinalName = "123";
    //Make publicStaticName a static final constant or non-public and provide accessors if needed.
    public static String publicStaticName = "123";
    //常量【publicFinalStaticName】命名应全部大写并以下划线分隔 
    public static final String publicFinalStaticName = "123";
 
    //Make publicNames a static final constant or non-public and provide accessors if needed.
    public List publicNames = Arrays.asList("123", "234");
    public final List publicFinalNames = Arrays.asList("123", "234");
    //Make publicNames a static final constant or non-public and provide accessors if needed.
    public static List publicStaticNames = Arrays.asList("123", "234");
    //常量【publicFinalStaticNames】命名应全部大写并以下划线分隔 
    public static final List publicFinalStaticNames = Arrays.asList("123", "234");
 
    //Make publicPhoneArea a static final constant or non-public and provide accessors if needed
    public PhoneArea publicPhoneArea;
    //Make publicPhoneArea a static final constant or non-public and provide accessors if needed
    public PhoneArea publicPhoneAreaNew = new PhoneArea("China");
    //Variable 'publicFinalPhoneArea' might not have been initialized
    public final PhoneArea publicFinalPhoneArea;
    //Make this final field static too.
    public final PhoneArea publicFinalPhoneAreaNew=null;
    //Make publicStaticPhoneAreaNew a static final constant or non-public and provide accessors if needed.
    public static PhoneArea publicStaticPhoneArea;
    //Variable 'publicStaticFinalPhoneArea' might not have been initialized
    public static final PhoneArea publicStaticFinalPhoneArea;
    //常量【publicStaticFinalPhoneAreaNew】命名应全部大写并以下划线分隔 
    public static final PhoneArea publicStaticFinalPhoneAreaNew=null;
}

总结

私有属性

1. 常量时final需要和static共同使用，引用对象时可以单独使用，都一定要初始化。
2. static final 属性命名应全部大写并以下划线分隔，为常量

公有属性

1. 常量都需要是static final
2. 引用类型static final修饰为常量，可以只有final,不静态
3. static final 属性命名应全部大写并以下划线分隔
4. final修饰的都要初始化

最好的使用

    private String name = "123";
    private static String staticName = "123";
    private static final String FINAL_STATIC_NAME = "123";
    private final List finalNames=new ArrayList<>();
 
    public static final String PUBLIC_FINAL_STATIC_NAME = "123";
    static final List PUBLIC_FINAL_STATIC_NAMES = Arrays.asList("123", "234");
    public final List publicFinalNames = Arrays.asList("123", "234");

使可变性最小化

不可变类是指其实例不能被修改的类，除非一定要可变，否则都应该是不可变的

域除非一定要变，否则有设为final，不要为每个get方法编写一个set方法

复合优先于继承

继承打破了封闭性

复合：增加一个私有域，引用现有类的一个实例。对应设计模式中装饰器模式和代理模式

package codeTemplate.effectiveJava.bean;
 
import java.util.List;
 
public class BasicBean {
    private String name;
    private List functions;
 
    public BasicBean(String name, List functions) {
        this.name = name;
        this.functions = functions;
    }
 
    public void addFunction(String function) {
        this.functions.add(function);
    }
}

继承

package codeTemplate.effectiveJava.bean;
 
import java.util.List;
 
public class RedBean extends BasicBean {
    private int number = 0;
 
    public RedBean(String name, List function) {
        super(name, function);
    }
 
    @Override
    public void addFunction(String function) {
        number++;
        super.addFunction(function);
    }
}

复合

package codeTemplate.effectiveJava.bean;
 
public class YellowBean {
    private int number = 0;
    private final BasicBean basicBean;
 
    public YellowBean(BasicBean basicBean) {
        this.basicBean = basicBean;
    }
 
    public void addFunction(String function) {
        number++;
        basicBean.addFunction(function);
    }
}

要么设计继承并提供文档说明，要么禁止继承

接口优于抽象类

接口使得安全地增强类的功能成为可能

为接口提供一个抽象的骨架实现类（抽象类），即模板方法模式

接口只用于定义类型

常量接口模式是对接口的不良使用，应使用不可实例化额工具类，静态导入

常量接口

package codeTemplate.effectiveJava.bean;
 
public interface Consts {
    String BOOK_NAME = "asds";
}

不可实例化的工具类

package codeTemplate.effectiveJava.bean;
 
public class Constants {
    private Constants() {
    }
 
    public static final String BOOK_NAME = "asds";
}

类层次优于标签类

使用继承生成多个类，优于多个构造器生成不同的对象

静态成员类（内部类）优于非静态成员类

嵌套类存在的目的应该只是为它的外围类提供服务

永远不要将多个顶级类或者接口放在一个源文件中

顶级类：非成员类（内部类）