Effective Java知识点整理

Effective Java（第2版）学习笔记

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Chapter 2 Creating and Destroying Objects

Item 1: Consider static factory methods instead of constructors

Item 2: Consider a builder when faced with many constructor parameters

可以参考这篇文章：使用Builder解决构造函数参数过多的问题

Item 3: Enforce the singleton property with a private constructor or an enum type

可以参考这篇文章： Java设计模式–单例模式

Item 4: Enforce noninstantiability with a private constructor

Item 5: Avoid creating unnecessary objects

String s = new String("abcdefg");   //bad

String s = "abcdefg";               //good

Item 6: Eliminate obsolete object references

当引用不再需要的时候，赋给其null

public Object pop(){
    if(size == 0){
        throw new EmptyStackException();
    }
    Object result = elments[--size];
    elements[size] = null;   //删除不再需要的引用
    return result;
}   

Item 7: Avoid finalizers

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Chapter 3 Methods Common to All Objects

Item 8: Obey the general contract when overriding equals

若x，y，z都是非null引用：
1. Reflexive（自反性）：x.equals(x) 必须返回true
2. Symmetric（对称性）：x.equals(y)返回true，当且仅当y.equals(x)返回true
3. Transitive（传递性）：x.equals(y)返回true，且y.equals(z)返回true，那么x.equals(z)也返回true
4. Consistent（一致性）：多次调用x.equals(y)后，要么都返回true，要么都返回false

Item 9: Always override hashCode when you override equals

Item10: Always override toString

Item 11: Override clone judiciously

复制构造函数：

public Yum(Yum yum);

复制工厂方法：

public static Yum newInstance(Yum yum);

Item 12: Consider implementing Comparable

对于基本类型的比较，用”<”和”>”
对于float和double型，用Float.compare和Double.compare

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Chapter 4 Classes and Interfaces

Item 13: Minimize the accessibility of classes and members

访问性由低到高：
private->default->protected->public

• 子类中方法的可访问性必须大于等于父类中所覆盖(override)的方法
• 如果一个类实现了一个接口，则与接口中同名的方法必须为public

Item 14: In public classes, use accessor methods, not public fields

accessor methods: getters
mutators: setters

Item 15: Minimize mutability

在多步操作的时候（如循环中），用可变类型代替不可变类型，这样可以提高性能。
例如，在for循环中，用StringBuilder代替String

Item 16: Favor composition over inheritance

Item 17: Design and document for inheritance or else prohibit it

Item 18: Prefer interfaces to abstract classes

Item 19: Use interfaces only to define types

Item 20: Prefer class hierarchies to tagged classes

Item 21: Use function objects to represent strategies

Item 22: Favor static member classes over nonstatic

非静态内部类实例必须通过外部实例来创建

Chapter 5 Generics

Item 23: Don’t use raw type in new code

用List< E>而不是List。
List< String>是List的子类，但不是List< Object>的子类。

Item 24: Eliminate unchecked warnings

Item 25: Prefer lists to arrays

• array（数组）是协变类型，如果Sub是Super的子类，那么Sub[]也是Super[]的子类。

• Generics（泛型）是非协变类型，List< Type1>gen Listt< Type2>没有任何关系。

    Object[] objectArray = new Long[1];
    objectArray[0] = "abc";                   //运行时才报错

    List<Object> ol = new ArrayList<long>();  //编译时就报错
    ol.add("abc");

Item 26: Favor generic types

Item 27: Favor generic methods

如果函数的返回类型是泛型，则需要在函数修饰符和返回类型之间加上< E>：

public static <E> Set<E> union(){
    Set<E> set = new HashSet<E>();
    ...
    return set;
}

Item 28: Use bounded wildcards to increase API flexibility

public void pushAll(Iterable<? extends E> src){
    ...
}

public void popAll(Collection<? super E> des){
    ...
}

<? extends E>和<? super E>都是限界通配符（bounded wildcard），extends用于指定上界，super用于指定下界。

两条准则：

• PECS：生产者（producer）用extends，消费者（consumer）用super

• 所有的comparable和comparator都是消费者

Item 29: Consider typesafe heterogeneous containers

String.class的泛型是Class<String>
Integer.class的泛型是Class<Integer>

//Typesafe heterogeneous container pattern - API
public class Favorites{
    
    
    public <T> void putFavorite(Class<T> type, T instance);
    public <T> T getFavorite(Class<T> type);
}

Chapter 6 Enums and Annotations

Item 30: Use enums instead of int constants

Item 31: Use instance fields instead of ordinals

Item 32: Use EnumSet instead of bit fields

Item 33: Use EnumMap instead of ordinal indexing

Item 34: Emulate extensible enums with interfaces

Item 35: Prefer annotations to naming patterns

Item 36: Consistently use the Override annotation

Item 37: Use marker interfaces to define types

Chapter 7 Methods

Item 38: Check parameters for validity

Item 39: Make defensive copies when needed

Item 40: Design method signatures carefully

Item 41: Use overloading judiciously

在overload方法中做选择是静态的，在override方法中做选择是动态的。

Item 42: Use varargs judiciously

Item 43: Return empty arrays or collections, not nulls

Item 44: Write doc comments for all exposed API elements

Chapter 8 General Programming

Item 45: Minimize the scope of local variables

Item 46: Prefer for-each loops to traditional for loops

Item 47: Know and use the libraries

Item 48: Avoid float and double if exact answers are required

float和double的结果是准确近似，并不精确。
如果需要精确的结果，应该使用BigDecimal、int或long

Item 49: Prefer primitive types to boxed primitives

对包装类使用==，基本都是错的。

Item 50: Avoid strings where other types are more appropriate

Item 51: Beware the performance of string concatenation

如果改动次数较多，应该使用StringBuilder

Item 52: Refer to objects by their interfaces

Item 53: Prefer interfaces to reflection

Item 54: Use native methods judiciously

Item 55: Optimize judiciously

Strive to write good programs rather than fast ones.

Item 56: Adhere to generally accepted naming conventions

Chapter 9 Exceptions

Item 57: Use exceptions only for exceptional conditions

Item 58: Use checked exceptions for reconverable conditions and runtimeexceptions for programming errors

三种throwable：

1. checked exceptions
2. runtime exceptions
3. errors

调用者如果能恢复，应该使用checked exceptions
用runtime exceptions来提示编程错误

Item 59: Avoid unneccessary use of checked exceptions

Item 60: Favor the use of standard exceptions

Item 61: Throw exceptions appropriate to the abstraction

Item 62: Document all exceptions thrown by each method

Item 63: Include failure-capture information in detail messages

Item 64: Strive for failure atomicity

Item 65: Don’t ignore exceptions

Chapter 10 Concurrency

Item 66: Synchronize access to shared mutable data

Item 67: Avoid excessive synchronization

Item 68: Prefer executors and tasks to threads

Item 69: Prefer concurrency utilities to wait and notify

Item 70: Document thread safety

Item 71: Use lazy initialization judiciously

Item 72: Don’t depend on the thread scheduler

Item 73: Avoid thread groups

Chapter 11 Serialization

Item 74: Implement Serializable judiciously

反序列化是一个隐式的构造函数。

Item 75: Consider using a custom serialized form

Item 76: Write readObject methods defensively

Item 77: For instance control, prefer enum types to readResolve

Item 78: Consider serialization proxies instead of serialized instances