僅僅在異常狀況下才使用Exceptions

• 不要把Exceptions用于ordinary control flow
• 不要寫(xiě)這樣的API: 迫使別人把Exceptions用于ordinary control flow
• 應(yīng)該寫(xiě)這樣的API:

have a separate “state-testing” method indicating 
whether it is appropriate to invoke the state-dependent method
比如:
the Iterator interface has the state-dependent method next 
and the corresponding state-testing method hasNext 

have the state-dependent method return an empty optional 
or a distinguished value such as null 
if it cannot perform the desired computation

java throwable有哪些？

checked exceptions, runtime exceptions, errors
其中 runtime exceptions, errors 都屬于unchecked exceptions

在使用java的throwable時(shí)，有哪些準(zhǔn)則？

• throw checked exceptions for recoverable conditions
• 為你的checked exceptions提供methods，便于caller recoverable
• throw unchecked exceptions for programming errors
• 只定義checked exceptions 或 runtime exceptions，不要定義Error及其子類(lèi)或其他的throwable及其子類(lèi)
• 不知道throw哪種exceptions時(shí)，就throw RuntimeException
• Avoid unnecessary use of checked exceptions，一種消除 a checked exception的方式是return an optional of the desired result type
• 盡量document all exceptions thrown by each method
• exception的detail message應(yīng)該包含導(dǎo)致該exception的所有parameters 和 fields 的值，比如IndexOutOfBoundsException的detail message包括 the lower bound, the upper bound, and the index value that failed to lie between the bounds

為什么擁抱使用standard exceptions

• 能否獲得高度的code reuse是區(qū)分expert programmers和一般programmers的重要屬性之一
• Reusing standard exceptions能使你的API更容易learn, use, and read
• fewer exception classes意味著smaller memory footprint 和 less time spent loading classes
• 不要直接 reuse: Exception, RuntimeException, Throwable, or Error
• 常被reused exceptions: IllegalArgumentException,IllegalStateException,NullPointerException,IndexOutOfBoundsException,ConcurrentModificationException,UnsupportedOperationException
• 不要過(guò)度使用exception translation和exception chaining
• 不要忽視異常，在try catch中，一個(gè)空的catch塊破壞了exception的目的

對(duì)exception translation的理解

• higher layers should catch lower-level exceptions and, in their place, throw exceptions that can be explained in terms of the higher-level abstraction

// Exception Translation
try {
    ... // Use lower-level abstraction to do our bidding
} catch (LowerLevelException e) {
    throw new HigherLevelException(...);
}

對(duì)exception chaining的理解

• The lower-level exception (the cause) is passed to the higher-level exception

// Exception Chaining
try {
    ... // Use lower-level abstraction to do our bidding
} catch (LowerLevelException cause) {
    throw new HigherLevelException(cause);
}
// Exception with chaining-aware constructor
class HigherLevelException extends Exception {
    HigherLevelException(Throwable cause) {
        super(cause);
    }
}

說(shuō)說(shuō)對(duì)原子性失敗(failure-atomic)的理解

• 一次失敗的方法調(diào)用應(yīng)該使該對(duì)象處于調(diào)用之前的狀態(tài)，具有這種屬性的方法，稱(chēng)為failure-atomic
• 使用failure atomicity時(shí)，不應(yīng)該significantly 增加代碼的cost和complexity

如何實(shí)現(xiàn)failure-atomic效果？

• design immutable objects
• 在修改對(duì)象狀態(tài)之前，先計(jì)算并check parameters的正確性
• 臨時(shí)copy object，在copy object 計(jì)算完成，然后替換掉原來(lái)object的內(nèi)容
• 寫(xiě)recovery code，使該object的狀態(tài)roll back到原始狀態(tài)