爲什麼java.util.concurrent.TimeUnit.convert拋出的AbstractMethodError，而不是被抽象

Question

java.util.concurrent.TimeUnit有這個來源：

public long convert(long sourceDuration, TimeUnit sourceUnit) { 
    throw new AbstractMethodError(); 
} 

爲什麼不是這樣的一個abstract方法類似

abstract int excessNanos(long d, long m); 

Answer 1

單上面的方法聲明說明如下，

// To maintain full signature compatibility with 1.5, and to improve the 
// clarity of the generated javadoc (see 6287639: Abstract methods in 
// enum classes should not be listed as abstract), method convert 
// etc. are not declared abstract but otherwise act as abstract methods. 

這裏，6287639是錯誤ID它說，

JDK-6287639：枚舉類抽象方法不應該被列爲抽象

現在考慮下面的enum將其視爲類，每枚舉常量爲Object，很顯然，如果我們創建的抽象的東西Object我們必須提供執行和避免這種convert不是abstract，

enum Blocks { 
    A1, B1, C1; 
    // It will say enum constant must implement 
    // the abstract method test 
    abstract int test(); 
}