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JavaSE Subclass Method Throw Exception

Posted on Edited on In Disqus:

JavaSE Overriding Methods With Exception

Scenario A

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class CanNotHopException extends Exceotion{ }

class Hopper {
	public void hop() throw Exception{
	
	}
}

class Bunny extends Hopper {
	public void hop() throws CanNotHopException{
		
	}
}

Result:

Report error.
Rule 1 - Java knows hop() isn’t allowed to throw any checked exceptions because the superclass Hoppper doesn’t declare any.

Scenario B

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class CanNotHopException extends Exceotion{ }

class Hopper {
	public void hop() throws CanNotHopException {
	
	}
}

class Bunny extends Hopper {
	public void hop(){
		
	}
}

Result:

Okay.
Rule 2 - A subclass is allowed to declare fewer exceptions than the superclass or interface.

Scenario C

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class CanNotHopException extends Exceotion{ }

class Hopper {
	public void hop() throws Exception {
	
	}
}

class Bunny extends Hopper throws CanNotHopException {
	public void hop(){
		
	}
}

Result:

Okay.
Rule 3 - A subclass is allowed to declare a subclass of an exception type because the superclass or interface has already taken care of a broader type.

These above-mentioned rules only apply to checked exceptions.

Scenario D

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class Bunny extends Hopper {
	public void hop() throws IllegalStateException{ }
}

Result:

Okay.
Rule 4 - declare new runtime exception in a subclass method that the declaration is redundant

Conclusion

  • checked exception, never subclass throws more exception than superclass
  • unchecked exception, no limitations
  • 用中文总结:
    子类在覆盖父类方法的时候，父类的引用是可以调用该方法的，如果父类的引用调用子类的方法，那么这个多抛出来的异常，就可能处于一种无法被处理的状态。一个修理家电的人，他能够修理冰箱，电脑，洗衣机，电视机。 一个年轻人从他这里学的技术，就只能修理这些家电，或者更少。你不能要求他教出来的徒弟用从他这里学的技术去修理直升飞机。

Reference