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Core Java 面接の質問と回答

Test your skills through the online practice test: Core Java Quiz Online Practice Test

質問 206. What are wrapped classes?

Wrapped classes are classes that allow primitive types to be accessed as objects.

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質問 207. What is casting?

There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

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質問 208. What are E and PI?

E is the base of the natural logarithm and PI is mathematical value pi.

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質問 209. What happens when you add a double value to a String?

The result is a String object.

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質問 210. Why can't I say just abs() or sin() instead of Math.abs() and Math.sin()?

The import statement does not bring methods into your local name space. It lets you abbreviate class names, but not get rid of them altogether. That's just the way it works, you'll get used to it. It's really a lot safer this way.
However, there is actually a little trick you can use in some cases that gets you what you want. If your top-level class doesn't need to inherit from anything else, make it inherit from java.lang.Math. That *does* bring all the methods into your local name space. But you can't use this trick in an applet, because you have to inherit from java.awt.Applet. And actually, you can't use it on java.lang.Math at all, because Math is a '??final'?? class which means it can't be extended.

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