Q021:

How can one prove that the array is not null but empty using one line of code?
如何能够用一行代码证明该array不是null而是空值？（接上问）

A:

Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.
打印args.length, 它将打印0。这意味着它是空的。如果它是空的话，该行代码将会抛出NullPointerException

Q022:

What environment variables do I need to set on my machine in order to be able to run Java programs?
为了能够运行Java程序，我需要设置哪些环境变量？

A:

CLASSPATH and PATH are the two variables.
需要设置的两个环境变量是CLASSPATH和PATH

Q023:

Can an application have multiple classes having main method?
同一个应用中可以允许多个类都有main方法么？

A:

Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.
这有可能。当我们用指定的类名来运行程序的时候，JVM将只在该类中寻找main方法来运行。因此，多个类都含有的main方法不会相互冲突。

Q024:

Can I have multiple main methods in the same class?
在同个类中指定多个main方法可以么？

A:

No the program fails to compile. The compiler says that the main method is already defined in the class.
不行，程序将编译失败。编译器会报main方法已经在类中定义过了。

Q025:

Do I need to import java.lang package any time? Why ?
我需要在什么时候import java.lang包么，为什么？

A:

No. It is by default loaded internally by the JVM.
不，该包会被JVM默认装载进来。

Q026:

Can I import same package/class twice? Will the JVM load the package twice at runtime?
我可以把一个包或者类import两次么？JVM会在运行时装载它两次么？

A:

One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.
我们可以多次import同一个包或者同一个类。编译器和JVM都不会报错。但是，在内部JVM只会将其导入一次。

Q027:

What are Checked and UnChecked Exception?
什么查核的异常和未查核的异常？

A:

A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses.
一个查核的异常是Exception的子类或者是Exception，不过不包括类RuntimeException和它的子类。
Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method·

查核异常要求客户端程序员处理异常将被抛出的可能性。例如：IOException可能会被java.io.FileInputStream中的read()方法抛出
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the
exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.
未查核的异常是RuntimeException和它的任何子类。Error类和它的子类也是未查核的。对于未查核的异常，编译器并不要求程序员catch它或是在throws语言中声明它。实际上，客户端程序员可能根本不知道有异常会被抛出。例如: StringIndexOutOfBoundsException被String的charAt()方法抛出。查核的异常必须在编译期被捕获。运行时异常不需要在编译器被捕获，而Error往往也不能在编译器被捕获。

Q028:

What is Overriding?
什么是覆盖？

A:

When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.

当一个类定义了一个方法，该方法和它的父类中的同名方法有着相同的返回类型和相同的参数，那么该类的这个方法覆盖了它父类中的那个同名方法。
当这个方法被该类的对象所调用的时候，这个新定义的方法会被调用，而不是其父类定义的那个方法。方法的覆盖必须是更public的，而不能更private。

Q029:

Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
import会在编译期进行有效性检查么？例如，包含import java.lang.ABCD语句的代码能编译么？

A:

Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD;
是的，import将会在编译期进行语义检查。上面的代码将无法通过编译。它将会抛出如下的错误报告无法解析符号

symbol : class ABCD
location: package io
import java.io.ABCD;

Q030:

Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
当import某个包的时候会自动import它的子包么？例如，import com.MyTest.*会同时import com.MyTest.UnitTests.*么？

A:

No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.
不，你必须显式地import子包。import com.MyTest.*只会导入MyTest中的类，它不会导入任何子类中的包。