Q: Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
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; [ Received from Sandesh Sadhale]
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Q: Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
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. [ Received from Sandesh Sadhale]
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Q: What is the difference between declaring a variable and defining a variable?
A: In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions. [ Received from Sandesh Sadhale]
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Q: What is the default value of an object reference declared as an instance variable?
A: null unless we define it explicitly. [ Received from Sandesh Sadhale]
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Q: Can a top level class be private or protected?
A: No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected. [ Received from Sandesh Sadhale]
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Q: What type of parameter passing does Java support?
A: In Java the arguments are always passed by value . [ Update from Eki and Jyothish Venu]
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Q: Primitive data types are passed by reference or pass by value?
A: Primitive data types are passed by value. [ Received from Sandesh Sadhale]
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Q: Objects are passed by value or by reference?
A: Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object . [ Update from Eki and Jyothish Venu]