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Answers
Answer to Question 1)
1) float f=1/3;
2) int i=1/3;
4) double d=999d;
The fact that option 3 does not compile may be a surprise. The problem is because the default type for a number with a decimal component is a double and not a float. The additional trailing d in the option with 999 doesn't help, but it doesn't harm.
Answer to Question 2)
2) new
The option NULL (note the upper case letter) is definitely not a keyword. There is some discussion as to i. There is some discussion as to if null is a keyword but for the purpose of the exam you should probably assume it is a keyword.
The option instanceOf is a bit of a misleading option that would probably not occur on the exam. The real keyword is instanceof (note that the of has no capital letter O). I had the incorrect version in an earlier version of this tutorial as it looks more likely to my eyes. The instanceof keyword looks like a method, but it is actually an operator.
The option wend is probably valid in some other language to indicate the end of a while loop, but Java has no such keyword.
Answer to Question 3)
1) System.out.println(1+1);
2) int i=2+'2';
Option 3 is not valid because single quotes are used to indicate a character constant and not a string. Several people have emailed me to say that option 3 will compile. When they eventually compiled the exact code they have agreed, it will not compile. Let me re-state that
String s="on"+'one';
Will NOT compile.
Option 4 will not compile because 255 is out of the range of a byte
Answer to Question 4)
1) The garbage collection algorithm in Java is vendor implemented
Threading and garbage collection are two of the few areas that are platform dependent. This is one of the
reasons why Java is not suitable for realtime programming. It is not a good idea use it to control your
plane or nuclear power station. Once an instance of the Integer class has a value it cannot be changed.
Answer to Question 5)
(Not on the official sub objectives but this topic does come up on the exam)
2) The RandomAccessFile class allows you to move directly to any point a file.
4) The characteristics of an instance of the File class such as the directory separator, depend on the current underlying operating system
The File class can be considered to represent information about a file rather than a real file object. You can create a file in the underlying operating system by passing an instance of a file to a stream such as FileOutputStream. The file will be created when you call the close method of the stream.
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Answer to Question 12)
4) Compilation and output of hello
This type of question is particularly calculated to catch out C/C++ programmers who might expect parameter zero to be the name of the compiler.
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Answer to Question 13)
1) If a class has any abstract methods it must be declared abstract itself.
3) The final modifier means that a class cannot be sub-classed
4) transient and volatile are Java modifiers
An abstract class may have non abstract methods. Any class that descends from an abstract class must implement the abstract methods of the base class or declare them as abstract itself.
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Answer to Question 14)
2) public static void amethod(){}
4) static native void amethod();
Option 1 is not valid because it has braces and the native modifier means that the method can have no body. This is because the body must be implemented in some other language (often C/C++). Option 3 is not valid because private and protected contradict themselves.
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Answer to Question 15)
4) Constructors are not inherited
Constructors can be marked public, private or protected. Constructors do not have a return type.
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Answer to Question 16)
2) Compile time error
An error occurs when the class Severn attempts to call the zero parameter constructor in the class Base Because the Base class has an integer constructor Java does not provide the "behind the scenes" zero parameter constructor.
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Answer to Question 17)
1) static methods do not have access to the implicit variable called this
2) A static method may be called without creating an instance of its class
3) a static may not be overriden to be non-static
The implicit variable this refers to the current instance of a class and thus and by its nature a static method cannot have access to it.
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Answer to Question 18)
1)
char c='1';
System.out.println(c>>1);
4)
int i=1;
System.out.println(i<<1);
Be aware that Integer (not the upper case I) is a wrapper class and thus cannot be treated like a primitive. The fact that option 1 will compile may be a surprise, but although the char type is normally used to store character types, it is actually an unsigned integer type. The reason option 3 does not compile is that Java has a >>> operator but not a <<< operator. ;>> operator but not a <<< operator.
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Answer to Question 19)
2) An event listener may be removed from a component
3) The ActionListener interface has no corresponding Adapter class
A component may have multiple event listeners attached. Thus a field may need to respond to both the mouse and the keyboard, requiring multiple event handlers. The ActionListener has not matching Adapter class because it has only one method, the idea of the Adapter classes is to eliminate the need to create blank methods.
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Answer to Question 20)
3) transient
4) volatile
Option 1, sizeof is designed to catch out the C/C++ programmers. Java does not have a sizeof keyword as the size of primitives should be consistent on all Java implementations. Although a program needs a main method with the standard signature to start up it is not a keyword. The real keywords are less commonly used and therefore might not be so familiar to you.
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Answer to Question 21)
3) The default constructor takes no parameters
4) The default constructor is not created if the class has any constructors of its own.
Option 1 is fairly obviously wrong as constructors never have a return type. Option 2 is very dubious as well as Java does not offer void as a type for a method or constructor.
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Answer to Question 22)
1) All of the variables in an interface are implicitly static
2) All of the variables in an interface are implicitly final
3) All of the methods in an interface are implictly abstract
All the variables in an interface are implicitly static and final. Any methods in an interface have no body, so may not access any type of variable
Answer to Question 23)
2) The + operator is overloaded for concatenation for the String class
In Java Strings are implemented as a class within the Java.lang package with the special distinction that the + operator is overloaded. If you thought that the String class is implemented as a char array, you may have a head full of C/++ that needs emptying. There is not "wrapper class" for String as wrappers are only for primitive types.
If you are surprised that option 4 is not a correct answer it is because length is a method for the String class, but a property for and array and it is easy to get the two confused.
Answer to Question 24)
1) A method in an interface must not have a body
3) A class may extends one other class plus many interfaces
A class accesses an interface using the implements keyword (not uses) [1] [2] [3] 下一页
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