Online Java Programming Test - Java Programming Test - Random
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- Total number of questions: 20.
- Time allotted: 30 minutes.
- Each question carries 1 mark; there are no negative marks.
- DO NOT refresh the page.
- All the best!
Marks : 2/20
Test Review : View answers and explanation for this test.
Option A sets the String reference to null.
Option B is wrong because null cannot be in single quotes.
Option C is wrong because there are multiple characters between the single quotes ('abc').
Option D is wrong because you can't cast a char (primitive) to a String (object).
- float f1 = -343;
- float f2 = 3.14;
- float f3 = 0x12345;
- float f4 = 42e7;
- float f5 = 2001.0D;
- float f6 = 2.81F;
(1) and (3) are integer literals (32 bits), and integers can be legally assigned to floats (also 32 bits). (6) is correct because (F) is appended to the literal, declaring it as a float rather than a double (the default for floating point literals).
(2), (4),and (5) are all doubles.
Option C will not compile; the synchronized modifier applies only to methods.
Option A and B will compile because protected and transient are legal variable modifiers. Option D will compile because volatile is a proper variable modifier.
public class Outer
{
public void someOuterMethod()
{
//Line 5
}
public class Inner { }
public static void main(String[] argv)
{
Outer ot = new Outer();
//Line 10
}
}
Which of the following code fragments inserted, will allow to compile?
Option A compiles without problem.
Option B gives error - non-static variable cannot be referenced from a static context.
Option C package ot does not exist.
Option D gives error - non-static variable cannot be referenced from a static context.
default access is the "package oriented" access modifier.
Option A and C are wrong because public and protected are less restrictive. Option B and D are wrong because abstract and synchronized are not access modifiers.
class Super
{
public Integer getLength()
{
return new Integer(4);
}
}
public class Sub extends Super
{
public Long getLength()
{
return new Long(5);
}
public static void main(String[] args)
{
Super sooper = new Super();
Sub sub = new Sub();
System.out.println(
sooper.getLength().toString() + "," + sub.getLength().toString() );
}
}
Option D is correct, compilation fails - The return type of getLength( ) in the super class is an object of reference type Integer and the return type in the sub class is an object of reference type Long. In other words, it is not an override because of the change in the return type and it is also not an overload because the argument list has not changed.
- You can extend the Runnable interface as long as you override the public run() method.
- The class must contain a method called run() from which all code for that thread will be initiated.
- The class must contain an empty public void method named run().
- The class must contain a public void method named runnable().
- The class definition must include the words implements Threads and contain a method called run().
- The mandatory method must be public, with a return type of void, must be called run(), and cannot take any arguments.
(2) and (6). When a thread's run() method completes, the thread will die. The run() method must be declared public void and not take any arguments.
(1) is incorrect because classes can never extend interfaces. (3) is incorrect because the run() method is typically not empty; if it were, the thread would do nothing. (4) is incorrect because the mandatory method is run(). (5) is incorrect because the class implements Runnable.
class Test
{
public static void main(String [] args)
{
Test p = new Test();
p.start();
}
void start()
{
boolean b1 = false;
boolean b2 = fix(b1);
System.out.println(b1 + " " + b2);
}
boolean fix(boolean b1)
{
b1 = true;
return b1;
}
}
The boolean b1 in the fix() method is a different boolean than the b1 in the start() method. The b1 in the start() method is not updated by the fix() method.
int x = 3;
int y = 1;
if (x = y) /* Line 3 */
{
System.out.println("x =" + x);
}
Line 3 uses an assignment as opposed to comparison. Because of this, the if statement receives an integer value instead of a boolean. And so the compilation fails.
public class X
{
public static void main(String [] args)
{
try
{
badMethod();
System.out.print("A");
}
catch (RuntimeException ex) /* Line 10 */
{
System.out.print("B");
}
catch (Exception ex1)
{
System.out.print("C");
}
finally
{
System.out.print("D");
}
System.out.print("E");
}
public static void badMethod()
{
throw new RuntimeException();
}
}
A Run time exception is thrown and caught in the catch statement on line 10. All the code after the finally statement is run because the exception has been caught.
public class Foo
{
public static void main(String[] args)
{
try
{
return;
}
finally
{
System.out.println( "Finally" );
}
}
}
- An exception arising in the finally block itself.
- The death of the thread.
- The use of System.exit()
- Turning off the power to the CPU.
I suppose the last three could be classified as VM shutdown.
Option A is correct. If the class specified in the catch clause does have subclasses, any exception object that subclasses the specified class will be caught as well.
Option B is wrong. The error class is a subclass of Throwable and not Runtime Exception.
Option C is wrong. You do not catch this class of error.
Option D is wrong. An exception can be thrown to the next method higher up the call stack.
The char type is integral but unsigned. The range of a variable of type char is from 0 to 216-1 or 0 to 65535. Java characters are Unicode, which is a 16-bit encoding capable of representing a wide range of international characters. If the most significant nine bits of a char are 0, then the encoding is the same as seven-bit ASCII.
Option A is valid declaration of float.
Option B is incorrect because any literal number with a decimal point u declare the computer will implicitly cast to double unless you include "F or f"
Option C is incorrect because it is a String.
Option D is incorrect because "d" tells the computer it is a double so therefore you are trying to put a double value into a float variable i.e there might be a loss of precision.
public static void main(String[] args)
{
Object obj = new Object()
{
public int hashCode()
{
return 42;
}
};
System.out.println(obj.hashCode());
}
This code is an example of an anonymous inner class. They can be declared to extend another class or implement a single interface. Since they have no name you can not use the "new" keyword on them.
In this case the annoynous class is extending the Object class. Within the {} you place the methods you want for that class. After this class has been declared its methods can be used by that object in the usual way e.g. objectname.annoymousClassMethod()
class Test1
{
public int value;
public int hashCode() { return 42; }
}
class Test2
{
public int value;
public int hashcode() { return (int)(value^5); }
}
The so-called "hashing algorithm" implemented by class Test1 will always return the same value, 42, which is legal but which will place all of the hash table entries into a single bucket, the most inefficient setup possible.
Option A and D are incorrect because these classes are legal.
Option B is incorrect based on the logic described above.
Yes, always the elements in the collection are ordered.
Option B is correct because a method-local inner class can be abstract, although it means a subclass of the inner class must be created if the abstract class is to be used (so an abstract method-local inner class is probably not useful).
Option A is incorrect because a method-local inner class does not have to be declared final (although it is legal to do so).
C and D are incorrect because a method-local inner class cannot be made public (remember-you cannot mark any local variables as public), or static.
- Thread(Runnable r, String name)
- Thread()
- Thread(int priority)
- Thread(Runnable r, ThreadGroup g)
- Thread(Runnable r, int priority)
(1) and (2) are both valid constructors for Thread.
(3), (4), and (5) are not legal Thread constructors, although (4) is close. If you reverse the arguments in (4), you'd have a valid constructor.
class MyThread extends Thread
{
public static void main(String [] args)
{
MyThread t = new MyThread();
Thread x = new Thread(t);
x.start(); /* Line 7 */
}
public void run()
{
for(int i = 0; i < 3; ++i)
{
System.out.print(i + "..");
}
}
}
The thread MyThread will start and loop three times (from 0 to 2).
Option A is incorrect because the Thread class implements the Runnable interface; therefore, in line 7, Thread can take an object of type Thread as an argument in the constructor.
Option B and C are incorrect because the variable i in the for loop starts with a value of 0 and ends with a value of 2.