Computer Knowledge

Java Core Classes and Threads

1,473 Questions

Java core classes and threads form the foundation of object oriented programming and are crucial for IT officer and programming exams. This includes concepts like string mutability, collections, and multithreading. Solve these questions to test your practical coding and theoretical knowledge.

String and StringBuffer classesThread execution methodsJava collections frameworkCharacter streams input outputVariable serialization rules

Java Core Classes and Threads Questions

Multiple choice

Which two can be used to create a new Thread?

  1. Extend java.lang.Thread and override the run() method.
  2. Extend java.lang.Runnable and override the start() method.
  3. Implement java.lang.Thread and implement the run() method.
  4. Implement java.lang.Runnable and implement the run() method.
  5. Implement java.lang.Thread and implement the start() method.

  1. 1 and 2

  2. 2 and 3

  3. 1 and 4

  4. 3 and 4
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

There are two ways of creating a thread; extend (sub-class) the Thread class and implement the Runnable interface. For both of these ways you must implement (override and not overload) the public void run() method. (1) is correct - Extending the Thread class and overriding its run method is a valid procedure. (4) is correct - You must implement interfaces, and runnable is an interface and you must also include the run method. (2) is wrong - Runnable is an interface which implements not Extends. Gives the error: (No interface expected here) (3) is wrong - You cannot implement java.lang.Thread (This is a Class). (Implements Thread, gives the error: Interface expected). Implements expects an interface. (5) is wrong - You cannot implement java.lang.Thread (This is a class). You Extend classes, and Implement interfaces. (Implements Thread, gives the error: Interface expected)

Multiple choice

The following block of code creates a Thread using a Runnable target: Runnable target = new MyRunnable(); Thread myThread = new Thread(target); Which of the following classes can be used to create the target, so that the preceding code compiles correctly?

  1. public class MyRunnable extends Runnable{public void run(){}}

  2. public class MyRunnable extends Object{public void run(){}}

  3. public class MyRunnable implements Runnable{public void run(){}}

  4. public class MyRunnable implements Runnable{void run(){}}
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The class correctly implements the Runnable interface with a legal public void run()method. Option 1 is incorrect because interfaces are not extended; they are implemented. Option 2 is incorrect because even though the class would compile and it has a validpublic void run() method, it does not implement the Runnable interface, so the compiler would complain when creating a Thread with an instance of it. Option 4 is incorrect because the run() method must be public.

Multiple choice

Which of the following methods is not defined in ResultSetMetaData interface?

  1. getColumnClassName(int column)

  2. getScale(int column)

  3. isCaseSensitive(int column)

  4. getColumnSize(int column)

  5. isSearchable(int column)
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

This is the correct answer because there is no method named getColumnSize(int column) in ResultSetMetaData interface. The correct method name is getColumnDisplaySize(int column), which indicates the designated column's normal maximum width in characters.

Multiple choice

Which of the following methods is not defined in DatabaseMetaData interface?

  1. allProceduresAreCallable()

  2. allTablesAreSelectable()

  3. autoCommitFailureClosesAllResultSets()

  4. dataDefinitionCausesTransactionCommit()

  5. allTablesSelected()
Reveal answer Fill a bubble to check yourself
E Correct answer
Explanation

This is the correct option because there is no method named allTablesSelected() in DatabaseMetaData. The correct method is allTablesAreSelectable() and it retrieves whether the current user can call all the procedures returned by the method getProcedures or not.