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

String s = “Welcome To Java” s.lastIndexOf(“Java”,5);

What is the result of the given statements?
    • 1

  2. 11

  3. 6

  4. Compiler error : method lastIndexOf() not declared

  5. Compiler error : parameters to method not sufficient
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

lastIndexOf(String s, index i) method return the last occurrence of string "s" before the index value i. it Return -1 if string not found. So here the string to find is “Java” before the index value 5 which means before the character "m". but string “java” is not present before the index 5 i.e character "m" .so it will return -1. So this is the correct answer.

Multiple choice

Which of the following methods cannot be used with the object of String class?

  1. replace(char, char)

  2. replaceAll(String, String)

  3. append(String)

  4. Both (1) and (2)

  5. All of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

This is the correct answer. Because the String class objects are immutable (i.e once the object of the String class is created its contents cannot be changed). So we cannot append new characters or string in the old string. So this is the correct answer.

Multiple choice

StringBuffer s = new StringBuffer(“Hello”); s.capacity();

What will be returned by the function capacity()?

  1. 16

  2. 4

  3. 18

  4. 3

  5. 15
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The correct answer is 16. When stringbuffer object is created the default capacity of the object is always 16. If the number of characters in the string increases, the capacity will increase with the following formula:-2 * (previous capacity + 1). In this case hello has 5 characters, so the default capacity will not change. Hence, this is correct.

Multiple choice

Which of the following are not the Subclasses of RuntimeException?

  1. IllegalArgumentException

  2. NullPointerException

  3. IOException

  4. IndexOutOfBoundException

  5. ArithmeticException
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

IOException is not a subclass of RuntimeException class, this class is a Subclass of Exception class. This exception is thrown when invalid input/output are given. So, this is the correct answer as this is not a subclass of RuntimeException class.

Multiple choice

Which of the following methods cannot be invoked from the instance of Frame class in AWT?

  1. setLocation(int, int)

  2. setVisible(Boolean)

  3. setSize(int, int)

  4. setDefaultCloseOperation(int)

  5. Pack()
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

This function is not defined in the Frame class. This function is defined in JFrame class in Swings. So, this method cannot be invoked using the object of Frame class. But can be invoked using the Object of JFrame class. This method specifies the operation when the frame is closed. So, the following line would be false. Frame f1 = new frame(); f1.setDefaultCloseOperation(int); so, this is the correct answer.

Multiple choice

Which of the following methods is/are not necessary to define for a class if it implement the windowListener Interface?

  1. windowMoved()

  2. windowClosing()

  3. windowIconified()

  4. windowActivated()

  5. windowDeiconified()
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

If a class implements an interface then it is necessary for that class to define body to all the methods defined in the interface it is implementing. But, windowMoved() method is not present in windowListener interface. So, when a class will implement this interface this method need not to be define in the class. So, this option is correct.

Multiple choice

How many parameters are passed to the constructor of the Font class in java?

  1. 5

  2. 4

  3. 3

  4. 2

  5. 1
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

This is correct choice because the constructor of the Font class takes Three parameters:- First:- first parameter is of String type which specifies the name of the Font. Second:- second parameter is of type int which specifies the style of the font like Font.BOLD. Third:- third parameter is of type int which specifies the size of the font. ExampleFont f1 = new Font(“Serif”, Font.BOLD, 32); So, this option is true.