Java Strings - Basics and Operations

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Questions and Answers

What is the most direct way to create a string in Java?

String greeting = new String("Hello world!");

String greeting = 'Hello world!'; (correct)

String greeting = String('Hello world!');

String greeting = new String('Hello world!');

How does the Java compiler handle string literals?

It does not recognize them during compilation.

It creates a StringBuilder object from them.

It treats them as primitive types.

It creates a String object automatically. (correct)

Which method is used to find the length of a string in Java?

length() (correct)

getLength()

lengthof()

size()

What will be the output of the following code? 'String name = "I am in BCA"; int len = name.length(); System.out.println(len);'

String Length is: 11 Signup and view all the answers

What is the result of the following operation: 'string1.concat(string2)'?

It returns a new string with string1 and string2 concatenated. Signup and view all the answers

What is the initial capacity of a newly created StringBuffer instance without any arguments?

16 Signup and view all the answers

How can you create a String object using an array of characters?

String myString = new String(charArray); Signup and view all the answers

What will be the output of the command 'buff.setLength(6)' if buff is initialized as new StringBuffer('ManeshPatel')?

Manesh Signup and view all the answers

What would be the output of the following code: 'char helloArray[] = { 'H', 'e', 'l', 'l', 'o', '.' }; String helloString = new String(helloArray); System.out.println(helloString);'?

Hello. Signup and view all the answers

Which constructor of the String class allows the use of a character array to create a string?

String(char[]) Signup and view all the answers

What does the charAt() method of StringBuffer return?

The character at the specified index Signup and view all the answers

If 'br' is a StringBuffer initialized with 'Patel Manesh', what is the result of 'br.charAt(6)'?

M Signup and view all the answers

What happens when you use the setCharAt() method on a StringBuffer?

It sets a character at a specified index Signup and view all the answers

Which output corresponds to System.out.println(buff.length()) right after initializing buff with 'ManeshPatel'?

11 Signup and view all the answers

After executing buff.setLength(6) on buff initialized with 'ManeshPatel', what will the content of buff be?

Manesh Signup and view all the answers

If the current content of StringBuffer br is 'Hello World' and you call br.setCharAt(6, 'J'), what will the new content be?

Hello Jorld Signup and view all the answers

What is the output of the following code: String s = "I am in BCA"; s = s.concat (" Shayona Campus"); System.out.println(s);

I am in BCA Shayona Campus Signup and view all the answers

What does the method endsWith do when applied to a string?

Checks if the string ends with a specified substring Signup and view all the answers

What will be the result of calling indexOf on the string "Welcome to Shayona BCA College" with 'o'?

7 Signup and view all the answers

Which of the following statements about the StringBuffer class is true?

StringBuffer is preferred for frequent string modifications. Signup and view all the answers

What is the initial capacity of a StringBuffer created with StringBuffer()?

16 characters Signup and view all the answers

What will be the output of System.out.println(s6.charAt(8)); if s6 is "Patel Manesh kumar"?

M Signup and view all the answers

What does the replace method do in the context of the String "Welcome to MissionClasses.com"?

Replaces all occurrences of 'o' with 'M' Signup and view all the answers

Which of these methods can be used to determine if a String variable starts with a specific prefix?

startsWith() Signup and view all the answers

What is the primary purpose of creating a custom exception in Java?

To handle specific exceptions with tailored solutions Signup and view all the answers

In the provided code, what is the output when an OwnException is caught?

Caught a user defined exception Signup and view all the answers

Which statement is true regarding the class OwnException?

It allows passing a custom error message upon instantiation Signup and view all the answers

Why might a developer need to create a business logic exception?

To manage exceptions related to specific workflows and logic Signup and view all the answers

What happens when the line 'throw obj;' is executed in the main method?

The exception is thrown and caught in the next catch block Signup and view all the answers

Which statement accurately describes unchecked exceptions in Java?

They can occur at runtime without special handling. Signup and view all the answers

What is the purpose of the 'try' keyword in Java exception handling?

To specify blocks of code for exception handling. Signup and view all the answers

What is the function of the 'finally' block in exception handling?

To execute important code regardless of whether an exception occurred. Signup and view all the answers

Which of the following exceptions occurs when attempting to divide by zero in Java?

ArithmeticException Signup and view all the answers

What keyword is used to declare that a method may throw exceptions?

throws Signup and view all the answers

Which statement correctly identifies a scenario that throws a NullPointerException?

Accessing a method on a null variable. Signup and view all the answers

In the context of Java exceptions, what does the term 'unchecked' signify?

Exceptions that are not required to be caught or declared. Signup and view all the answers

What exception is thrown when there is a mismatch in number formatting?

NumberFormatException Signup and view all the answers

Study Notes

Creating Strings

In Java, strings are treated as objects and can be created using the String class.
The most direct way to create a string is by assigning a string literal to a variable.
For example, String greeting = "Hello world!"; declares a string variable greeting and assigns the value "Hello world!" to it.
The compiler automatically creates a String object with the specified value when it encounters a string literal.
You can also create String objects using the new keyword and a constructor from the String class.
The String class offers eleven constructors that allow you to initialize the string with values from various sources, such as character arrays.

String Length

The length() method can be used to determine the number of characters in a string.
It returns an integer value representing the string's length.
Use it like this: int len = stringVariable.length();

Concatenating Strings

The concat() method is used to combine two strings.
It takes another string as an argument and returns a new string containing both strings concatenated.
For example, string1.concat(string2) results in a new string that has string2 added to the end of string1.

String Operations (Example Code)

The provided code demonstrates various string operations and methods:
- Concatenating strings using the concat() method, e.g., s = s.concat(" Shayona Campus");
- Checking if a string ends with a specific substring using endsWith(), e.g., retVal = Str.endsWith("immutable");
- Finding the first occurrence of a character within a string using indexOf(), e.g., System.out.println(Str2.indexOf('o'));
- Getting the length of a string using length(), e.g., System.out.println(Str3.length());
- Replacing characters in a string using replace(), e.g., System.out.println(Str5.replace('o', 'M'));
- Accessing a specific character in a string using charAt(), e.g., char result = s6.charAt(8);
- Checking if a string starts with a specific substring using startsWith(), e.g., System.out.println(s6.startsWith("Patel"));
- Converting the case of a string using toLowerCase() and toUpperCase(), e.g., System.out.println(s6.toLowerCase());

StringBuffer Class

The StringBuffer class creates mutable string objects, meaning the contents of the string can be changed after creation.
It represents a growable and writable character sequence.
Since String objects are immutable, using StringBuffer is advantageous when performing frequent modifications to string content, preventing memory leaks that can arise from creating numerous immutable string objects.
Four constructors are provided by StringBuffer:
- StringBuffer(): Creates an empty string buffer with a capacity of 16 characters.
- StringBuffer(int size): Creates a string buffer with the given initial capacity.
- StringBuffer(String str): Creates a string buffer initialized with the specified string.
- StringBuffer(charSequence[] ch): Creates a string buffer initialized with the given character array.

setLength() Method

The setLength() method modifies the length of a StringBuffer object.
If the new length is less than the current length, the string buffer is truncated.
If the new length is greater than the current length, the string buffer is extended with null characters.

charAt() Method

The charAt() method returns the character at a specified index within a StringBuffer object.
The index is passed as an argument to the method.

setCharAt() Method

The setCharAt() method allows changing the character at a specific index within a StringBuffer object.
The index and the new character to be inserted are passed as arguments to the method.

Java Exceptions

Exceptions are runtime errors that can occur during program execution.
Java provides a mechanism for handling exceptions using the try, catch, and finally keywords.

Types of Exceptions

Checked Exceptions: Are compiled-time errors that must be handled explicitly using try-catch blocks.
Unchecked Exceptions: Are runtime errors that don't require explicit handling, but can still cause program termination if not managed.

Exception Keywords

try: Defines a block of code that might throw an exception.
catch: Used to handle specific types of exceptions thrown in the try block.
finally: Executes a block of code regardless of whether an exception occurred or was caught.
throw: Manually throws an exception object, allowing for controlled error reporting.
throws: Declares that a method might throw a particular exception, allowing the calling code to handle it properly.

Scenario for ArithmeticException

Dividing a number by zero results in an ArithmeticException, for example: int a = 50/0;.

Scenario for NullPointerException

Accessing members of a null variable results in a NullPointerException, for example: String s = null; System.out.println(s.length());.

Scenario for NumberFormatException

Attempting to convert a string to a number with incorrect formatting results in a NumberFormatException, for example: int number = Integer.parseInt("abc");.

Custom Exceptions

Custom exceptions, also known as user-defined exceptions, are created by extending the Exception class.
They allow you to create specific exceptions for your application's custom business logic.

Why Use Custom Exceptions?

To specifically catch and handle a subset of existing Java exceptions.
To represent exceptions related to business logic and workflows.
To provide informative error messages for application users and developers.

Example of a Custom Exception

The provided code creates a custom exception class named OwnException and demonstrates throwing and catching it within a try-catch block.
This allows for controlled error handling that is specific to the custom exception scenario.

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Description

This quiz covers the essential concepts of strings in Java, including creation, length determination, and concatenation methods. Test your knowledge on how strings are treated as objects, and learn about the various constructors available in the String class. Perfect for beginners looking to understand Java string handling.