String Class in Java

Java String Class

The String class in Java is a fundamental part of the language, providing numerous built-in methods to manipulate and analyze string data. Below is an overview of some common String methods with examples.

Character Access

charAt(int index)

• Description: Returns the character at the specified index.
• Example:

  String str = "Hello";
  char ch = str.charAt(1); // 'e'

String Comparison

equals(Object anObject)

• Description: Compares this string to the specified object.
• Example:

  String str1 = "Hello";
  String str2 = "Hello";
  boolean isEqual = str1.equals(str2); // true

equalsIgnoreCase(String anotherString)

• Description: Compares this string to another string, ignoring case considerations.
• Example:

  String str1 = "Hello";
  String str2 = "hello";
  boolean isEqualIgnoreCase = str1.equalsIgnoreCase(str2); // true

String Search

contains(CharSequence s)

• Description: Returns true if and only if this string contains the specified sequence of char values.
• Example:

  String str = "Hello World";
  boolean contains = str.contains("World"); // true

indexOf(int ch)

• Description: Returns the index within this string of the first occurrence of the specified character.
• Example:

  String str = "Hello";
  int index = str.indexOf('e'); // 1

indexOf(String str)

• Description: Returns the index within this string of the first occurrence of the specified substring.
• Example:

  String str = "Hello World";
  int index = str.indexOf("World"); // 6

lastIndexOf(String str)

• Description: Returns the index within this string of the last occurrence of the specified substring.
• Example:

  String str = "Hello World, World!";
  int index = str.lastIndexOf("World"); // 13

String Modification

concat(String str)

• Description: Concatenates the specified string to the end of this string.
• Example:

  String str1 = "Hello";
  String str2 = "World";
  String result = str1.concat(str2); // "HelloWorld"

replace(char oldChar, char newChar)

• Description: Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.
• Example:

  String str = "Hello";
  String newStr = str.replace('l', 'L'); // "HeLLo"

substring(int beginIndex)

• Description: Returns a new string that is a substring of this string.
• Example:

  String str = "Hello World";
  String subStr = str.substring(6); // "World"

toLowerCase()

• Description: Converts all of the characters in this String to lower case.
• Example:

  String str = "Hello";
  String lowerStr = str.toLowerCase(); // "hello"

toUpperCase()

• Description: Converts all of the characters in this String to upper case.
• Example:

  String str = "Hello";
  String upperStr = str.toUpperCase(); // "HELLO"

trim()

• Description: Returns a string whose value is this string, with any leading and trailing whitespace removed.
• Example:

  String str = "  Hello World  ";
  String trimmedStr = str.trim(); // "Hello World"

String Conversion

valueOf(Object obj)

• Description: Converts different types of values into a string.
• Example:

  String number = String.valueOf(19); // "19"

toString()

• Description: Returns the string representation of the object.
• Example:

  Integer num = 10;
  String str = num.toString(); // "10"

String Concatenation

Using + Operator

• Description: Concatenates two strings using the + operator.
• Example:

  String str1 = "Hello";
  String str2 = "World";
  String result = str1 + str2; // "HelloWorld"

Using concat() Method

• Description: Concatenates two strings using the concat() method.
• Example:

  String str1 = "Hello";
  String str2 = "World";
  String result = str1.concat(str2); // "HelloWorld"

String Splitting

split(String regex)

• Description: Splits this string around matches of the given regular expression.
• Example 1:

  String str = "Hello World";
  String[] result = str.split(" "); // ["Hello", "World"]

• Example 2:

  String str = "apple,banana;cherry#grape$orange";
  String[] result = str.split("[,;#$]"); // ["apple", "banana", "cherry", "grape", "orange"]

• Example 3:

  String str = "one:two:three:four";
  String[] result = str.split(":", 3); // ["one", "two", "three:four"]

Retrieving Individual Characters in a String

• Do not use: message[0]
• Use: message.charAt(index)
• Note: Index starts from 0.

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Access Functions

• String is an array of characters, but we can’t access it using s[0].
• Use: charAt(index) function. Certainly! The toString() method in Java is often overridden to provide a string representation of an object. This allows the object to be printed directly using System.out.println(), which internally calls the toString() method. Here’s a corrected and improved version of your code snippet with an explanation:

class Student {
    String name;
    int rollNo;
 
    // Constructor to initialize the Student object
    Student(String name, int rollNo) {
        this.name = name;
        this.rollNo = rollNo;
    }
 
    // Overriding the toString() method to provide a string representation of the Student object
    @Override
    public String toString() {
        return "Student{name=" + name + ", rollNo=" + rollNo + "}";
    }
 
    public static void main(String[] args) {
        // Creating instances of Student
        Student stu1 = new Student("xxx", 10);
        Student stu2 = new Student("yyy", 20);
 
        // Printing the Student objects directly, which calls the overridden toString() method
        System.out.println("Student 1: " + stu1);
        System.out.println("Student 2: " + stu2);
    }
}

Explanation

• toString() Method: The toString() method is overridden in the Student class to return a string that represents the object’s state. This method is called automatically when an object is passed to System.out.println().

• Direct Printing: When System.out.println("Student 1: " + stu1); is executed, Java internally calls stu1.toString() to get the string representation of stu1. This allows the object to be printed directly without explicitly calling toString().

• Output: The output of the above code will be:

  Student 1: Student{name=xxx, rollNo=10}
  Student 2: Student{name=yyy, rollNo=20}
  

This demonstrates how overriding toString() can be useful for providing meaningful string representations of objects, making it easier to print and debug object states.

References

• Authors:
• Date: 2025.02.17
• Time: 08:08