Find Length, Concatenate and Replace String in Java Program

class StringMethods {
  public static void main(String args[]) {
    int n;
    String s = "Java programming", t = "", u = "";
    System.out.println(s);
    // Find length of string
    n = s.length();
    System.out.println("Number of characters = " + n);
    // Replace characters in string
    t = s.replace("Java", "C++");
    System.out.println(s);
    System.out.println(t);
    // Concatenating string with another string
    u = s.concat(" is fun");
    System.out.println(s);
    System.out.println(u);
  }
}

Step-by-Step Breakdown:

1. Class and Main Method Declaration:

class StringMethods {

    public static void main(String args[]) {

A class StringMethods is declared.

The main is the point at which the program gets executed.

2. String Initialization:

int n;

String s = “Java programming”, t = “”, u = “”;

• A string s is initialized with the value “Java programming”.
• Two more string variables t and u are initialized as empty strings (“”).
• The variable n is declared to hold the length of the string.

3. Print the Original String:

System.out.println(s);

The string s is printed. 

Output will be: 

Java programming

4. Counting the Length of the String:

n = s.length();

System.out.println(“Number of characters = ” + n);

• The method length() is invoked for the string s. It returns the count of characters within the string. This count also includes spaces.
• The count of Java programming is 17 characters.
• The program prints: 

Number of characters = 17

5. Replacing Part of the String:

t = s.replace(“Java”, “C++”);

System.out.println(s);

System.out.println(t);

• The replace() method is invoked on string s. It replaces the substring “Java” with “C++”.
• The result is assigned to t.
• s is printed again, which is still “Java programming” because strings in Java are immutable, meaning the original string is not modified.
• t is printed, which now holds the value “C++ programming”.

The output will be:

Java programming

C++ programming

6. Concatenating Another String:

u = s.concat(” is fun”);

System.out.println(s);

System.out.println(u);

• The concat() method is used to append the string  ” is fun” to the original string s. This means to create a new string and store it in u.
• s is again printed, which is  “Java programming”.
• u is printed, which now contains the value  “Java programming is fun”.
• The above line of console will print the following output.

Java programming

Java programming is fun

Program Output:

Here is the complete output from the program.

Java programming

Number of characters = 17

Java programming

C++ programming

Java programming

Java programming is fun

Summary:

• length(): This method returns the number of characters in a string, including spaces.
• replace(): This method creates a new string where a specified substring is replaced by another substring.
• concat(): This method concatenates (joins) a given string to the end of another string.
• String Is Immutable: String in Java are immutable. Any method, replace(), concat() etc return another string; thus, s itself cannot be modified in the method invocation.