1. Remove trailing space (password issue)
The trim() method removes unwanted spaces from the beginning and end of a string. It is commonly used to clean user input.
String pwd = " Admin123 ";
pwd = pwd.trim();
System.out.println(pwd);
2. Compare username safely (ignore case)
The equalsIgnoreCase() method compares two strings while ignoring case differences. It is useful for username validation.
String s1 = "Vijay";
String s2 = "vijay";
System.out.println(s1.equalsIgnoreCase(s2));
3. == vs equals()
== compares memory references, whereas equals() compares actual values. This is a very important concept for interviews.
String s1 = "Java";
String s2 = new String("Java");
System.out.println(s1 == s2); // false (memory check)
System.out.println(s1.equals(s2)); // true (value check)
4. Check email contains '@' and '.'
The contains() method checks whether a string contains a specific substring. It is useful in validations like email checking.
String email = "test@gmail.com";
if(email.contains("@") && email.contains(".")) { System.out.println("Valid");
}
else {
System.out.println("invalid");
}
5. Convert String to int
The Integer.parseInt() method converts a string into an integer. It is commonly used in form input processing.
String num = "12345";
int n = Integer.parseInt(num);
System.out.println(n+20);
6. Remove spaces only start & end
The split() method divides a string into multiple parts based on a delimiter. It is widely used in CSV parsing and data processing.
String str = " Welcome to Java ";
System.out.println(str.trim());
7. Count "Java" occurrences
The length() method returns the length of a string. It is very useful when working with loops.
String str = "Java is easy. Java is powerful.";
int count = 0;
String[] words = str.split(" ");
for(String w : words){
if(w.equals("Java")) count++;
}
System.out.println(count);
8. Reverse string (without StringBuilder)
The charAt() method returns the character at a specific index. It is used for string traversal.
String str = "Java";
String rev = "";
for(int i = str.length()-1; i >= 0; i--){
rev += str.charAt(i);
}
System.out.println(rev);
## 9. Check palindrome
The indexOf() method returns the position of a character or substring. It is useful for checking duplicates.
String str = "madam";
String rev = "";
for(int i = str.length()-1; i >= 0; i--){
rev += str.charAt(i);
}
if(str.equals(rev))
System.out.println("Palindrome:"+rev);
else
System.out.println("Not Palindrome");
10. Remove duplicate characters
The equals() method compares the actual content of two strings. It is commonly used in palindrome checks.
String str = "aabbccdd";
String result = "";
for(int i=0; i<str.length(); i++){
char c = str.charAt(i);
if(result.indexOf(c) == -1){
result += c;
}
}
System.out.println(result);
11. Extract message from log
The replace() method replaces one part of a string with another. It is useful for modifying text.
String log = "ERROR: File not found";
String result = log.split(":")[1].trim();
System.out.println(result);
12. CSV extract
The matches() method uses regular expressions to validate patterns. It is commonly used for mobile number or password validation.
String data = "Vijay,25,Chennai";
String[] arr = data.split(",");
System.out.println("Name: " + arr[0]);
System.out.println("Age: " + arr[1]);
System.out.println("City: " + arr[2]);
13. Replace word
The + operator is used to combine strings. It is often used in building new strings like in reverse logic.
String str = "I love Java programming";
str = str.replace("Java", "Spring Boot");
System.out.println(str);
14. Validate mobile number (10 digits)
After using split(), array indexing is used to access individual values.
String num = "9876543210";
if(num.matches("\\d{10}")){
System.out.println("Valid");
}else{
System.out.println("Invalid");
}
15. Split string
Loops are used to traverse strings. This is the base for solving most string problems.
String str = "apple#banana#mango";
String[] arr = str.split("#");
for(String s : arr){
System.out.println(s);
}
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