String Methods in Java

String Methods in Java

In Java, String is one of the most frequently used classes.
Almost every Java program deals with text — names, input, passwords, messages, file data, etc.

Java provides many built-in String methods to perform operations like:

• Finding length
• Comparing strings
• Extracting characters
• Modifying text
• Searching content

👉 All these methods belong to the String class.

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Important Thing to Remember About Strings ⚠️

Strings in Java are immutable.

This means:

• Once a String object is created, it cannot be changed
• Any operation on a String creates a new String

String s = "Java";
s.concat(" Language");

System.out.println(s);

Output:

Java

Why?
Because concat() creates a new String, but we didn’t store it.

Correct way:

s = s.concat(" Language");
System.out.println(s);

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length() Method

Returns the number of characters in a string.

String name = "Ajay";
System.out.println(name.length());

Output:

4

📌 Important:

• length() is a method
• For arrays, we use length (without parentheses)

❌ Common confusion:

name.length;   // ❌ error

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charAt() Method

Returns the character at a given index.

String word = "Java";
System.out.println(word.charAt(0));
System.out.println(word.charAt(2));

Output:

J
v

📌 Index starts from 0.

❌ Common mistake:

word.charAt(4); // ❌ StringIndexOutOfBoundsException

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toUpperCase() and toLowerCase()

Used to change case.

String text = "Java Programming";

System.out.println(text.toUpperCase());
System.out.println(text.toLowerCase());

Output:

JAVA PROGRAMMING
java programming

⚠️ Original string remains unchanged.

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trim() Method

Removes leading and trailing spaces.

String input = "   Java   ";
System.out.println(input.trim());

Output:

Java

📌 Does NOT remove spaces between words.

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equals() Method

Used to compare content of two strings.

String a = "Java";
String b = "Java";

System.out.println(a.equals(b));

Output:

true

✅ Always use equals() for content comparison.

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Common Mistake: == vs equals()

String a = new String("Java");
String b = new String("Java");

System.out.println(a == b);       // false
System.out.println(a.equals(b));  // true

Explanation:

• == compares memory references
• equals() compares actual content

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equalsIgnoreCase()

Ignores case while comparing.

String a = "java";
String b = "JAVA";

System.out.println(a.equalsIgnoreCase(b));

Output:

true

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contains() Method

Checks if a string contains a sequence.

String sentence = "Java is powerful";

System.out.println(sentence.contains("Java"));
System.out.println(sentence.contains("python"));

Output:

true
false

📌 Case-sensitive.

---

startsWith() and endsWith()

String file = "report.pdf";

System.out.println(file.startsWith("rep"));
System.out.println(file.endsWith(".pdf"));

Output:

true
true

Very useful in:

• File handling
• URL validation
• Input checking

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indexOf() Method

Returns index of first occurrence.

String text = "Java Programming";

System.out.println(text.indexOf("a"));
System.out.println(text.indexOf("Program"));

Output:

1
5

If not found:

System.out.println(text.indexOf("Python")); // -1

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lastIndexOf() Method

Returns index of last occurrence.

String text = "banana";
System.out.println(text.lastIndexOf("a"));

Output:

5

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substring() Method

Extracts part of a string.

String word = "Programming";

System.out.println(word.substring(0, 7));

Output:

Program

📌 End index is exclusive.

Another example:

System.out.println(word.substring(3));

Output:

gramming

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replace() Method

Replaces characters or words.

String msg = "Java is hard";

System.out.println(msg.replace("hard", "easy"));

Output:

Java is easy

📌 Creates a new string.

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isEmpty() Method

Checks if string length is 0.

String s = "";

System.out.println(s.isEmpty());

Output:

true

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isEmpty() vs null Confusion ⚠️

String s = null;
s.isEmpty(); // ❌ NullPointerException

Correct check:

if (s != null && !s.isEmpty()) {
    System.out.println("Valid string");
}

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isBlank() Method (Java 11+)

Checks if string is empty or contains only spaces.

String s = "   ";

System.out.println(s.isBlank());

Output:

true

Difference:

• isEmpty() → length = 0
• isBlank() → spaces allowed

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split() Method

Splits a string into an array.

String line = "Java,Python,C++";
String[] langs = line.split(",");

for (String lang : langs) {
    System.out.println(lang);
}

Output:

Java
Python
C++

📌 Very useful for CSV data.

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valueOf() Method

Converts other data types into String.

int num = 100;
String s = String.valueOf(num);

System.out.println(s);

Output:

100

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compareTo() Method

Used for lexicographical comparison.

String a = "Apple";
String b = "Banana";

System.out.println(a.compareTo(b));

Output:

negative number

Meaning:

• 0 → equal
• Negative → first comes before
• Positive → first comes after

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Common String Errors & Mistakes 🚫

1. Forgetting immutability
2. Using == instead of equals()
3. Calling methods on null
4. Confusing length and length()
5. Ignoring case sensitivity
6. Assuming methods modify original string

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Quick String Methods Summary 📌

• length() → size
• charAt() → character access
• equals() → content comparison
• substring() → extract
• replace() → replace content
• split() → convert to array
• trim() → remove spaces
• contains() → search
• isEmpty() / isBlank() → validation