Java Programming Lesson 6 Part-2 | String Class & Methods Explained

LESSON - 6: STRINGS

String Class & Methods (Practical + Clear Concepts)

✅ Here is Lesson-6: Strings — String Class & Methods (Practical + Clear Concepts)

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📘 Lesson-6: Strings in Java

✅ What is a String?

A String is a sequence of characters.
In Java, String is a Class in java.lang package and is immutable (cannot be changed once created).

Example:

String name = "Java";

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✅ Ways to Create Strings:

✅ 1️⃣ Using String Literal (Recommended)

String s1 = "Hello";

✅ 2️⃣ Using new keyword

String s2 = new String("Hello");

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✅ Common Useful String Methods

Method	Description	Example
length()	Count characters	s.length()
charAt()	Returns character at index	s.charAt(0)
toUpperCase()	Convert to uppercase	s.toUpperCase()
toLowerCase()	Convert to lowercase	s.toLowerCase()
contains()	Check substring	s.contains("Java")
equals()	Compares values	s1.equals(s2)
equalsIgnoreCase()	Compare without case sensitivity	s1.equalsIgnoreCase(s2)
trim()	Removes side spaces	" Java ".trim()
substring()	Extract part of string	s.substring(1,4)
replace()	Replace characters	s.replace('a','o')

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✅ Example Program

public class Main {
    public static void main(String[] args) {
        String txt = "  Java Programming  ";

        System.out.println("Original: '" + txt + "'");
        System.out.println("Uppercase: " + txt.toUpperCase());
        System.out.println("Lowercase: " + txt.toLowerCase());
        System.out.println("Trimmed: '" + txt.trim() + "'");
        System.out.println("Length: " + txt.length());
        System.out.println("Substring(2, 6): " + txt.substring(2, 6));
        System.out.println("Replace: " + txt.replace("Java", "Hello"));
        System.out.println("CharAt(4): " + txt.charAt(4));
        System.out.println("Contains 'Pro': " + txt.contains("Pro"));
    }
}

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✅ String Comparison

❌ Wrong: == → compares memory location
✅ Correct: .equals() → compares content

String a = "Java";
String b = "Java";
System.out.println(a == b);       // false (sometimes true but unsafe)
System.out.println(a.equals(b)); // ✅ true

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✅ Mutable Alternatives (StringBuilder / StringBuffer)

When working with many modifications, use:

StringBuilder sb = new StringBuilder("Hello");
sb.append(" Java");
System.out.println(sb);

✅ Faster than creating new Strings every time!

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📝 Exercise for You

1️⃣ Take your name as input
2️⃣ Print:

• Uppercase

• Length

• First 3 characters (substring)

• last character (charAt)

Example Output:

Enter name: JITU
UPPER: JITU
Length: 4
SUBSTRING: JIT
Last Char: U

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✅ Lesson-6 Completed!

➡ NEXT Lesson-7 PART-1 Strings (String class & methods)