In Java, strings are compared using different methods depending on the type of comparison you want to perform. Here’s a detailed guide with examples:

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1. Using equals() for Content Comparison

The equals() method compares the actual content of two strings for equality.

Example:

public class StringComparison {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "Hello";
        String str3 = "hello";

        // Case-sensitive content comparison
        System.out.println(str1.equals(str2)); // true
        System.out.println(str1.equals(str3)); // false
    }
}

Key Points:

• equals() is case-sensitive.
• Returns true if the content is the same; otherwise, false.

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2. Using equalsIgnoreCase() for Case-Insensitive Comparison

The equalsIgnoreCase() method compares strings, ignoring case differences.

Example:

public class StringComparison {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "hello";

        // Case-insensitive content comparison
        System.out.println(str1.equalsIgnoreCase(str2)); // true
    }
}

Key Points:

• Ignores case while comparing strings.
• Useful for case-insensitive checks like user inputs.

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3. Using compareTo() for Lexicographical Comparison

The compareTo() method compares strings lexicographically (dictionary order).

Example:

public class StringComparison {
    public static void main(String[] args) {
        String str1 = "Apple";
        String str2 = "Banana";
        String str3 = "Apple";

        // Lexicographical comparison
        System.out.println(str1.compareTo(str2)); // Negative (-1 because "Apple" < "Banana")
        System.out.println(str1.compareTo(str3)); // Zero (0 because "Apple" == "Apple")
        System.out.println(str2.compareTo(str1)); // Positive (1 because "Banana" > "Apple")
    }
}

Key Points:

• Returns:
  • 0 if the strings are equal.
  • Negative value if the first string is lexicographically less than the second.
  • Positive value if the first string is lexicographically greater than the second.

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4. Using compareToIgnoreCase() for Case-Insensitive Lexicographical Comparison

The compareToIgnoreCase() method compares strings lexicographically, ignoring case differences.

Example:

public class StringComparison {
    public static void main(String[] args) {
        String str1 = "Apple";
        String str2 = "apple";

        // Lexicographical comparison ignoring case
        System.out.println(str1.compareToIgnoreCase(str2)); // 0 (ignores case)
    }
}

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5. Using == for Reference Comparison

The == operator checks if two string references point to the same memory location.

Example:

public class StringComparison {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "Hello"; // Points to the same string in the string pool
        String str3 = new String("Hello"); // Creates a new string object

        // Reference comparison
        System.out.println(str1 == str2); // true (same reference)
        System.out.println(str1 == str3); // false (different references)
    }
}

Key Points:

• == compares object references, not content.
• Use equals() for content comparison instead of ==.

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6. Checking if a String Contains Another String

Use the contains() method to check if one string contains another.

Example:

public class StringComparison {
    public static void main(String[] args) {
        String str1 = "Hello, world!";
        String str2 = "world";

        // Check if str1 contains str2
        System.out.println(str1.contains(str2)); // true
    }
}

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7. Checking if a String Starts or Ends with a Substring

Use startsWith() or endsWith() for this purpose.

Example:

public class StringComparison {
    public static void main(String[] args) {
        String str = "Hello, world!";

        // Check starting and ending
        System.out.println(str.startsWith("Hello")); // true
        System.out.println(str.endsWith("world!")); // true
    }
}

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8. Checking for Null or Empty Strings

Use isEmpty() or isBlank() (Java 11+).

Example:

public class StringComparison {
    public static void main(String[] args) {
        String str1 = "";
        String str2 = "   ";

        // Check for empty or blank strings
        System.out.println(str1.isEmpty()); // true
        System.out.println(str2.isEmpty()); // false
        System.out.println(str2.isBlank()); // true (Java 11+)
    }
}

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Best Practices for String Comparison

1. Always use equals() or equalsIgnoreCase() for content comparison.
2. Avoid using == for string comparison unless checking references.
3. Use compareTo() or compareToIgnoreCase() for sorting or lexicographical comparisons.
4. Handle null strings to avoid NullPointerException.