Introduction

String comparison in Java is an essential operation when working with text-based data. However, strings in Java are objects, not primitive types, so you need to understand how Java compares them. There are several methods available to compare strings in Java, each with its purpose and use case.

In this guide, we’ll walk through the common methods of comparing strings, including equals(), compareTo(), and the == operator, and explain when to use each.

1. Using the equals() Method

The equals() method is used to compare the actual contents of two strings. It checks if the values inside both strings are identical, and it returns true if they are equal and false otherwise.

Example:

String str1 = "hello";
String str2 = "hello";
String str3 = "world";

System.out.println(str1.equals(str2)); // true
System.out.println(str1.equals(str3)); // false

• When to use: Always use equals() when comparing the content of two strings. The equals() method is case-sensitive, so "Hello" and "hello" will not be considered equal.

2. Using the == Operator

The == operator compares the reference (memory address) of the two string objects, not the content. This means it checks if both string variables point to the exact same object in memory.

Example:

String str1 = "hello";
String str2 = "hello";
String str3 = new String("hello");

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

• When to use: You should rarely use == to compare string content. It’s mostly used when you are sure the strings refer to the same object in memory (such as with string literals or interned strings).

3. Using the compareTo() Method

The compareTo() method compares two strings lexicographically (i.e., based on the Unicode value of each character in the string). It returns:

• 0 if the strings are equal.
• A negative integer if the first string is lexicographically less than the second string.
• A positive integer if the first string is lexicographically greater than the second string.

Example:

String str1 = "apple";
String str2 = "banana";
String str3 = "apple";

System.out.println(str1.compareTo(str2)); // Negative value (apple < banana)
System.out.println(str1.compareTo(str3)); // 0 (equal)
System.out.println(str2.compareTo(str1)); // Positive value (banana > apple)

• When to use: Use compareTo() when you need to determine the lexicographical order of strings, such as sorting them alphabetically.

4. Using the equalsIgnoreCase() Method

The equalsIgnoreCase() method compares two strings without considering case sensitivity. It returns true if the strings are equal ignoring case differences, and false otherwise.

Example:

String str1 = "hello";
String str2 = "HELLO";

System.out.println(str1.equalsIgnoreCase(str2)); // true

• When to use: Use equalsIgnoreCase() when you want to compare two strings and don’t care about case sensitivity (e.g., user input comparisons).

5. Using the regionMatches() Method

The regionMatches() method is used to compare specific parts (regions) of two strings. This method allows you to specify a starting position and compare substrings.

Example:

String str1 = "hello world";
String str2 = "world";

System.out.println(str1.regionMatches(6, str2, 0, 5)); // true

• When to use: Use regionMatches() when you need to compare a substring of one string with a substring of another string.

Conclusion

String comparison is a fundamental operation in Java, and it's important to understand the various methods available to do it properly. The equals() method is the most common and should be used for comparing string content. The == operator is generally used to check reference equality, while compareTo() helps when determining the lexicographical order of strings. 

Additionally, methods like equalsIgnoreCase() and regionMatches() offer specialized comparisons depending on the use case. By using these methods correctly, you can ensure your Java code handles string comparisons accurately and efficiently.