String comparison in Java

Introduction

We compare strings to check if two pieces of text are the same or to find their order.

Checking if a user entered the correct password.

Sorting a list of names alphabetically.

Finding if two words are equal in a game.

Comparing input commands in a program.

Syntax

Java

string1.equals(string2)
string1.equalsIgnoreCase(string2)
string1.compareTo(string2)

Use equals() to check if two strings have exactly the same characters.

Use equalsIgnoreCase() to compare strings ignoring uppercase or lowercase differences.

Use compareTo() to find the alphabetical order between two strings.

Examples

Checks if both strings are exactly the same.

Java

String a = "hello";
String b = "hello";
boolean result = a.equals(b);

Checks if strings are the same ignoring case differences.

Java

String a = "Hello";
String b = "hello";
boolean result = a.equalsIgnoreCase(b);

Returns a negative number because "apple" comes before "banana" alphabetically.

Java

String a = "apple";
String b = "banana";
int result = a.compareTo(b);

Sample Program

This program compares two strings using three methods and prints the results.

Java

public class Main {
    public static void main(String[] args) {
        String word1 = "Java";
        String word2 = "java";
        
        System.out.println("Using equals: " + word1.equals(word2));
        System.out.println("Using equalsIgnoreCase: " + word1.equalsIgnoreCase(word2));
        System.out.println("Using compareTo: " + word1.compareTo(word2));
    }
}

Important Notes

The equals() method is case sensitive.

The compareTo() method returns 0 if strings are equal, a negative number if the first string is alphabetically before the second, and a positive number if after.

Always use equals() or equalsIgnoreCase() to compare string content, not the == operator.

Summary

Use equals() to check if two strings are exactly the same.

Use equalsIgnoreCase() to ignore letter case when comparing.

Use compareTo() to find alphabetical order between strings.

Practice

1. Which method should you use in Java to check if two strings have exactly the same characters and case?
String s1 = "Hello";
String s2 = "hello";

easy

A. s1 == s2

B. s1.compareTo(s2)

C. s1.equalsIgnoreCase(s2)

D. s1.equals(s2)

Solution

1. Step 1: Understand string equality in Java

   Using == compares references, not content, so it is not reliable for string content comparison.

2. Step 2: Choose the method for exact match including case

   equals() checks if two strings have exactly the same characters and case, which is what is asked.

3. Final Answer:

   s1.equals(s2) -> Option D

4. Quick Check:

   Use equals() for exact string match [OK]

Hint: Use equals() for exact string match including case [OK]

Common Mistakes:

• Using == instead of equals() for string content comparison
• Using equalsIgnoreCase() when case matters
• Using compareTo() for equality check

2. Which of the following is the correct syntax to compare two strings s1 and s2 ignoring case in Java?

easy

A. s1.compareToIgnoreCase(s2)

B. s1.equals(s2)

C. s1.equalsIgnoreCase(s2)

D. s1 == s2

Solution

1. Step 1: Identify method to compare strings ignoring case

   Java provides equalsIgnoreCase() to compare strings ignoring letter case differences.

2. Step 2: Confirm syntax correctness

   s1.equalsIgnoreCase(s2) is the correct method call syntax.

3. Final Answer:

   s1.equalsIgnoreCase(s2) -> Option C

4. Quick Check:

   Use equalsIgnoreCase() to ignore case in string comparison [OK]

Hint: Use equalsIgnoreCase() to compare strings ignoring case [OK]

Common Mistakes:

• Using equals() when ignoring case is needed
• Using == for string comparison
• Using non-existent compareToIgnoreCase() method

3. What will be the output of the following Java code?

String a = "apple";
String b = "banana";
int result = a.compareTo(b);
System.out.println(result > 0 ? "a > b" : "a <= b");

medium

A. a > b

B. a <= b

C. 0

D. Compilation error

Solution

1. Step 1: Understand compareTo() behavior

   compareTo() returns a negative number if the first string is alphabetically before the second.

2. Step 2: Compare "apple" and "banana" alphabetically

   "apple" comes before "banana", so a.compareTo(b) returns a negative value, making result > 0 false.

3. Final Answer:

   a <= b -> Option B

4. Quick Check:

   compareTo() negative means first string is less [OK]

Hint: compareTo() negative means first string is alphabetically before second [OK]

Common Mistakes:

• Assuming compareTo() returns boolean
• Confusing positive and negative return values
• Expecting 0 when strings differ

4. The following code is intended to print "Strings are equal" if two strings match ignoring case. What is wrong?

String s1 = "Java";
String s2 = "java";
if (s1 == s2) {
    System.out.println("Strings are equal");
} else {
    System.out.println("Strings are not equal");
}

medium

A. Using == compares references, not content

B. Missing semicolon after if condition

C. Strings cannot be compared in Java

D. Should use compareTo() instead of ==

Solution

1. Step 1: Identify how string comparison works

   Using == compares if both variables point to the same object, not if their text is equal.

2. Step 2: Correct way to compare ignoring case

   Use s1.equalsIgnoreCase(s2) to compare string content ignoring case.

3. Final Answer:

   Using == compares references, not content -> Option A

4. Quick Check:

   Use equalsIgnoreCase() for content ignoring case [OK]

Hint: Never use == for string content comparison [OK]

Common Mistakes:

• Using == instead of equalsIgnoreCase()
• Thinking == compares string content
• Using compareTo() for equality check

5. You want to sort an array of strings alphabetically ignoring case. Which approach correctly compares two strings s1 and s2 for sorting?

hard

A. s1.compareToIgnoreCase(s2)

B. s1.equalsIgnoreCase(s2)

C. s1.compareTo(s2)

D. s1 == s2

Solution

1. Step 1: Understand sorting with string comparison

   Sorting requires a method that returns an integer indicating order, not just equality.

2. Step 2: Identify method ignoring case for order

   compareToIgnoreCase() compares strings alphabetically ignoring case and returns an int for sorting.

3. Step 3: Confirm method availability

   Java's String class has compareToIgnoreCase() for this purpose.

4. Final Answer:

   s1.compareToIgnoreCase(s2) -> Option A

5. Quick Check:

   Use compareToIgnoreCase() for case-insensitive sorting [OK]

Hint: Use compareToIgnoreCase() to sort strings ignoring case [OK]

Common Mistakes:

• Using equalsIgnoreCase() which returns boolean, not order
• Using compareTo() which is case sensitive
• Using == which compares references