Since String is a popular data type in Java, the string comparison is probably one of the most commonly used operations. Strings can be compared based on their content as well as their reference.
In this tutorial, we will learn about the following ways to compare two strings with each other:
- By comparison (
==) operator - By
equals()method - By
Objects.equals()method - By
compareTo()method
1. Using Comparison (==) Operator
The comparison operator compares two strings by their reference. It does not take into account strings' values and the only checks the referential equality of two strings. It returns true if both strings refer to the same object, otherwise false.
String str1 = "Spring Boot";
String str2 = "Spring Boot";
String str3 = new String("Spring Boot");
System.out.println(str1 == str2); // true
System.out.println(str2 == str3); // false
Look at the above examples. The comparison between 1st and 2nd string returns true because both these variables refer to the same string literal.
On the other hand, when we compare the 2nd string with 3rd string, it returns false. This is because both these strings are pointing to different objects (literal vs object).
Be careful while using comparison (==) operator for matching strings. It can potentially lead to unexpected results if you are not sure about the string types.
2. Using equals() method
The equals() method is part of String class inherited from Object class. This method compares two strings based on their contents — character by characters, ignoring their references.
It returns true if both strings have equal length and all characters are in the same order:
String str1 = "Spring Boot";
String str2 = "Spring Boot";
String str3 = new String("Spring Boot");
String str4 = new String("SPRING BOOT");
System.out.println(str1.equals(str2)); // true
System.out.println(str2.equals(str3)); // true
System.out.println(str2.equals(str4)); // false
In the above examples, the first two comparisons are true because str1, str2, and str3 all have the same contents irrespective of their references. Although str4 has the same content in uppercase equals() returns false as it is case-sensitive.
Unlike
==operator which handles null strings well, callingequals()method from a null string will cause aNullPointerExceptionexception. However, if the string passed toequals()method is null, it returns false.
equalsIgnoreCase() method
If you want to ignore the content case, use equalsIgnoreCase() method instead. This method is similar to equals() but does not consider the casing in characters while comparing strings:
System.out.println(str2.equalsIgnoreCase(str4)); // true
3. Using compareTo() method
The compareTo() method is a part of String class and compares the strings character by character lexicographically and returns an integer value that indicates whether the first string is less than (< 0 value), equal to (0 value), or greater than (> 0 value) the second string:
String str1 = "Spring Boot";
String str2 = "Spring Boot";
String str3 = new String("Spring Boot");
String str4 = new String("SPRING BOOT");
System.out.println(str1.compareTo(str2)); // 0 (true)
System.out.println(str2.compareTo(str3)); // 0 (true)
System.out.println(str1.compareTo(str4)); // 32 (false)
If any of the two strings is null, the
compareTo()method throws aNullPointerExceptionexception.
compareToIgnoreCase() method
The compareToIgnoreCase() method is similar to compareTo() method except that it ignores characters case:
System.out.println(str1.compareToIgnoreCase(str4)); // 0 (true)
4. Using Objects.equals() method
The Objects class is a part of the Java utility package which contains a static equals() method that can be used to compare two strings.
This method returns true if both strings are equal to each other and false otherwise. Consequently, if both strings are null, true is returned and if exactly one string is null, false is returned. Otherwise, equality is determined by using the equals() method of the first string.
String str1 = "Spring Boot";
String str2 = "Spring Boot";
String str3 = new String("Spring Boot");
System.out.println(Objects.equals(str1, str2)); // true
System.out.println(Objects.equals(str1, str3)); // true
System.out.println(Objects.equals(null, str3)); // false
System.out.println(Objects.equals(null, null)); // true
Since Objects.equals() internally calls String class's equals() method, it is case-sensitive.
Bonus: Using Apache Commons
The StringUtils class from Apache Commons Lang library has some very good methods for performing string-related operations. The equals() method of StringUtils class is a null-safe version of the equals() method of String class, which also handles null values.
The StringUtils class also includes equalsIgnoreCase(), compare(), and compareIgnoreCase() methods:
// use `equals()` and `equalsIgnoreCase()` methods
System.out.println(StringUtils.equals("Spring Boot", "Spring Boot")); // true
System.out.println(StringUtils.equalsIgnoreCase("Spring Boot", "SPRING BOOT")); // true
// use `compare()` and `compareIgnoreCase()` methods
System.out.println(StringUtils.compare("Spring Boot", "Spring Boot")); // true
System.out.println(StringUtils.compareIgnoreCase("Spring Boot", "SPRING BOOT")); // true
// `null` values
System.out.println(StringUtils.equals(null, "SPRING BOOT")); // false
System.out.println(StringUtils.equals(null, null)); // true
Before you start using StringUtils utility class methods, make sure that you have added Apache Commons Lang dependency to your project's pom.xml file:
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-lang3</artifactId>
<version>3.9</version>
</dependency>
If you are using Gradle build tool, add the following to your project's build.gradle file:
implementation 'org.apache.commons:commons-lang3:3.9'
Conclusion
That's folks for comparing strings in Java. We discussed 4 different ways to compare two strings with each other. You should always use Objects.equals() as it is null-safe and performs better.
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