Java string equals method null

How to compare strings in Java

In this article, we will show you a few ways to compare strings in Java.

1. Using equals(), compare strings case-sensitive

1.1 In Java, we can use .equals() to compare strings.

 package com.mkyong.string.compare; public class StringCompareEquals < public static void main(String[] args) < String str1 = "apple"; boolean result = str1.equals("apple"); System.out.println(result); // true >> 

1.2 The .equals() is case-sensitive in string comparison.

 package com.mkyong.string.compare; public class StringCompareEquals2 < public static void main(String[] args) < String str1 = "apple"; boolean result = str1.equals("Apple"); System.out.println(result); // false >> 

2. Using equalsIgnoreCase(), compare strings case-insensitive

2.1 Below example uses .equalsIgnoreCase() to compare strings, ignore case, case-insensitive.

 package com.mkyong.string.compare; public class StringCompareEqualsIgnoreCase < public static void main(String[] args) < String str1 = "apple"; boolean result = str1.equalsIgnoreCase("Apple"); System.out.println(result); // true >> 

3. Handle null in strings comparison

3.1 If we use .equals to compare string with a null value, it throws NullPointerException .

 package com.mkyong.string; public class StringCompareNull < public static void main(String[] args) < String str1 = null; if (str1.equals("hello")) < System.out.println("equals"); >> > 

 Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.equals(Object)" because "str1" is null 

3.2 To fix it, check the null before compare strings.

 package com.mkyong.string; public class StringCompareNull < public static void main(String[] args) < String str1 = null; if (str1 != null && str1.equals("hello")) < System.out.println("equals"); >> > 

4. Using contentEquals(), compare strings for CharSequence

• The .equals() can compare string value only if the object is an instance of String.class .
• The contentEquals() can compare string value to any CharSequence implementations like StringBuffer or StringBuilder .

 package com.mkyong.string.compare; public class StringCompareContentEquals < public static void main(String[] args) < String str1 = "apple"; String str2 = "apple"; StringBuilder sb = new StringBuilder("apple"); StringBuffer buffer = new StringBuffer("apple"); // true System.out.println(str1.equals(str2)); // false, .equals cant compare StringBuilder System.out.println(str1.equals(sb)); // .contentEquals supports CharSequence // CharSequence implementations: StringBuffer, StringBuilder, String, etc. // true System.out.println(str1.contentEquals(sb)); // true System.out.println(str1.contentEquals(buffer)); >> 

5. Using compareTo(), compare strings lexicographically

In Java, we can use compareTo() method compares two strings lexicographically (alphabetical order). It returns a positive number, negative number, or zero.

 s1.compareTo(s2) // for example "a".compareTo("b") 

 System.out.println("a".compareTo("b")); // -1 System.out.println("a".compareTo("c")); // -2 System.out.println("1".compareTo("2")); // -1 System.out.println("1".compareTo("3")); // -2 System.out.println("b".compareTo("a")); // 1 System.out.println("c".compareTo("a")); // 2 System.out.println("2".compareTo("1")); // 1 System.out.println("3".compareTo("1")); // 2 System.out.println("a".compareTo("a")); // 0 System.out.println("1".compareTo("1")); // 0 

Note
More Java String compareTo() examples.

6. Using Objects.equals() to compare strings

Since Java 1.7 and later, we can use Objects.equals() to ensure both objects are equal, and it works for String.class as well.

 package com.mkyong.string.compare; import java.util.Objects; public class StringCompareObjectsEquals < public static void main(String[] args) < String str1 = "apple"; String str2 = "banana"; // false System.out.println(Objects.equals(str1, str2)); // true System.out.println(Objects.equals(str1, new String("apple"))); // false System.out.println(Objects.equals(null, str2)); // false System.out.println(Objects.equals(str1, null)); // true System.out.println(Objects.equals(null, null)); >> 

 false true false false true 

7. Why “==” sometimes work to compare strings?

Why below == operator return true for string comparison, are we supposed to use .equals() to compare strings?

 String str1 = "apple"; String str2 = "apple"; // true, why true? why we can use == to compare strings? System.out.println(str1 == str2); 

To answer the above question, we need to understand how String interning works in Java.

7.1 String interning, how it works?

In Java, String interning is a method of storing only one copy of each distinct string value into a String intern pool, String constant pool, or String pool. The entire JVM shares a single String pool.

In Java, if JVM sees String literal like below:

 // String literal String str1 = "apple"; 

String interning process starts and decides the following two tasks:

• If the String pool contains this string, then the string from the pool is returned.
• If the String pool does not contain this string, add it to the pool, and return a reference to this String object.

Read code comments to understand how to String interning works in Java.

 // JVM checks if this string exists in the string pool? The answer is NO. // JVM add this string object to the string pool and returns a reference // str1 has a reference "111" (example), which points to "apple" in the string pool String str1 = "apple"; // JVM checks if this string exists in string pool? The answer is YES. // JVM returns the reference "111", which points to "apple" String str2 = "apple"; // Now str1 and str2 both have the same reference "111" to the "apple" // and that is why it return true here. System.out.println(str1 == str2); // move on for the following example // JVM see "new", ignore string pool and create a new reference or address "222" (example) String str3 = new String("apple"); // str = address "111" // str3 = address "222" // false System.out.println(str1 == str3); // move on for the following example // we can use `intern` to add this to the string pool // JVM see this "apple" already existed in the string pool // JVM returns the reference "111" to str4 String str4 = str3.intern(); // true System.out.println(str1 == str4); // still false, str3 is still has the reference "222" System.out.println(str1 == str3); 

7.2 Why String interning?

String interning speeds up string comparisons without examining every character of both. Read this Wikipedia – String interning.

8. Download Source Code

9. References

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