[#]: collector: (lujun9972) [#]: translator: ( ) [#]: reviewer: ( ) [#]: publisher: ( ) [#]: url: ( ) [#]: subject: (How to compare strings in Java) [#]: via: (https://opensource.com/article/19/9/compare-strings-java) [#]: author: (Girish Managoli https://opensource.com/users/gammayhttps://opensource.com/users/sethhttps://opensource.com/users/clhermansenhttps://opensource.com/users/clhermansen) How to compare strings in Java ====== There are six ways to compare strings in Java. ![Javascript code close-up with neon graphic overlay][1] String comparison is a fundamental operation in programming and is often quizzed during interviews. These strings are a sequence of characters that are _immutable_ which means unchanging over time or unable to be changed. Java has a number of methods for comparing strings; this article will teach you the primary operation of how to compare strings in Java. There are six options: 1. The == operator 2. String equals 3. String equalsIgnoreCase 4. String compareTo 5. String compareToIgnoreCase 6. Objects equals ### The == operator **==** is an operator that returns **true** if the contents being compared refer to the same memory or **false** if they don't. If two strings compared with **==** refer to the same string memory, the return value is **true**; if not, it is **false**. ``` [String][2] string1 = "MYTEXT"; [String][2] string2 = "YOURTEXT";                 [System][3].out.println("Output: " + (string1 == string2)); Output: false ``` The return value of **==** above is **false**, as "MYTEXT" and "YOURTEXT" refer to different memory. ``` [String][2] string1 = "MYTEXT"; [String][2] string6 = "MYTEXT";                 [System][3].out.println("Output: " + (string1 == string6)); Output: true ``` In this case, the return value of **==** is **true**, as the compiler internally creates one memory location for both "MYTEXT" memories, and both variables refer to the same memory location. ``` [String][2] string1 = "MYTEXT"; [String][2] string7 = string1; [System][3].out.println("Output: " + (string1 == string7)); Output: true ``` If you guessed right, you know string7 is initialized with the same memory location as string1 and therefore **==** is true. ``` [String][2] string1 = "MYTEXT"; [String][2] string4 = new [String][2]("MYTEXT"); [System][3].out.println("Output: " + (string1 == string4)); Output: false ``` In this case, the compiler creates a new memory location, even though the value is the same for string4 and string1. ``` [String][2] string1 = "MYTEXT"; [String][2] string5 = new [String][2](string1); [System][3].out.println("Output: " + (string1 == string4)); Output: false ``` Here, string5 is a new string object initialized with string1; hence, **string1 == string4** is not true. ### String equals The string class has a **String equals** method to compare two strings. String comparison with **equals** is case-sensitive. According to the [docs][4]: ```     /**      * Compares this string to the specified object.  The result is {@code      * true} if and only if the argument is not {@code null} and is a {@code      * String} object that represents the same sequence of characters as this      * object.      *      * @param  anObject      *         The object to compare this {@code String} against      *      * @return  {@code true} if the given object represents a {@code String}      *          equivalent to this string, {@code false} otherwise      *      * @see  #compareTo(String)      * @see  #equalsIgnoreCase(String)      */     public boolean equals(Object anObject) { ... } ``` Let's see a few examples: ``` [String][2] string1 = "MYTEXT"; [String][2] string2 = "YOURTEXT"; [System][3].out.println("Output: " + string1.equals(string2)); Output: false ``` If the strings are not the same, the output of the **equals** method is obviously **false**. ``` [String][2] string1 = "MYTEXT"; [String][2] string3 = "mytext"; [System][3].out.println("Output: " + string1.equals(string3)); Output: false ``` These strings are the same in value but differ in case; hence, the output is **false**. ``` [String][2] string1 = "MYTEXT"; [String][2] string4 = new [String][2]("MYTEXT"); [System][3].out.println("Output: " + string1.equals(string4)); Output: true [/code] [code] [String][2] string1 = "MYTEXT"; [String][2] string5 = new [String][2](string1); [System][3].out.println("Output: " + string1.equals(string5)); Output: true ``` The examples in both these cases are **true**, as the two values are the same. Unlike with **==**, the second example above returns **true**. The string object on which **equals** is called should obviously be a valid string object and non-null. ``` [String][2] string1 = "MYTEXT"; [String][2] string8 = null; [System][3].out.println("Output: " + string8.equals(string1)); [Exception][5] in thread _____  java.lang.[NullPointerException][6] ``` The above evidently is not a good code. ``` [System][3].out.println("Output: " + string1.equals(string8)); Output: false ``` This is alright. ### String equalsIgnoreCase The behavior of **equalsIgnoreCase** is identical to **equals** with one difference—the comparison is not case-sensitive. The [docs][4] say: ```     /**      * Compares this {@code String} to another {@code String}, ignoring case      * considerations.  Two strings are considered equal ignoring case if they      * are of the same length and corresponding characters in the two strings      * are equal ignoring case.      *      * <p> Two characters {@code c1} and {@code c2} are considered the same      * ignoring case if at least one of the following is true:      * <ul>      *   <li> The two characters are the same (as compared by the      *        {@code ==} operator)      *   <li> Applying the method {@link      *        java.lang.Character#toUpperCase(char)} to each character      *        produces the same result      *   <li> Applying the method {@link      *        java.lang.Character#toLowerCase(char)} to each character      *        produces the same result      * </ul>      *      * @param  anotherString      *         The {@code String} to compare this {@code String} against      *      * @return  {@code true} if the argument is not {@code null} and it      *          represents an equivalent {@code String} ignoring case; {@code      *          false} otherwise      *      * @see  #equals(Object)      */     public boolean equalsIgnoreCase(String anotherString) { ... } ``` The second example in **equals** (above) is the only difference from the comparison in **equalsIgnoreCase**. ``` [String][2] string1 = "MYTEXT"; [String][2] string3 = "mytext"; [System][3].out.println("Output: " + string1.equalsIgnoreCase(string3)); Output: true ``` This returns **true** because the comparison is case-independent. All other examples under **equals** remain the same as they are for **equalsIgnoreCase**. ### String compareTo The **compareTo** method compares two strings lexicographically (i.e., pertaining to alphabetical order) and case-sensitively and returns the lexicographical difference in the two strings. The [docs][4] describe lexicographical order computation as: ``` /**      * Compares two strings lexicographically.      * The comparison is based on the Unicode value of each character in      * the strings. The character sequence represented by this      * {@code String} object is compared lexicographically to the      * character sequence represented by the argument string. The result is      * a negative integer if this {@code String} object      * lexicographically precedes the argument string. The result is a      * positive integer if this {@code String} object lexicographically      * follows the argument string. The result is zero if the strings      * are equal; {@code compareTo} returns {@code 0} exactly when      * the {@link #equals(Object)} method would return {@code true}.      * <p>      * This is the definition of lexicographic ordering. If two strings are      * different, then either they have different characters at some index      * that is a valid index for both strings, or their lengths are different,      * or both. If they have different characters at one or more index      * positions, let <i>k</i> be the smallest such index; then the string      * whose character at position <i>k</i> has the smaller value, as      * determined by using the &lt; operator, lexicographically precedes the      * other string. In this case, {@code compareTo} returns the      * difference of the two character values at position {@code k} in      * the two string -- that is, the value:      * <blockquote><pre>      * this.charAt(k)-anotherString.charAt(k)      * </pre></blockquote>      * If there is no index position at which they differ, then the shorter      * string lexicographically precedes the longer string. In this case,      * {@code compareTo} returns the difference of the lengths of the      * strings -- that is, the value:      * <blockquote><pre>      * this.length()-anotherString.length()      * </pre></blockquote>      *      * @param   anotherString   the {@code String} to be compared.      * @return  the value {@code 0} if the argument string is equal to      *          this string; a value less than {@code 0} if this string      *          is lexicographically less than the string argument; and a      *          value greater than {@code 0} if this string is      *          lexicographically greater than the string argument.      */     public int compareTo(String anotherString) { ... } ``` Let's look at some examples. ``` [String][2] string1 = "A"; [String][2] string2 = "B"; [System][3].out.println("Output: " + string1.compareTo(string2)); Output: -1 [System][3].out.println("Output: " + string2.compareTo(string1)); Output: 1 [/code] [code] [String][2] string1 = "A"; [String][2] string3 = "a"; [System][3].out.println("Output: " + string1.compareTo(string3)); Output: -32 [System][3].out.println("Output: " + string3.compareTo(string1)); Output: 32 [/code] [code] [String][2] string1 = "A"; [String][2] string6 = "A";                         [System][3].out.println("Output: " + string1.compareTo(string6)); Output: 0 [/code] [code] String string1 = "A"; String string8 = null;                 System.out.println("Output: " + string8.compareTo(string1)); Exception in thread ______  java.lang.NullPointerException at java.lang.String.compareTo(String.java:1155) String string1 = "A"; String string10 = "";                 System.out.println("Output: " + string1.compareTo(string10)); Output: 1 ``` ### String compareToIgnoreCase The behavior of **compareToIgnoreCase** is identical to **compareTo** with one difference: the strings are compared without case consideration. ``` [String][2] string1 = "A"; [String][2] string3 = "a"; [System][3].out.println("Output: " + string1.compareToIgnoreCase(string3)); Output: 0 ``` ### Objects equals The **Objects equals** method invokes the overridden **String equals** method; its behavior is the same as in the **String equals** example above. ``` [String][2] string1 = "MYTEXT"; [String][2] string2 = "YOURTEXT"; [System][3].out.println("Output: " + Objects(string1, string2)); Output: false [/code] [code] [String][2] string1 = "MYTEXT"; [String][2] string3 = "mytext"; [System][3].out.println("Output: " + Objects(string1, string3)); Output: false [/code] [code] [String][2] string1 = "MYTEXT"; [String][2] string6 = "MYTEXT"; [System][3].out.println("Output: " + Objects(string1, string6)); Output: true [/code] [code] [String][2] string1 = "MYTEXT"; [String][2] string8 = null; [System][3].out.println("Output: " + Objects.equals(string1, string8)); Output: false [System][3].out.println("Output: " + Objects.equals(string8, string1)); Output: false [/code] [code] [String][2] string8 = null; [String][2] string9 = null; [System][3].out.println("Output: " + Objects.equals(string8, string9)); Output: true ``` The advantage here is that the **Objects equals** method checks for null values (unlike **String equals**). The implementation of **Object equals** is: ``` public static boolean equals([Object][7] a, [Object][7] b) { return (a == b) || (a != null && a.equals(b)); } ``` ### Which method to use? There are many methods to compare two strings. Which one should you use? As a common practice, use **String equals** for case-sensitive strings and **String equalsIgnoreCase** for case-insensitive comparisons. However, one caveat: take care of NPE (**NullPointerException**) if one or both strings are null. The source code is available on [GitLab][8] and [GitHub][9]. -------------------------------------------------------------------------------- via: https://opensource.com/article/19/9/compare-strings-java 作者：[Girish Managoli][a] 选题：[lujun9972][b] 译者：[译者ID](https://github.com/译者ID) 校对：[校对者ID](https://github.com/校对者ID) 本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出 [a]: https://opensource.com/users/gammayhttps://opensource.com/users/sethhttps://opensource.com/users/clhermansenhttps://opensource.com/users/clhermansen [b]: https://github.com/lujun9972 [1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/code_javascript.jpg?itok=60evKmGl (Javascript code close-up with neon graphic overlay) [2]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+string [3]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+system [4]: http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/lang/String.java [5]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+exception [6]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+nullpointerexception [7]: http://www.google.com/search?hl=en&q=allinurl%3Adocs.oracle.com+javase+docs+api+object [8]: https://gitlab.com/gammay/stringcomparison [9]: https://github.com/gammay/stringcompare