TranslateProject/sources/tech/20190920 How to compare strings in Java.md

How to compare strings in Java

There are six ways to compare strings in Java.

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:

    /**
     * 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 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 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 < 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 and GitHub.

---

via: https://opensource.com/article/19/9/compare-strings-java

作者：Girish Managoli 选题：lujun9972 译者：译者ID 校对：校对者ID

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