Java| ==、equals()、compareTo()、equalsIgnoreCase() 和 compare()

在Java中有很多方法可以比较两个字符串：

• 使用 ==运算符
• 使用 equals() 方法
• 使用 compareTo() 方法
• 使用 compareToIgnoreCase() 方法
• 使用 compare() 方法
  • 方法一：使用 ==运算符
    双等号运算符用于比较两个或两个以上的对象，如果它们引用的是同一个对象，则返回 true，否则返回 false。字符串在Java中是不可变的。当两个或多个对象在没有 new 关键字的情况下创建时，两个对象引用相同的值。双等号运算符实际上比较对象引用。
    

• 下面的示例说明了在Java中使用 == 进行字符串比较：
  

JAVA

// Java program to demonstrate
// use of == operator in Java
 
class GFG {
    public static void main(String[] args)
    {
 
        // Get some Strings to compare
        String s1 = "A";
        String s2 = "A";
        String s3 = "A";
        String s4 = new String("A");
 
        // Compare s1 and s2
        // It should return true as they both
        // refer to same object in memory
        System.out.println(s1 + " == " + s2
                         + ": " + (s1 == s2));
 
        // Compare s1 and s3
        // It should return true as they both
        // refer to same object in memory
        System.out.println(s1 + " == " + s3
                         + ": " + (s1 == s3));
 
        // Compare s2 and s3
        // It should return true as they both
        // refer to same object in memory
        System.out.println(s2 + " == " + s3
                         + ": " + (s2 == s3));
 
        // Compare s1 and s4
        // It should return false as they both
        // refer to different object in memory
        System.out.println(s1 + " == " + s4
                         + ": " + (s1 == s4));
    }
}

输出：

A == A: true
A == A: true
A == A: true
A == A: false

• 方法2：使用equals()方法
  在Java中，字符串equals() 方法根据字符串的数据/内容比较两个给定的字符串。如果两个字符串的所有内容都相同，则返回 true。如果所有字符都不匹配，则返回 false。
  

• 下面的示例说明了在Java中使用 .equals 进行字符串比较：
  

Java

// Java program to demonstrate
// use of .equals operator in Java
 
class GFG {
    public static void main(String[] args)
    {
 
        // Get some Strings to compare
        String s1 = "A";
        String s2 = "A";
        String s3 = "a";
        String s4 = new String("A");
 
        // Compare s1 and s2
        // It should return true as they both
        // have the same content
        System.out.println(s1 + " .equals " + s2
                         + ": " + s1.equals(s2));
 
        // Compare s1 and s3
        // It should return false as they both
        // have the different content
        System.out.println(s1 + " .equals " + s3
                         + ": " + s1.equals(s3));
 
        // Compare s2 and s3
        // It should return false as they both
        // have the different content
        System.out.println(s2 + " .equals " + s3
                         + ": " + s2.equals(s3));
 
        // Compare s1 and s4
        // It should return true as they both
        // have the same content
        System.out.println(s1 + " .equals " + s4
                         + ": " + s1.equals(s4));
    }
}

输出：

A .equals A: true
A .equals a: false
A .equals a: false
A .equals A: true

• 方法 3：使用 compareTo() 方法
  在Java Comparable 接口中比较值并返回一个 int，这些 int 值可能小于、等于或大于。 Java比较两个字符串是基于字符串中每个字符的Unicode值。如果两个字符串不同，则它们在某个索引处具有不同的字符，该索引是两个字符串的有效索引，或者它们的长度不同，或两者兼而有之。
  假设索引 'i' 是字符不同的地方，那么 compareTo() 将返回firstString.charAt(i)-secondString.charAt(i) 。
  

• 下面的示例说明了在Java中使用 .compareTo 进行字符串比较：
  

Java

// Java program to demonstrate
// use of .compareTo operator in Java
 
class GFG {
    public static void main(String[] args)
    {
 
        // Get some Strings to compare
        String s1 = "A";
        String s2 = "A";
        String s3 = "a";
        String s4 = new String("A");
 
        // Compare s1 and s2
        // It should return 0 as they both
        // have the same ASCII value
        System.out.println(s1 + " .compareTo " + s2
                         + ": " + s1.compareTo(s2));
 
        // Compare s1 and s3
        // It should return -32 as they both
        // have the different ASCII value
        System.out.println(s1 + " .compareTo " + s3
                         + ": " + s1.compareTo(s3));
 
        // Compare s3 and s2
        // It should return 32 as they both
        // have the different ASCII value
        System.out.println(s3 + " .compareTo " + s2
                         + ": " + s3.compareTo(s2));
 
        // Compare s1 and s4
        // It should return 0 as they both
        // have the same ASCII value
        System.out.println(s1 + " .compareTo " + s4
                         + ": " + s1.compareTo(s4));
    }
}

输出：

A .compareTo A: 0
A .compareTo a: -32
a .compareTo A: 32
A .compareTo A: 0

• 方法四：使用equalsIgnoreCase()方法
  Java String equalsIgnoreCase() 方法与 equals() 方法非常相似，不同之处在于忽略大小写，如上面示例中的 String 对象 s4 与 s3 比较，然后 equals() 方法返回 false，但在 equalsIgnoreCase() 的情况下，它将返回 true .因此 equalsIgnoreCase() 方法不区分大小写。
  

• 下面的示例说明了在Java中使用 .equalsIgnoreCase 进行字符串比较：
  

Java

// Java program to demonstrate
// use of .equalsIgnoreCase operator in Java
 
class GFG {
    public static void main(String[] args)
    {
 
        // Get some Strings to compare
        String s1 = "A";
        String s2 = "A";
        String s3 = "a";
        String s4 = new String("A");
 
        // Compare s1 and s2
        // It should return true as they both
        // have the same content
        System.out.println(s1 + " .equalsIgnoreCase " + s2
                         + ": " + s1.equalsIgnoreCase(s2));
 
        // Compare s1 and s3
        // It should return true as they both
        // have the same content being case insensitive
        System.out.println(s1 + " .equalsIgnoreCase " + s3
                         + ": " + s1.equalsIgnoreCase(s3));
 
        // Compare s2 and s3
        // It should return true as they both
        // have the same content being case insensitive
        System.out.println(s2 + " .equalsIgnoreCase " + s3
                         + ": " + s2.equalsIgnoreCase(s3));
 
        // Compare s1 and s4
        // It should return true as they both
        // have the same content
        System.out.println(s1 + " .equalsIgnoreCase " + s4
                         + ": " + s1.equalsIgnoreCase(s4));
    }
}

输出：

A .equalsIgnoreCase A: true
A .equalsIgnoreCase a: true
A .equalsIgnoreCase a: true
A .equalsIgnoreCase A: true

• 方法 5：使用 compare() 方法
  在Java中进行特定于语言环境的比较，应该使用Java.text包中的Collator 类。 Collator 类的一个最重要的特性是能够定义我们自己的自定义比较规则。
  

• 下面的例子说明了在Java中使用 compare() 方法来比较字符串：
  

Java

// Java program to demonstrate
// use of collator.compare operator in Java
 
import java.text.Collator;
 
class GFG {
    public static void main(String[] args)
    {
 
        // Get Collator instance
        Collator collator = Collator.getInstance();
 
        // Get some Strings to compare
        String s1 = "A";
        String s2 = "A";
        String s3 = "a";
        String s4 = new String("A");
 
        // Compare s1 and s2
        // It should return 0 as they both
        // have the same ASCII value
        System.out.println(s1 + " collator.compare " + s2
                         + ": " + collator.compare(s1, s2));
 
        // Compare s1 and s3
        // It should return 1
        System.out.println(s1 + " collator.compare " + s3
                         + ": " + collator.compare(s1, s3));
 
        // Compare s3 and s2
        // It should return -1
        System.out.println(s3 + " collator.compare " + s2
                         + ": " + collator.compare(s3, s2));
 
        // Compare s1 and s4
        // It should return 0 as they both
        // have the same ASCII value
        System.out.println(s1 + " collator.compare " + s4
                         + ": " + collator.compare(s1, s4));
    }
}

输出：

A collator.compare A: 0
A collator.compare a: 1
a collator.compare A: -1
A collator.compare A: 0