Java String compareTo()

The Java String class compareTo() method compares the given string with the current string lexicographically. It returns a positive number, negative number, or 0.

It compares strings on the basis of the Unicode value of each character in the strings.

If the first string is lexicographically greater than the second string, it returns a positive number (difference of character value). If the first string is less than the second string lexicographically, it returns a negative number, and if the first string is lexicographically equal to the second string, it returns 0.

if s1 > s2, it returns positive number
if s1 < s2, it returns negative number
if s1 == s2, it returns 0

Syntax

The method accepts a parameter of type String that is to be compared with the current string.

It returns an integer value. It throws the following two exceptions:

ClassCastException: If this object cannot get compared with the specified object.

NullPointerException: If the specified object is null.

Internal implementation

int compareTo(String anotherString) {
    int length1 = value.length;
    int length2 = anotherString.value.length;
    int limit = Math.min(length1, length2);
    char v1[] = value;
    char v2[] = anotherString.value;
 
    int i = 0;
    while (i < limit) {
        char ch1 = v1[i];
        char ch2 = v2[i];
        if (ch1 != ch2) {
            return ch1 - ch2;
        }
        i++;
    }
    return length1 - length2;
}

Java String compareTo() Method Example

FileName: CompareToExample.java

public class CompareToExample{
public static void main(String args[]){
String s1="hello";
String s2="hello";
String s3="meklo";
String s4="hemlo";
String s5="flag";
System.out.println(s1.compareTo(s2));//0 because both are equal
System.out.println(s1.compareTo(s3));//-5 because "h" is 5 times lower than "m"
System.out.println(s1.compareTo(s4));//-1 because "l" is 1 times lower than "m"
System.out.println(s1.compareTo(s5));//2 because "h" is 2 times greater than "f"
}}

Test it Now

Output:

0
-5
-1
2

Java String compareTo(): empty string

When we compare two strings in which either first or second string is empty, the method returns the length of the string. So, there may be two scenarios:

If first string is an empty string, the method returns a negative
If second string is an empty string, the method returns a positive number that is the length of the first string.

FileName: CompareToExample2.java

public class CompareToExample2{
public static void main(String args[]){
String s1="hello";
String s2="";
String s3="me";
System.out.println(s1.compareTo(s2));
System.out.println(s2.compareTo(s3));
}}

Test it Now

Output:

5
-2

Java String compareTo(): case sensitive

To check whether the compareTo() method considers the case sensitiveness of characters or not, we will make the comparison between two strings that contain the same letters in the same sequence.

Suppose, a string having letters in uppercase, and the second string having the letters in lowercase. On comparing these two string, if the outcome is 0, then the compareTo() method does not consider the case sensitiveness of characters; otherwise, the method considers the case sensitiveness of characters.

FileName: CompareToExample3.java

public class CompareToExample3
{
// main method
public static void main(String argvs[])
{

// input string in uppercase
String st1 = new String("INDIA IS MY COUNTRY");

// input string in lowercase
String st2 = new String("india is my country");

System.out.println(st1.compareTo(st2));
}
}

Output:

-32

Conclusion: It is obvious by looking at the output that the outcome is not equal to zero. Hence, the compareTo() method takes care of the case sensitiveness of characters.

Java String compareTo(): ClassCastException

The ClassCastException is thrown when objects of incompatible types get compared. In the following example, we are comparing an object of the ArrayList (al) with a string literal ("Sehwag").

FileName: CompareToExample4.java

// import statement
import java.util.*;

class Players 
{
     
private String name;

// constructor of the class
public Players(String str)
{
name = str;
}
 
}

public class CompareToExample4
{
 
// main method
public static void main(String[] args) 
{
 
Players ronaldo = new Players("Ronaldo");
Players sachin = new Players("Sachin");
Players messi = new Players("Messi");
ArrayList<Players> al = new ArrayList<>();

al.add(ronaldo);
al.add(sachin);
al.add(messi);
 
// performing binary search on the list al
Collections.binarySearch(al, "Sehwag", null);
}
     
}

Output:

Exception in thread "main" java.lang.ClassCastException: class Players cannot be cast to class java.lang.Comparable

Java String compareTo(): NullPointerException

The NullPointerException is thrown when a null object invokes the compareTo() method. Observe the following example.

FileName: CompareToExample5.java

public class CompareToExample5
{
// main method
public static void main(String[] args) 
{
 
String str = null;

// null is invoking the compareTo method. Hence, the NullPointerException
// will be raised
int no =  str.compareTo("India is my country.");

System.out.println(no);
}
}

Output:

Exception in thread "main" java.lang.NullPointerException
	at CompareToExample5.main(CompareToExample5.java:9)

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Java String compareTo()

Syntax

Internal implementation

Java String compareTo() Method Example

Java String compareTo(): empty string

Java String compareTo(): case sensitive

Java String compareTo(): ClassCastException

Java String compareTo(): NullPointerException

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