String Compare in Java | Equals, CompareTo Methods

In Java, two strings can be compared on the basis of content and reference. We cannot compare two strings by using relational operators like >, <, >=,<=, and !=. 

The string equals() method is used to compare the original content of string. It compares two strings for equality. If any character is not matching, it returns false. If all the characters are the same, it will return true. The string class overrides the equals() method of the object class. 

This method performs the compersion between two strings by ignoring case difference. It only checks the content characters. When it compares two strings, it considers the uppercase (A-Z) to be the same as the lowercase (a-z). If any character is not matched, it returns false otherwise, it returns true.
For example:

String str = "HELLO JAVA";
boolean result = str.equals("hello java");

Program source code 1:

package stringPrograms; 
public class EqualsMethodTest 
{ 
public static void main(String args[])
{ 
 String s1 = "Hello"; 
 String s2 = "Hello"; 
 String s3 = new String("Good bye"); 
 String s4 = new String("Hello"); 
System.out.println(s1.equals(s2)); // true because content and case is same. 
System.out.println(s1.equals(s3)); // false because content is different. 
System.out.println(s1.equals(args)); // false. 
System.out.println(s1.equals(null)); // false. 
if (s2.equals(s4)) 
{ 
  System.out.println("Both strings are equal"); 
 } 
 else { 
 System.out.println("Both strings are unequal"); 
 } 
}

Output: 
       true 
       false 
       false 
       false 
       Both strings are equal

Program source code 2:

package stringPrograms; 
public class EqualsIgnoreCaseTest 
{ 
public static void main(String args[])
{ 
 String s1 = "GOOD BYE"; 
 String s2 = new String("Good bye"); 
System.out.println(s1.equals(s2)); // false because content is same but case is different. 
System.out.println(s1.equalsIgnoreCase(s2)); // true. 
  } 
}

Output: 
       false 
       true

Although the above two methods are the easiest ways to compare strings. But the string class gives you many other options.

String Compare by = = Operator in Java

package stringPrograms; 
public class DoubleEqualOperatorTest 
{ 
public static void main(String args[])
{ 
 String s1 = "Cricket"; 
 String s2 = "Cricket"; 
 String s3 = new String("Cricket"); 
System.out.println(s1==s2); // true 
System.out.println(s1==s3); // false 
 } 
}

Output: 
       true 
       false

Let’s see the memory details to understand the output of this program very clearly.

Explanation of allotting memory for storing objects: 

1. Let us take the first statement: String s1 = “Cricket”;
When JVM executes the first statement, it will create an object in the string constant pool and store “Cricket” in it. The reference variable s1 which contains the address of object says 365a25 is allocated to this object in the memory.

Program source code 4:

package stringPrograms; 
public class DoubleEqualOperatorTest 
{ 
public static void main(String args[])
{ 
 String s1 = new String("Football"); 
 String s2 = new String("Football"); 
 String s3 = "Football"; 
 String s4 = "Football"; 
System.out.println(s1==s2); // false 
System.out.println(s1==s3); // false 
System.out.println(s1==s4); // false 
System.out.println(s3==s4); // true 
 } 
}

Output: 
       false 
       false 
       false 
       true

To understand the output of this program, look at the memory details.

Explanation of allotting memory for storing objects:

The string compareTo() method compares the current string object with specified string object in the method argument lexicographically and returns an integer value.

Basically, it compares two strings on the basis of the Unicode value of each character in the string. Each character of both strings is converted into a Unicode value for comparison. The general form of this method is given below:

public int compareTo(String str)

Here, str is a specified string object being compared with calling string object.

For example:
 1. s1.compareTo(s2); where s1 and s2 are two reference variables of string literals.
➲ If s1 = = s2, this method returns zero. It means that two strings are equal.
➲ If s1 > s2, +ve value. It means that first string is lexicographically greater than second string. 
➲ If s1 < s2, -ve value. It means that first string is lexicographically less than second string. 

2: s1.compareTo(“Hello Java”); where s1 is a string literal and its value is compared with string specified in the method argument.

Let’s create a program where we will compare two strings using compareTo() method.
Program source code 5: