compareTo() vs. equals()

equals() checks whether two strings are equal or not.It gives boolean value. compareTo() checks whether string object is equal to,greater or smaller to the other string object.It gives result as : 1 if string object is greater 0 if both are equal -1 if string is smaller than other string

eq:

String a = "Amit";
String b = "Sumit";
String c = new String("Amit");
System.out.println(a.equals(c));//true
System.out.println(a.compareTo(c)); //0
System.out.println(a.compareTo(b)); //1

This is an experiment in necromancy :-)

Most answers compare performance and API differences. They miss the fundamental point that the two operations simply have different semantics.

Your intuition is correct. x.equals(y) is not interchangeable with x.compareTo(y) == 0. The first compares identity, while the other compares the notion of 'size'. It is true that in many cases, especially with primitive types, these two co-align.

The general case is this:

If x and y are identical, they share the same 'size': if x.equals(y) is true => x.compareTo(y) is 0.

However, if x and y share the same size, it does not mean they are identical.

if x.compareTo(y) is 0 does not necessarily mean x.equals(y) is true.

A compelling example where identity differs from size would be complex numbers. Assume that the comparison is done by their absolute value. So given two complex numbers: Z1 = a1 + b1*i and Z2 = a2 + b2*i:

Z1.equals(z2) returns true if and only if a1 = a2 and b1 = b2.

However Z1.compareTo(Z2) returns 0 for and infinite number of (a1,b1) and (a2,b2) pairs as long as they satisfy the condition a1^2 + b1^2 == a2^2 + b2^2.

String s1 = "a";
String s2 = "c";

System.out.println(s1.compareTo(s2));
System.out.println(s1.equals(s2));

This prints -2 and false

String s1 = "c";
String s2 = "a";
System.out.println(s1.compareTo(s2));
System.out.println(s1.equals(s2));

This prints 2 and false

String s1 = "c";
String s2 = "c";
System.out.println(s1.compareTo(s2));
System.out.println(s1.equals(s2));

This prints 0 and true

equals returns boolean if and only if both strings match.

compareTo is meant to not just tell if they match but also to tell which String is lesser than the other, and also by how much, lexicographically. This is mostly used while sorting in collection.

Equals -

1- Override the GetHashCode method to allow a type to work correctly in a hash table.

2- Do not throw an exception in the implementation of an Equals method. Instead, return false for a null argument.

3-

  x.Equals(x) returns true.

  x.Equals(y) returns the same value as y.Equals(x).

  (x.Equals(y) && y.Equals(z)) returns true if and only if x.Equals(z) returns true.

Successive invocations of x.Equals(y) return the same value as long as the object referenced by x and y are not modified.

x.Equals(null) returns false.

4- For some kinds of objects, it is desirable to have Equals test for value equality instead of referential equality. Such implementations of Equals return true if the two objects have the same value, even if they are not the same instance.

For Example -

   Object obj1 = new Object();
   Object obj2 = new Object();
   Console.WriteLine(obj1.Equals(obj2));
   obj1 = obj2; 
   Console.WriteLine(obj1.Equals(obj2)); 

Output :-

False
True

while compareTo -

Compares the current instance with another object of the same type and returns an integer that indicates whether the current instance precedes, follows, or occurs in the same position in the sort order as the other object.

It returns -

Less than zero - This instance precedes obj in the sort order. Zero - This instance occurs in the same position in the sort order as obj. Greater than zero - This instance follows obj in the sort order.

It can throw ArgumentException if object is not the same type as instance.

For example you can visit here.

So I suggest better to use Equals in place of compareTo.

I believe equals and equalsIgnoreCase methods of String return true and false which is useful if you wanted to compare the values of the string object, But in case of implementing compareTo and compareToIgnoreCase methods returns positive, negative and zero value which will be useful in case of sorting.