How to compare Strings in Java

Question

I have an array of Strings. I want unique Strings in that array. How to check the equality between two Strings?

Answer 1

In Java, Objects.equals() method is used to check the equality between Strings.

Objects.equals() method checks for the null before calling .equals()

String s1="Hello";

String s2="Hello";

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

Output of this code will be true. If the Strings are not equal, Output will be false.

Answer 2

String fooString1 = new String("foo");
String fooString2 = new String("foo");

// Evaluates to false
fooString1 == fooString2;

// Evaluates to true
fooString1.equals(fooString2);

// Evaluates to true, because Java uses the same object
"bar" == "bar";

Answer 3

There are two ways to compare strings in Java :

• Use equals() 
  • If both strings are same the equals() method will return "true"  and else it will return "false".
  • This method is case sensitive, thus if you compare "String" & "string" it will return false.
  • You can use equalsIgnoreCase() in above case.
• Use "==" operator
  • This operator compares the objects, not values.
  • It is Case sensitive.

There is another method compareTo() which compares strings lexicographically.

• string1 == string2 : return 0
• string1 > string2   : return any positive value
• string1 < string2   : return any negative value

Answer 4

The == operator check if the two references point to the same object or not. .equals() check for the actual string content (value).

Note that the .equals() method belongs to class Object (super class of all classes). You need to override it as per you class requirement, but for String it is already implemented, and it checks whether two strings have the same value or not.

• Case 1

  String s1 = "Stack Overflow";
  String s2 = "Stack Overflow";
  s1 == s2;      //true
  s1.equals(s2); //true

  Reason: String literals created without null are stored in the String pool in the permgen area of heap. So both s1 and s2 point to same object in the pool.

• Case 2

  String s1 = new String("Stack Overflow");
  String s2 = new String("Stack Overflow");
  s1 == s2;      //false
  s1.equals(s2); //true

  Reason: If you create a String object using the new keyword a separate space is allocated to it on the heap.

Answer 5

1. == This will always return false
2. String1.equals(String2) returns true when both string are same.