Compare Strings in Java

The String class provides a method equals() to compare string values. The double equal sign == only compares the references, not the values.

String a = new String("test");
String b = new String("test");
System.out.println(a.equals(b));    // true
System.out.println(a == b);         // false

Here “a” and “b” are pointing to two different String objects. The equals() method will check the value of the two strings, while == only checks the references (the memory addresses of the two string objects).

-----         ----------
| a |   -->   | "test" |
-----         ----------
-----         ----------
| b |   -->   | "test" |
-----         ----------
new String("test").equals("test");            // true 
new String("test") == "test";                 // false 
new String("test") == new String("test");     // false 

However, string literals are “interned” and they refer to the same object if they have the same value.

System.out.println("test" == "test");         // true 
String a = "test";
String b = "test";
System.out.println(a == b);                   // true

Concatenation of string literals are done at compile time. This may result in the same literals.

System.out.println("test" == "te" + "st");    // true 

Be aware of null values. The double-equal sign == compares null references as well, but .equals() can not be invoked from a null string.

String a = null;
String b = null;
String c = "test";

System.out.println(a == b);                   // true
System.out.println(a == c);                   // false
System.out.println(a.equals(c));              // throws exception
System.out.println(c.equals(a));              // false

The String class also provides some other methods comparing strings.

  • public boolean equalsIgnoreCase(String anotherString) Compares strings ignoring case considerations.
  • public int compareTo(String anotherString) Compares two strings lexicographically. compareTo returns 0 exactly when the equals(Object) method would return true.
String a = “Hello World”;
String b = a;
String c = new String(“Hello World”);

System.out.println(c.equals(a));                         // true
System.out.println(b.equals(a));                         // true
System.out.println(c.equalsIgnoreCase("hello world"));   // true
System.out.println(c.compareTo(a));                      // 0
System.out.println(c.compareTo("Iello World"));
System.out.println(c.compareTo("Hello Xorld"));
System.out.println(c.compareTo("Aello World"));



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Posted in Java