Java · Theory

String Comparison in Java

Theory

75/270

String Comparison in Java

== compares references (same object in memory). equals compares characters. new String("Hi") twice → == is false, equals is true.

Literals "Hi" == "Hi" can be true because of the String Pool. User input and new String(...) almost never match with ==. Always equals for content.

equalsIgnoreCase ignores capital letters. compareTo gives ordering, not just true/false.

Let's take this on the board with one tiny Main class — no extra files. String Comparison in Java — output: false then true. new String("Hi") twice = two objects (== false). equals checks the text Hi. Start from main, go line by line, and stop at each print. That output is the proof for String Comparison.

Diagram

new String("Hi")   new String("Hi")
       │                  │
       └─ == false
       └─ equals true

Exam tip

Show new String("Hi") twice: == false, equals true. That is the whole answer.

Example

public class Main {
  public static void main(String[] args) {
    String a = new String("Hi");
    String b = new String("Hi");
    System.out.println(a == b);      // false (refs)
    System.out.println(a.equals(b)); // true (content)
  }
}

String Comparison in Java — output: false then true. new String("Hi") twice = two objects (== false). equals checks the text Hi.

Short notes

Def== = same object. equals = same characters.
RuleUser input → equals, not ==.
RememberPool can make two literals ==. new String breaks that.
TrapUsing == on strings from the keyboard / database.

Java · Theory

String Comparison in Java

← All stacks

Theory

75/270

String Comparison in Java

== compares references (same object in memory). equals compares characters. new String("Hi") twice → == is false, equals is true.

Literals "Hi" == "Hi" can be true because of the String Pool. User input and new String(...) almost never match with ==. Always equals for content.

equalsIgnoreCase ignores capital letters. compareTo gives ordering, not just true/false.

Diagram

new String("Hi")   new String("Hi")
       │                  │
       └─ == false
       └─ equals true

Exam tip

Show new String("Hi") twice: == false, equals true. That is the whole answer.

Example

public class Main {
  public static void main(String[] args) {
    String a = new String("Hi");
    String b = new String("Hi");
    System.out.println(a == b);      // false (refs)
    System.out.println(a.equals(b)); // true (content)
  }
}

String Comparison in Java — output: false then true. new String("Hi") twice = two objects (== false). equals checks the text Hi.

Short notes

Def== = same object. equals = same characters.
RuleUser input → equals, not ==.
RememberPool can make two literals ==. new String breaks that.
TrapUsing == on strings from the keyboard / database.

String Comparison in Java

Example

Short notes

Questions

String Comparison in Java

Example

Short notes

Questions