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Easyjavastrings

~75s

Comparing strings with == and equals

What does this Java program print?

public class Main {
    public static void main(String[] args) {
        String a = "code";
        String b = "code";
        String c = new String("code");
        System.out.println((a == b) + " " + (a == c) + " " + a.equals(c));
    }
}
Show the answer and the worked solution

Correct answer: B.

true false true

Worked solution

  1. a and b are both the literal "code", which the compiler places in the string pool once, so a == b compares two references to the same object and is true.
  2. new String("code") deliberately builds a separate object, so a == c compares different references and is false.
  3. equals compares contents rather than references, so a.equals(c) is true, and the output is true false true.

Answer: true false true.

The idea

== on objects asks whether two references point at the same object; equals asks whether the contents match. String literals are interned, which is why the first comparison surprises people.

Shortcut. equals compares contents, so the third value is true; new String always makes a fresh object, so a == c is false; pooled literals make a == b true.

Common trap. "false false true" assumes == on two separately declared Strings always compares different objects, missing that identical literals are interned as one.

Programming Fundamentals

17 of 40

Level

Easy

Target time

~75s

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Comparing strings with == and equals — Aptitude Question

Comparing strings with == and equals: What does this Java program print? Answer and worked solution on CodeKairo.

Question

What does this Java program print?

public class Main {
    public static void main(String[] args) {
        String a = "code";
        String b = "code";
        String c = new String("code");
        System.out.println((a == b) + " " + (a == c) + " " + a.equals(c));
    }
}

Options

  1. true true true
  2. true false true
  3. false false true
  4. true false false
Show the answer and the worked solution

Correct answer: B. true false true

Worked solution

  1. a and b are both the literal "code", which the compiler places in the string pool once, so a == b compares two references to the same object and is true.
  2. new String("code") deliberately builds a separate object, so a == c compares different references and is false.
  3. equals compares contents rather than references, so a.equals(c) is true, and the output is true false true.

Answer: true false true.

The idea

== on objects asks whether two references point at the same object; equals asks whether the contents match. String literals are interned, which is why the first comparison surprises people.

Shortcut. equals compares contents, so the third value is true; new String always makes a fresh object, so a == c is false; pooled literals make a == b true.

Common trap. "false false true" assumes == on two separately declared Strings always compares different objects, missing that identical literals are interned as one.

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