Why ‘==’ and ‘equals’ Differ for Java Strings: 5 Interview Questions Explained

st1 and st2 Equality?

public class Demo2_String {
  public static void main(String[] args) {
    String st1 = "abc";
    String st2 = "abc";
    System.out.println(st1 == st2);
    System.out.println(st1.equals(st2));
  }
}

First line: true

Second line: true

In Java, the == operator compares primitive values directly, but for reference types it compares the memory addresses of the objects. Since strings are reference types, st1 and st2 point to the same interned constant "abc", both == and equals return true.

The equals method in String is overridden to compare character sequences, returning true only when the sequences are identical.

How Many Objects Are Created?

String st1 = new String("abc");

Two objects are created: one in the heap (a copy of the constant) and one in the string constant pool.

The constant pool holds the literal "abc"; the new String call creates a separate heap object that is a copy of that literal, as documented in the JDK API for String(String original).

st1 and st2 Equality with Concatenation

public class Demo2_String {
  public static void main(String[] args) {
    String st1 = "a" + "b" + "c";
    String st2 = "abc";
    System.out.println(st1 == st2);
    System.out.println(st1.equals(st2));
  }
}

Both == and equals output true because the compiler optimizes the constant concatenation into the literal "abc", which is interned in the constant pool.

Comparing st2 and st3

public class Demo2_String {
  public static void main(String[] args) {
    String st1 = "ab";
    String st2 = "abc";
    String st3 = st1 + "c";
    System.out.println(st2 == st3);
    System.out.println(st2.equals(st3));
  }
}

false

true

The == comparison is false because st3 is created at runtime using StringBuilder (or StringBuffer) and therefore resides at a different memory address than the interned literal st2. The equals comparison is true because the character sequences match.

Summary

This set of interview questions tests understanding of Java's string handling, constant pool behavior, reference equality versus content equality, and the effects of the '+' operator, all clarified through code examples and memory diagrams.