Difference between Object equals() method and ==

Question

Hi,
Any one help me to clarify my confusion.
stud s=new stud();
stud s1=new stud();
System.out.println("Equals======>"+s.equals(s1));
System.out.println("== --------->"+(s==s1));
Result:
Equals ======> false
== ------------> false
Can you please explain what is the difference between equals method in Object class and == operator.
In which situation we use Object equals() method and == operator.
Regards,
Saravanan.K

Answer 1

corlettk wrote:
I'm not sure, but I suspect that the later Java compilers might actually generate the same byte code for both versions, i.e. I suspect the compiler has gotten smart enough to devine that && other!=null is a no-op and ignore it... Please could could someone who understands bytecode confirm or repudiate my guess?Don't need deep understanding of bytecode
Without !=null
C:>javap -v SomeClass
Compiled from "SomeClass.java"
class SomeClass extends java.lang.Object
  SourceFile: "SomeClass.java"
  minor version: 0
  major version: 49
  Constant pool:
const #1 = Method       #4.#15; //  java/lang/Object."<init>":()V
const #2 = class        #16;    //  SomeClass
const #3 = Field        #2.#17; //  SomeClass.field:Ljava/lang/Object;
const #4 = class        #18;    //  java/lang/Object
const #5 = Asciz        field;
const #6 = Asciz        Ljava/lang/Object;;
const #7 = Asciz        <init>;
const #8 = Asciz        ()V;
const #9 = Asciz        Code;
const #10 = Asciz       LineNumberTable;
const #11 = Asciz       equals;
const #12 = Asciz       (Ljava/lang/Object;)Z;
const #13 = Asciz       SourceFile;
const #14 = Asciz       SomeClass.java;
const #15 = NameAndType #7:#8;//  "<init>":()V
const #16 = Asciz       SomeClass;
const #17 = NameAndType #5:#6;//  field:Ljava/lang/Object;
const #18 = Asciz       java/lang/Object;
SomeClass();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 1: 0
public boolean equals(java.lang.Object);
  Code:
   Stack=2, Locals=2, Args_size=2
   0:   aload_1
   1:   instanceof      #2; //class SomeClass
   4:   ifeq    25
   7:   aload_1
   8:   checkcast       #2; //class SomeClass
   11:  getfield        #3; //Field field:Ljava/lang/Object;
   14:  aload_0
   15:  getfield        #3; //Field field:Ljava/lang/Object;
   18:  if_acmpne       25
   21:  iconst_1
   22:  goto    26
   25:  iconst_0
   26:  ireturn
  LineNumberTable:
   line 6: 0
}With !=null
C:>javap -v SomeClass
Compiled from "SomeClass.java"
class SomeClass extends java.lang.Object
  SourceFile: "SomeClass.java"
  minor version: 0
  major version: 49
  Constant pool:
const #1 = Method       #4.#15; //  java/lang/Object."<init>":()V
const #2 = class        #16;    //  SomeClass
const #3 = Field        #2.#17; //  SomeClass.field:Ljava/lang/Object;
const #4 = class        #18;    //  java/lang/Object
const #5 = Asciz        field;
const #6 = Asciz        Ljava/lang/Object;;
const #7 = Asciz        <init>;
const #8 = Asciz        ()V;
const #9 = Asciz        Code;
const #10 = Asciz       LineNumberTable;
const #11 = Asciz       equals;
const #12 = Asciz       (Ljava/lang/Object;)Z;
const #13 = Asciz       SourceFile;
const #14 = Asciz       SomeClass.java;
const #15 = NameAndType #7:#8;//  "<init>":()V
const #16 = Asciz       SomeClass;
const #17 = NameAndType #5:#6;//  field:Ljava/lang/Object;
const #18 = Asciz       java/lang/Object;
SomeClass();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 1: 0
public boolean equals(java.lang.Object);
  Code:
   Stack=2, Locals=2, Args_size=2
   0:   aload_1
   1:   instanceof      #2; //class SomeClass
   4:   ifeq    29
   7:   aload_1
   8:   ifnull  29
   11:  aload_1
   12:  checkcast       #2; //class SomeClass
   15:  getfield        #3; //Field field:Ljava/lang/Object;
   18:  aload_0
   19:  getfield        #3; //Field field:Ljava/lang/Object;
   22:  if_acmpne       29
   25:  iconst_1
   26:  goto    30
   29:  iconst_0
   30:  ireturn
  LineNumberTable:
   line 6: 0
}