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all doubts about == and equals

 
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there has been tremendous number of posts regarding the differences effects between == and equals. Comparing this and that returns true? or false? why?

can you, as expertised in SCJP or general java knowledge, provide a summary of using == and equals and the approach working out the result?

there has been a lot of helpful users providing excellent explanation, but there are too all over posts.

pls help all starters like me!
 
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I'll give it a try...

The concept to keep in mind is that == is a simple comparison of values.
  • For primitives, values are straightforward (see note below regarding NaN).
  • For objects, the "values" are references to the instances. So in comparing objects using ==, the result will be true if and only if the instances are the same.
  • There are special cases in which the same instance is "re-used." For example, the String literal pool (see Strings Literally) and autoboxed wrappers of booleans or non-floating values within the range of -128 through 127 (see JLS - 5.1.7).

    The Object class implements equals as a comparison of instances (like ==). Therefore, if equals is not overridden, then this is the default implementation. Otherwise, how equals works depends entirely on how it's implemented for a specific type.

    Note: Equality testing of the primitive NaN (not a number) always results in false. The only exception is NaN != NaN, which returns true.
     
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