String comparison with "==" is not reliable?

Hi,

I always thought I can use "==" to compare if two Strings are equal(with same case too). I know there is a method .equals(Object o) or equalsIgnoreCase(String s). But I always thought "==" is good enough.

This morning, I found out that I was wrong.

I have two Strings, str1 and str2. They received values from some where else. When I print them out to screen, they look same.(I am aware of some unvisible thing, so I used trim()

//str1 and str2
if(str1.trim() == str2.trim())
{
System.out.println("equal");
}
else
{
System.out.println("Not equal");
}

Above code print out "Not equal". But the following code print out "equal".

if(str1.trim().equalsIgnoreCase(str2.trim()))
{
System.out.println("equal");
}
else
{
System.out.println("Not equal");
}

Can you give me some hint? Thank you.
.

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