java字符串连接奇怪的行为

Question

I am getting strange string concat behavior, please help me understand this behavior .

String s3 = "ABC";
String s5 = new String(s3);

System.out.println("s5 == s3 "+ s5 == s3); // output: false
System.out.println("s5 == s3 "+ (s5 == s3)); // output: s5 == s3 false

Should first print s5 == s3 false instead of false ?

Answer 1

Here's a modified version of your first expression, which prints false :

System.out.println(("s5 == s3 " + s5) == s3); // "s5 == s3 ABC" == "ABC"

== has lower precedence than + , so concatenation is done first, then comparison follows.

To make it produce your expected output, you need to override this operator precedence, just as you did in your second sysout , which will concatenate the result of the comparison to the string.

Answer 2

So Java or any language has operator precedence, meaning some operations happen before others. In the first line, the "==" has a lower precedence (happens later) than "+", which happens earlier, so the output is the result of the "==" operation, which is false

see more on operator precedence here: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html

Answer 3

s5 == s3 checks if references are same are not.

s3 in created is string pool and s5 is new object and not referring to s3.