连接运算符和 concat() 如何执行任何任务

Question

I can use either concatenation operator (+) or concat() method for string concatenation

String myData = "a"+"b";            // using concatenation operator
String myData = "a".concat("b");    // using concat() of String

But to concate a string with integer I cannot use concat() directly. SO I have to use either of the following logic

String myData = "a"+5;
String myData = "a".concat(String.valueOf(5));

But I found some thing strange in the following line when I want to use concatenation operator and concat

String myData = "a"+null;              //output =  anull
    String myData = "a".concat(String.valueOf(5)); // output = NullPointerException
or
    String myData = "a".concat(null); // output = NullPointerException

I have below question arised in my mind

1) How concat() method and concatenation operator works what is the difference in their logic of performing any task?
2) Can we really concat a null using (+) if so why concat() method cannot achieve the same

Thanks

Answer 1

1) The + operator (to produce String s) always goes through an intermediate StringBuilder (or StringBuffer if targeting old platforms before 1.5 (released 7 (seven) years ago)). For concatenating two or perhaps three String s concat will generally be faster due to the lack of intermediate. However, for longer concatenations + will win because there will be fewer intermediate allocations.

2) null generally indicates an error (quite possibly a design error). In general, errors should be reported as early as possible, which String.concat does. However, + or StringBuilder concatenation is often used to produce debug strings, so the null is tolerated and produces a result suitable for debugging (but not UI.).

Answer 2

There's no difference in the semantics, you just got your example wrong.

String myData = "a"+null;              //output =  anull

String myData = "a".concat(String.valueOf( (Object)null ) ); // output = anull

And this will compile and work. + behaves as if String.valueOf() was invoked for every operand.