Java – string instantiation and StringBuffer instantiation

I can't figure out if

String ab = "hello";        //straight initialization

String ab_1 = new String ("hello_1");  //initializing using new

Both work, but

StringBuffer bfr = new StringBuffer("hi");   //works only with new

Only valid when created with new

Why can string be instantiated directly, but StringBuffer needs new operator Someone can explain my main reason

Solution

All objects need to be instantiated with new Primitives can only be instantiated from text (int i = 0;)

The only exceptions are:

>String, allowing special initialization structures:

String s = "abc"; //can be instantiated from a literal,like primitives

> null instantiation：Object o = null;

It is defined in Java language specification #3.10:

Note: arrays also have a special initialization mode, but this is not text:

int[][] a = { { 00,01 },{ 10,11 } };