Why can not use a String to initialize a StringBuilder object?

Question

StringBuilder sb = "asd";

In Java, this statement is obviously wrong. IDEs like eclipse will tell you that :

cannot convert from String to StringBuilder

However, a String object could be initialized by = operator.

I'd like to know some reasons related with the memory allocation.

Answer 1

Because StringBuilder is an Object and it needs to be constructed. You're getting the error because String is not a StringBuilder.

String is a special, it's designed to be between primitive and class¹. You can assign a string literal directly into a String variable, instead of calling the constructor to create a String instance.

¹interesting topic:

The designers of Java decided to retain primitive types in an object-oriented language, instead of making everything an object, so as to improve the performance of the language. Primitives are stored in the call stack, which require less storage spaces and are cheaper to manipulate. On the other hand, objects are stored in the program heap, which require complex memory management and more storage spaces.

For performance reason, Java's String is designed to be in between a primitive and a class.

More reading:

• JLS
• Java tutorials

Answer 2

"xxx" is defined as a String literal by the specification of the language* and is a String object. So you can write:

String s = "abc";        //ok we got it
Object o = "abc";        //a String is an Object
CharSequence cs = "abc"; //a String is also a CharSequence

But a String is not a StringBuilder...

_{*Quote from the JLS: "A string literal is always of type String"}

Answer 3

StringBuilder is an object not a wrapper.